Return to The Medical Biochemistry Page
Fatty Acid Synthesis
One might predict that the pathway for the synthesis of fatty acids would be the reversal of the oxidation pathway. However, this would not allow distinct regulation of the two pathways to occur even given the fact that the pathways are separated within different cellular compartments.
The pathway for fatty acid synthesis occurs in the cytoplasm, whereas, oxidation occurs in the mitochondria. The other major difference is the use of nucleotide co-factors. Oxidation of fats involves the reduction of FADH+ and NAD+. Synthesis of fats involves the oxidation of NADPH. However, the essential chemistry of the two processes are reversals of each other. Both oxidation and synthesis of fats utilize an activated two carbon intermediate, acetyl-CoA. However, the acetyl-CoA in fat synthesis exists temporarily bound to the enzyme complex as malonyl-CoA.
The synthesis of malonyl-CoA is the first committed step of fatty acid synthesis and the enzyme that catalyzes this reaction, acetyl-CoA carboxylase (ACC), is the major site of regulation of fatty acid synthesis. Like other enzymes that transfer CO2 to substrates, ACC requires a biotin co-factor.
The rate of fatty acid synthesis is controlled by the equilibrium between monomeric ACC and polymeric ACC. The activity of ACC requires polymerization. This conformational change is enhanced by citrate and inhibited by long-chain fatty acids. ACC is also controlled through hormone mediated phosphorylation (see below).
The acetyl groups that are the products of fatty acid oxidation are linked to CoASH. As you should recall, CoA contains a phosphopantetheine group coupled to AMP. The carrier of acetyl groups (and elongating acyl groups) during fatty acid synthesis is also a phosphopantetheine prosthetic group, however, it is attached a serine hydroxyl in the synthetic enzyme complex. The carrier portion of the synthetic complex is called acyl carrier protein, ACP. This is somewhat of a misnomer in eukaryotic fatty acid synthesis since the ACP portion of the synthetic complex is simply one of many domains of a single polypeptide. The acetyl-CoA and malonyl-CoA are transferred to ACP by the action of acetyl-CoA transacylase and malonyl-CoA transacylase, respectively. The attachment of these carbon atoms to ACP allows them to enter the fatty acid synthesis cycle.
The synthesis of fatty acids from acetyl-CoA and malonyl-CoA is carried out by fatty acid synthase, FAS. The active enzyme is a dimer of identical subunits.
All of the reactions of fatty acid synthesis are carried out by the multiple enzymatic activities of FAS. Like fat oxidation, fat synthesis involves 4 enzymatic activities. These are, β-keto-ACP synthase, β-keto-ACP reductase, 3-OH acyl-ACP dehydratase and enoyl-CoA reductase. The two reduction reactions require NADPH oxidation to NADP+.
The primary fatty acid synthesized by FAS is palmitate. Palmitate is then released from the enzyme and can then undergo separate elongation and/or unsaturation to yield other fatty acid molecules.
back to the topOrigin of Cytoplasmic Acetyl-CoA
Acetyl-CoA is generated in the mitochondria primarily from two sources, the pyruvate dehydrogenase (PDH) reaction and fatty acid oxidation. In order for these acetyl units to be utilized for fatty acid synthesis they must be present in the cytoplasm. The shift from fatty acid oxidation and glycolytic oxidation occurs when the need for energy diminishes. This results in reduced oxidation of acetyl-CoA in the TCA cycle and the oxidative phosphorylation pathway. Under these conditions the mitochondrial acetyl units can be stored as fat for future energy demands.
Acetyl-CoA enters the cytoplasm in the form of citrate via the tricarboxylate transport system (see Figure). In the cytoplasm, citrate is converted to oxaloacetate and acetyl-CoA by the ATP driven ATP-citrate lyase reaction. This reaction is essentially the reverse of that catalyzed by the TCA enzyme citrate synthase except it requires the energy of ATP hydrolysis to drive it forward. The resultant oxaloacetate is converted to malate by malate dehydrogenase (MDH).
 |
Pathway for the movement of acetyl-CoA units from within the mitochondrion to the cytoplasm for use in lipid and cholesterol biosynthesis. Note that the cytoplasmic malic enzyme catalyzed reaction generates NADPH which can be used for reductive biosynthetic reactions such as those of fatty acid and cholesterol synthesis. |
The malate produced by this pathway can undergo oxidative decarboxylation by malic enzyme. The co-enzyme for this reaction is NADP+ generating NADPH. The advantage of this series of reactions for converting mitochondrial acetyl-CoA into cytoplasmic acetyl-CoA is that the NADPH produced by the malic enzyme reaction can be a major source of reducing co-factor for the fatty acid synthase activities.
back to the topRegulation of Fatty Acid Metabolism
One must consider the global organismal energy requirements in order to effectively understand how the synthesis and degradation of fats (and also carbohydrates) needs to be exquisitely regulated. The blood is the carrier of triacylglycerols in the form of VLDLs and chylomicrons, fatty acids bound to albumin, amino acids, lactate, ketone bodies and glucose. The pancreas is the primary organ involved in sensing the organisms dietary and energetic states via glucose concentrations in the blood. In response to low blood glucose, glucagon is secreted, whereas, in response to elevated blood glucose insulin is secreted. The regulation of fat metabolism occurs via two distinct mechanisms. One is short term regulation which is regulation effected by events such as substrate availability, allosteric effectors and/or enzyme modification.
ACC is the rate-limiting (committed) step in fatty acid synthesis. There are two major isoforms of ACC in mammalian tissues. These are identified as ACC1 and ACC2. ACC1 is strictly cytosolic and is enriched in liver, adipose tissue and lactating mammary tissue. ACC2 was originally discovered in rat heart but is also expressed in liver and skeletal muscle. ACC2 has an N-terminal extension that contains a mitochondrial targeting motif and is found associated with carnitine palmitoyltransferase I (CPT I) allowing for rapid regulation of CPT I by the malonyl-CoA produced by ACC. Both isoforms of ACC are allosterically activated by citrate and inhibited by palmitoyl-CoA and other short- and long-chain fatty acyl-CoAs. Citrate triggers the polymerization of ACC1 which leads to significant increases in its activity. Although ACC2 does not undergo significant polymerization (presumably due to its mitochondrial association) it is allosterically activated by citrate. Glutamate and other dicarboxylic acids can also allosterically activate both ACC isoforms.
ACC activity can also be affected by phosphorylation. Both ACC1 and ACC2 contain at least eight sites that undergo phosphorylation. The sites of phosphorylation in ACC2 have not been as extensively studied as those in ACC1. Phosphorylation of ACC1 at three serine residues (S79, S1200, and S1215) by AMPK leads to inhibition of the enzyme. Glucagon-stimulated increases in cAMP and subsequently to increase PKA activity also lead to phosphorylation of ACC and ACC2 is a better substrate for PKA than is ACC1. The activating effects of insulin on ACC are complex and not completely resolved. It is known that insulin leads to the dephosphorylation of the serines in ACC1 that are AMPK targets in the heart enzyme. This insulin-mediated effect has not been observed in hepatocytes or adipose tissues cells. At least a portion of the activating effects of insulin are related to changes in cAMP levels. Early evidence has shown that phosphorylation and activation of ACC occurs via the action of an insulin-activated kinase. However, contradicting evidence indicates that although there is insulin-mediated phosphorylation of ACC this does not result in activation of the enzyme. Activation of α-adrenergic receptors in liver and skeletal muscle cells inhibits ACC activity as a result of phosphorylation by an as yet undetermined kinase.
Control of a given pathways' regulatory enzymes can also occur by alteration of enzyme synthesis and turn-over rates. These changes are long term regulatory effects. Insulin stimulates ACC and FAS synthesis, whereas, starvation leads to decreased synthesis of these enzymes. Adipose tissue lipoprotein lipase levels also are increased by insulin and decreased by starvation. However, in contrast to the effects of insulin and starvation on adipose tissue, their effects on heart lipoprotein lipase are just the inverse. This allows the heart to absorb any available fatty acids in the blood in order to oxidize them for energy production. Starvation also leads to increases in the levels of fatty acid oxidation enzymes in the heart as well as a decrease in FAS and related enzymes of synthesis.
Adipose tissue contains hormone-sensitive lipase (HSL), that is activated by PKA-dependent phosphorylation leading to increased fatty acid release to the blood. The activity of HSL is also affected through the action of AMPK, however, AMPK-mediated phosphorylation of HSL is inhibitory. The phosphorylation and inhibition of HSL by AMPK may seem paradoxical since the release of fatty acids stored in triglycerides would seem necessary to promote the production of ATP via fatty acid oxidation and the major function of AMPK is to shift cells to ATP production from ATP consumption. This paradigm can be explained if one considers that if the fatty acids that are released from triglycerides are not consumed they will be recycled back into triglycerides at the expense of ATP consumption. Thus, it has been proposed that inhibition of HSL by AMPK mediated-phosphorylation is a mechanism to ensure that the rate of fatty acid release does not exceed the rate at which they are utilized either by export or oxidation.
In the liver the net result of activation of HSL (due to increased acetyl-CoA levels) is the production of ketone bodies. This would occur under conditions where insufficient carbohydrate stores and gluconeogenic precursors were available in liver for increased glucose production. The increased fatty acid availability in response to glucagon or epinephrine is assured of being completely oxidized since both PKA and AMPK also phosphorylate (and as a result inhibits) ACC, thus inhibiting fatty acid synthesis.
Insulin, on the other hand, has the opposite effect to glucagon and epi leading to increased glycogen and triacylglyceride synthesis. One of the many effects of insulin is to lower cAMP levels which leads to increased dephosphorylation through the enhanced activity of protein phosphatases such as PP-1. With respect to fatty acid metabolism this yields dephosphorylated and inactive hormone sensitive lipase. Insulin also stimulates certain phosphorylation events. This occurs through activation of several cAMP-independent kinases.
Regulation of fat metabolism also occurs through malonyl-CoA induced inhibition of carnitine acyltransferase I. This functions to prevent the newly synthesized fatty acids from entering the mitochondria and being oxidized.
back to the topChREBP: Master Lipid Regulator in the Liver
The basic helix-loop-helix/leucine zipper (bHLH/LZ) transcription factor, carbohydrate-responsive element-binding protein (ChREBP) has emerged as a central regulator of lipid synthesis in liver. ChREBP was identified as a major glucose-responsive transcription factor and it is required for glucose-induced expression of the hepatic isozyme of the glycolytic enzyme pyruvate kinase (identified as L-PK). ChREBP acts in synergy with SREBP to induce lipogenic genes such as acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS).
The liver X receptors (LXRs) are members of the steroid/thyroid hormone superfamily of cytosolic ligand binding receptors that migrate to the nucleus upon ligand binding and regulate gene expression by binding to specific target sequences. There are two forms of the LXRs, LXRα and LXRβ. The LXRs form heterodimers with the retinoid X receptors (RXRs) and as such can regulate gene expression either upon binding oxysterols (e.g. 22R-hydroxycholesterol) or 9-cis-retinoic acid. LXRs are also important regulators of the lipogenic pathway. Recent evidence has demonstrated that ChREBP is a direct target of LXRs and that glucose itself can bind and activate LXRs.
Expression of the ChREBP gene is induced in the liver in response to increased glucose uptake. In addition to gene activation, the activity of ChREBP is regulated by post-translational modifications as well as sub-cellular localization. Under conditions of low (basal) glucose concentration, ChREBP is phosphorylated and resides in the cytosol. One key site of phosphorylation is serine 196 (S196) another is threonine 666 (T666). When glucose levels rise, protein phosphatase 2A (PP2A) is activated by xylulose 5-phosphate which is an intermediate in the pentose phosphate pathway. PP2A dephosphorylates S196 (which is phosphorylated by PKA) resulting in translocation of ChREBP into the nucleus. In the nucleus PP2A dephosphorylates T666 which allows ChREBP to bind to specific sequence elements in target genes termed carbohydrate response elements (ChoRE). However, ChREBP does not bind to ChoREs as a typical homodimeric bHLH transcription factor. ChREBP interacts with another bHLH protein identified as MAX-like protein X (MLX). MLX is a member of the MYC/MAX/MAD family of transcription factors that serve as interacting partners in transcription factor networks.
An emerging model of the role of ChREBP in overall glucose and lipid metabolism indicates that this transcription factor is a master regulator of glucose-mediated lipid homeostasis not only in the liver but also in adipose tissue.
back to the topElongation and Desaturation
The fatty acid product released from FAS is palmitate (via the action of palmitoyl thioesterase) which is a 16:0 fatty acid, i.e. 16 carbons and no sites of unsaturation. Elongation and unsaturation of fatty acids occurs in both the mitochondria and endoplasmic reticulum (microsomal membranes). The predominant site of these processes is in the ER membranes. Elongation involves condensation of acyl-CoA groups with malonyl-CoA. The resultant product is two carbons longer (CO2 is released from malonyl-CoA as in the FAS reaction) which undergoes reduction, dehydration and reduction yielding a saturated fatty acid. The reduction reactions of elongation require NADPH as co-factor just as for the similar reactions catalyzed by FAS. Mitochondrial elongation involves acetyl-CoA units and is a reversal of oxidation except that the final reduction utilizes NADPH instead of FADH2 as co-factor.
Desaturation occurs in the ER membranes as well and in mammalian cells involves 4 broad specificity fatty acyl-CoA desaturases (non-heme iron containing enzymes). These enzymes introduce unsaturation at C4, C5, C6 or C9. The electrons transferred from the oxidized fatty acids during desaturation are transferred from the desaturases to cytochrome b5 and then NADH-cytochrome b5 reductase. These electrons are un-coupled from mitochondrial oxidative-phosphorylation and, therefore, do not yield ATP.
Since these enzymes cannot introduce sites of unsaturation beyond C9 they cannot synthesize either linoleate (18:2Δ9,12) or linolenate (18:3Δ9,12,15). These fatty acids must be acquired from the diet and are, therefore, referred to as essential fatty acids. Linoleic is especially important in that it required for the synthesis of arachidonic acid. As we shall encounter later, arachindonate is a precursor for the eicosanoids (the prostaglandins and thromboxanes). It is this role of fatty acids in eicosanoid synthesis that leads to poor growth, wound healing and dermatitis in persons on fat free diets. Also, linoleic acid is a constituent of epidermal cell sphingolipids that function as the skins water permeability barrier.
back to the topSynthesis of Triglycerides
Fatty acids are stored for future use as triacylglycerols (TAGs) in all cells, but primarily in adipocytes of adipose tissue. TAGs constitute molecules of glycerol to which three fatty acids have been esterified. The fatty acids present in TAGs are predominantly saturated. The major building block for the synthesis of TAGs, in tissues other than adipose tissue, is glycerol. Adipocytes lack glycerol kinase, therefore, dihydroxyacetone phosphate (DHAP), produced during glycolysis, is the precursor for TAG synthesis in adipose tissue. This means that adipoctes must have glucose to oxidize in order to store fatty acids in the form of TAGs. DHAP can also serve as a backbone precursor for TAG synthesis in tissues other than adipose, but does so to a much lesser extent than glycerol.
 |
 |
Phosphatidic acid Synthesis |
Triglyceride Synthesis |
The glycerol backbone of TAGs is activated by phosphorylation at the C-3 position by glycerol kinase. The utilization of DHAP for the backbone is carried out through either of two pathways depending upon whether the synthesis of triglycerides is carried out in the mitochondria and ER or the ER and the peroxisomes. In the former case the action of glycerol-3-phosphate dehydrogenase, a reaction that requires NADH (the same reaction as that used in the glycerol-phosphate shuttle), converts DHAP to glycerol-3-phosphate. Glycerol-3-phosphate acyltransferase then esterifies a fatty acid to glycerol-3-phosphate generating the monoacylglycerol phosphate structure called lysophosphatidic acid. The second reaction pathway utilizes the peroxisomal enzyme DHAP acyltransferase to fatty acylate DHAP to acyl-DHAP which is then reduced by the NADPH-requiring enzyme acyl-DHAP reductase. An interesting feature of the latter pathway is that DHAP acyltransferase is one of only a few enzymes that are targeted to the peroxisomes through the recognition of a peroxisome targeting sequence 2 (PTS2) motif in the enzyme. Most peroxisomal enzymes contain a PTS1 motif. For more information on peroxisome enzymes see the Zellweger syndrome page.
The fatty acids incorporated into TAGs are activated to acyl-CoAs through the action of acyl-CoA synthetases. Two molecules of acyl-CoA are esterified to glycerol-3-phosphate to yield 1,2-diacylglycerol phosphate (commonly identified as phosphatidic acid). The phosphate is then removed, by phosphatidic acid phosphatase (PAP1), to yield 1,2-diacylglycerol, the substrate for addition of the third fatty acid. Intestinal monoacylglycerols, derived from the hydrolysis of dietary fats, can also serve as substrates for the synthesis of 1,2-diacylglycerols.
Recent studies have identified a critical role for the enzyme PAP1 in overall TAG and phospholipid homeostasis. In the yeast Saccharomyces cerevisiae, the PAP1 gene was identified as Smp2p and the encoded protein was shown to be the yeast ortholog of the mammalian protein called lipin-1. The fission yeast lipin-1 ortholog is identified as Ned1p. Lipin-1 is only one of four lipin proteins identified in mammals. The lipin-1 gene (symbol = LPN1) was originally identified in a mutant mouse called the fatty liver dystrophy (fld) mouse. The mutation causing this disorder was found to reside in the LPN1 gene. There are three lipin genes with the LPN1 gene encoding two isoforms derived through alternative splicing. These two lipin-1 isoforms are identified as lipin-1A and lipin-1B. Mutations in the LPN2 gene have recently been associated with Majeed syndrome which is characterized by chronic recurrent osteomyelitis, cutaneous inflammation, recurrent fever, and congenital dyserythropoietic anemia. In addition to the obvious role of lipin-1 in TAG synthesis, evidence indicates that the protein is also required for the development of mature adipocytes, coordination of peripheral tissue glucose and fatty acid storage and utilization, and serves as a transcriptional co-activator. The latter function has significance to diabetes as it has been shown that some of the effects of the thiazolidinedione (TZD) class of drugs used to treat the hyperglycemia associated with type 2 diabetes are exerted via the effects of lipin-1. Lipin-1 has been shown to interact with peroxisome proliferator-activated receptor-γ [PPARγ] co-activator 1α (PGC-1α) and PPARα. The interactions of lipin-1 with these other transcription factors leads to increased expression of fatty acid oxidizing genes such as carnitine palmitoyl transferase-1, acyl CoA oxidase, and medium-chain acylCoA dehydrogenase (MCAD).
back to the topPhospholipid Structures
Phospholipids are synthesized by esterification of an alcohol to the phosphate of phosphatidic acid (1,2-diacylglycerol 3-phosphate). Most phospholipids have a saturated fatty acid on C-1 and an unsaturated fatty acid on C-2 of the glycerol backbone. The most commonly added alcohols (serine, ethanolamine and choline) also contain nitrogen that may be positively charged, whereas, glycerol and inositol do not. The major classifications of phospholipids are:
 |
Phosphatidylcholine (PC) |
 |
Phosphatidylethanolamine (PE) |
 |
Phosphatidylserine (PS) |
 |
Phosphatidylinositol (PI) |
 |
Phosphatidylglycerol (PG) |
 |
Diphosphatidylglycerol (DPG) |
back to the top
Phospholipid Synthesis
Phospholipids can be synthesized by two mechanisms. One utilizes a CDP-activated polar head group for attachment to the phosphate of phosphatidic acid. The other utilizes CDP-activated 1,2-diacylglycerol and an inactivated polar head group.
PC:This class of phospholipids is also called the lecithins. At physiological pH, phosphatidylcholines are neutral zwitterions. They contain primarily palmitic or stearic acid at carbon 1 and primarily oleic, linoleic or linolenic acid at carbon 2. The lecithin dipalmitoyllecithin is a component of lung or pulmonary surfactant. It contains palmitate at both carbon 1 and 2 of glycerol and is the major (80%) phospholipid found in the extracellular lipid layer lining the pulmonary alveoli. Choline is activated first by phosphorylation and then by coupling to CDP prior to attachment to phosphatidic acid. PC is also synthesized by the addition of choline to CDP-activated 1,2-diacylglycerol. A third pathway to PC synthesis, involves the conversion of either PS or PE to PC. The conversion of PS to PC first requires decarboxylation of PS to yield PE; this then undergoes a series of three methylation reactions utilizing S-adenosylmethionine (SAM) as methyl group donor.
PE:These molecules are neutral zwitterions at physiological pH. They contain primarily palmitic or stearic acid on carbon 1 and a long chain unsaturated fatty acid (e.g. 18:2, 20:4 and 22:6) on carbon 2. Synthesis of PE can occur by two pathways. The first requires that ethanolamine be activated by phosphorylation and then by coupling to CDP. The ethanolamine is then transferred from CDP-ethanolamine to phosphatidic acid to yield PE. The second involves the decarboxylation of PS.
PS:Phosphatidylserines will carry a net charge of –1 at physiological pH and are composed of fatty acids similar to the phosphatidylethanolamines. The pathway for PS synthesis involves an exchange reaction of serine for ethanolamine in PE. This exchange occurs when PE is in the lipid bilayer of the a membrane. As indicated above, PS can serve as a source of PE through a decarboxylation reaction.
PI:These molecules contain almost exclusively stearic acid at carbon 1 and arachidonic acid at carbon 2. Phosphatidylinositols composed exclusively of non-phosphorylated inositol exhibit a net charge of –1 at physiological pH. These molecules exist in membranes with various levels of phosphate esterified to the hydroxyls of the inositol. Molecules with phosphorylated inositol are termed polyphosphoinositides. The polyphosphoinositides are important intracellular transducers of signals emanating from the plasma membrane. The synthesis of PI involves CDP-activated 1,2-diacylglycerol condensation with myo-inositol. PI subsequently undergoes a series of phosphorylations of the hydroxyls of inositol leading to the production of polyphosphoinositides. One polyphosphoinositide (phosphatidylinositol 4,5-bisphosphate, PIP2) is a critically important membrane phospholipid involved in the transmission of signals for cell growth and differentiation from outside the cell to inside.
PG:Phosphatidylglycerols exhibit a net charge of –1 at physiological pH. These molecules are found in high concentration in mitochondrial membranes and as components of pulmonary surfactant. Phosphatidylglycerol also is a precursor for the synthesis of cardiolipin. PG is synthesized from CDP-diacylglycerol and glycerol-3-phosphate. The vital role of PG is to serve as the precursor for the synthesis of diphosphatidylglycerols (DPGs).
DPG:These molecules are very acidic, exhibiting a net charge of –2 at physiological pH. They are found primarily in the inner mitochondrial membrane and also as components of pulmonary surfactant. One important class of diphosphatidylglycerols is the cardiolipins. These molecules are synthesized by the condensation of CDP-diacylglycerol with PG.
The fatty acid distribution at the C–1 and C–2 positions of glycerol within phospholipids is continually in flux, owing to phospholipid degradation and the continuous phospholipid remodeling that occurs while these molecules are in membranes. Phospholipid degradation results from the action of phospholipases. There are various phospholipases that exhibit substrate specificities for different positions in phospholipids.
In many cases the acyl group which was initially transferred to glycerol, by the action of the acyl transferases, is not the same acyl group present in the phospholipid when it resides within a membrane. The remodeling of acyl groups in phospholipids is the result of the action of phospholipase A1 (PLA1) and phospholipase A2 (PLA2).
Sites of Action of the Phospholipases A1, A2, C and D.
The products of these phospholipases are called lysophospholipids and can be substrates for acyl transferases utilizing different acyl-CoA groups. Lysophospholipids can also accept acyl groups from other phospholipids in an exchange reaction catalyzed by lysolecithin:lecithin acyltransferase (LLAT).
PLA2 is also an important enzyme, whose activity is responsible for the release of arachidonic acid from the C–2 position of membrane phospholipids. The released arachidonate is then a substrate for the synthesis of the eicosanoids. In fact there is not just a single PLA2 enzyme. At least 19 enzymes have been identified with PLA2 activity. There are 10 isozymes that are in the secretory pathway and these PLA2 isozymes are abbreviated sPLA2. These secretory enzymes are low molecular weight proteins that are Ca2+-requiring and are involved in numerous processes including modification of eicosanoid generation, host defense, and inflammation. The cytosolic PLA2 family (cPLA2) comprises three isozymes with cPLA2α being an essential component of the initiation of arachidonic acid metabolism. Like the sPLA2 enzymes, the cPLA2 enzymes are tightly regulated by Ca2+. In addition, this class of PLA2 enzyme is regulated by phosphorylation. There is an additional family of two PLA2 isozymes that are not dependent on Ca2+ for activity and they identified as iPLA2. This latter class of enzyme is involved primarily with the remodeling of phospholipids.
back to the topPlasmalogens
Plasmalogens are glycerol ether phospholipids. They are of two types, alkyl ether (–O–CH2–) and alkenyl ether (–O–CH=CH–). Dihydroxyacetone phosphate serves as the glycerol precursor for the synthesis of glycerol ether phospholipids. Three major classes of plasmalogens have been identified: choline, ethanolamine and serine plasmalogens. Ethanolamine plasmalogen is prevalent in myelin. Choline plasmalogen is abundant in cardiac tissue.
One particular choline alkyl ether plasmalogen (1-O-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine) has been identified as an extremely powerful biological mediator, capable of inducing cellular responses at concentrations as low as 10–11M. This molecule is called platelet activating factor, PAF. PAF functions as a mediator of hypersensitivity, acute inflammatory reactions and anaphylactic shock. PAF is synthesized in response to the formation of antigen-IgE complexes on the surfaces of basophils, neutrophils, eosinophils, macrophages and monocytes. The synthesis and release of PAF from cells leads to platelet aggregation and the release of serotonin from platelets. PAF also produces responses in liver, heart, smooth muscle, and uterine and lung tissues.
Platelet activating factor
back to the top
Metabolism of the Sphingolipids
The sphingolipids, like the phospholipids, are composed of a polar head group and two nonpolar tails. The core of sphingolipids is the long-chain amino alcohol, sphingosine. Amino acylation, with a long chain fatty acid, at carbon 2 of sphingosine yields a ceramide.
Top: Sphingosine
Bottom: Basic composition of a ceramide
"n" indicates any fatty acid may be N-acetylated at this position.
The sphingolipids include the sphingomyelins and glycosphingolipids (the cerebrosides, sulfatides, globosides and gangliosides). Sphingomyelins are the only sphingolipid that are phospholipids. Sphingolipids are a component of all membranes but are particularly abundant in the myelin sheath.
Sphingomyelins are sphingolipids that are also phospholipids. Sphingomyelins are important structural lipid components of nerve cell membranes. The predominant sphingomyelins contain palmitic or stearic acid N-acylated at carbon 2 of sphingosine.
The sphingomyelins are synthesized by the transfer of phosphorylcholine from phosphatidylcholine to a ceramide in a reaction catalyzed by sphingomyelin synthase.
A Sphingomyelin
Defects in the enzyme acid sphingomyelinase result in the lysosomal storage disease known as Niemann-Pick disease. There are in fact two major forms of Niemann-Pick (NP) disease. NP disease caused by acid sphingomyelinase deficiencies comprises types A and B, referred to as NP-A and NP-B. The other form of NP disease comprises types C1 and C2, the former due to defects in the NPC1 gene and the latter due presumably to defects in a gene identified as NPC2.
Glycosphingolipids, or glycolipids, are composed of a ceramide backbone with a wide variety of carbohydrate groups (mono- or oligosaccharides) attached to carbon 1 of sphingosine. The four principal classes of glycosphingolipids are the cerebrosides, sulfatides, globosides and gangliosides.
Cerebrosides have a single sugar group linked to ceramide. The most common of these is galactose (galactocerebrosides), with a minor level of glucose (glucocerebrosides). Galactocerebrosides are found predominantly in neuronal cell membranes. By contrast glucocerebrosides are not normally found in membranes, especially neuronal membranes; instead, they represent intermediates in the synthesis or degradation of more complex glycosphingolipids.
Galactocerebrosides are synthesized from ceramide and UDP-galactose. Excess lysosomal accumulation of glucocerebrosides is observed in Gaucher disease.
A Glucocerebroside
Sulfatides: The sulfuric acid esters of galactocerebrosides are the sulfatides. Sulfatides are synthesized from galactocerebrosides and activated sulfate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Excess accumulation of sulfatides is observed in metachromatic leukodystrophy (sulfatide lipodosis).
Globosides: Globosides represent cerebrosides that contain additional carbohydrates, predominantly galactose, glucose or GalNAc. Lactosyl ceramide is a globoside found in erythrocyte plasma membranes. Globotriaosylceramide (also called ceramide trihexoside) contains glucose and two moles of galactose and accumulates, primarily in the kidneys, of patients suffering from Fabry disease.
Gangliosides: Gangliosides are very similar to globosides except that they also contain NANA in varying amounts. The specific names for gangliosides are a key to their structure. The letter G refers to ganglioside, and the subscripts M, D, T and Q indicate that the molecule contains mono-, di-, tri and quatra(tetra)-sialic acid. The numerical subscripts 1, 2 and 3 refer to the carbohydrate sequence that is attached to ceramide; 1 stands for GalGalNAcGalGlc-ceramide, 2 for GalNAcGalGlc-ceramide and 3 for GalGlc-ceramide.
Structure of a GM2 Ganglioside
Deficiencies in lysosomal enzymes, which normally are responsible for the degradation of the carbohydrate portions of various gangliosides, underlie the symptoms observed in rare autosomally inherited diseases termed lysosomal storage diseases, (also called the sphingolipidoses or lipid storage diseases) many of which are listed below.
back to the topClinical Significances of Sphingolipids
Some of the most devastating inborn errors in metabolism are those associated with defects in the enzymes responsible for the lysosomal degradation of membrane glycosphingolipids which are particularly abundant in the membranes of neural cells. Many of these disorders lead to severe psycho-motor retardation and early lethality. Because the disorders are caused by defective lysosomal enzymes, with the result being lysosomal accumulation of pathway intermediates, these are often referred to as lysosomal storage diseases. The mucopolysaccharidoses are another class of disorders that are members of the lysosomal storage diseases.
The following figure shows several of the pathways and intermediates in glycosphingolipid metabolism. Enzymes are indicated in green and the disease(s) associated with defects in the indicated enzyme are shown in blue. SAP-A, SAP-B, SAP-C, and SAP-D are the saposins which are a family of small glycoproteins. The saposins (A, B, C, and D) are all derived from a single precursor, prosaposin. The mature saposins, as well as prosaposin, activate several lysosomal hydrolases involved in the metabolism of various sphingolipids. Prosaposin is proteolytically processed to saposins A, B, C and D, within lysosomes but also exists as an integral membrane protein not destined for lysosomal entry. Uncleaved prosaposin can be found in many biological fluids such as seminal plasma, human milk, and cerebrospinal fluid, where it appears to have a different function. Each of the disease names in the image can be clicked to go to a descriptive page of that disease. The Table below the Figure lists some additional lysosomal storage diseases caused by defective sphingolipid metabolism.
Disorders Associated with Abnormal Sphingolipid Metabolism
Disorder |
Enzyme Deficiency |
Accumulating Substance |
Symptoms |
| Tay-Sachs disease | HexA | GM2 ganglioside | infantile form: rapidly progressing mental retardation, blindness, early mortality |
| Sandhoff disease | HexA and HexB | globoside; GM2 ganglioside | infantile form: same symptoms as Tay-Sachs, progresses more rapidly |
| Tay-Sachs AB variant GM2 activator deficiency |
GM2 activator (GM2A) | GM2 ganglioside | infantile form: same symptoms as Tay-Sachs |
| Gaucher disease | acid β-glucosidase (glucocerebrosidase) |
glucocerebrosides | hepatosplenomegaly, mental retardation in infantile form, long bone degeneration |
| Fabry disease | α-galactosidase A | globotriaosylceramide; also called ceramide trihexoside (CTH) | kidney failure, skin rashes |
| Niemann-Pick diseases Types A and B Type C |
sphingomyelinase NPC1 protein |
sphingomyelins LDL-derived cholesterol |
type A is severe disorder with heptosplenomegaly, severe neurological involvement leading to early death, type B only visceral involvement |
| Krabbe disease; globoid cell leukodystrophy (GLD) | galactocerebrosidase | galactocerebrosides | mental retardation, myelin deficiency |
| GM1 gangliosidosis | β-galactosidase-1 | GM1 gangliosides | mental retardation, skeletal abnormalities, hepatomegaly |
| Metachromatic leukodystrophy; sulfatide lipodosis |
arylsulfatase A | sulfatides | mental retardation, metachromasia of nerves |
| Fucosidosis | α-fucosidase | pentahexosylfucoglycolipid | cerebral degeneration, thickened skin, muscle spasticity |
| Farber lipogranulomatosis | acid ceramidase | ceramides | hepatosplenomegaly, painful swollen joints |
One of the most clinically important classes of sphingolipids are those that confer antigenic determinants on the surfaces of cells, particularly the erythrocytes. The ABO blood group antigens are the carbohydrate moieties of glycolipids on the surface of cells as well as the carbohydrate portion of serum glycoproteins. When present on the surface of cells the ABO carbohydrates are linked to sphingolipid and are therefore of the glycosphingolipid class. When the ABO carbohydrates are associated with protein in the form of glycoproteins they are found in the serum and are referred to as the secreted forms. Some individuals produce the glycoprotein forms of the ABO antigens while others do not. This property distinguishes secretors from non-secretors, a property that has forensic importance such as in cases of rape.
 |
Structure of the ABO blood group carbohydratesR represents the linkage to protein in the secreted forms, sphingolipid (ceramide) in the cell-surface bound form, open square = GlcNAc, open diamond = galactose, filled square = fucose, filled diamond = GalNAc. The linkage in the glycolipid form may include a glucose in a β-1,3 or β-1,4 to the initial galactose residue. |
A significant cause of death in premature infants and, on occasion, in full term infants is respiratory distress syndrome (RDS) or hyaline membrane disease. This condition is caused by an insufficient amount of pulmonary surfactant. Under normal conditions the surfactant is synthesized by type II endothelial cells and is secreted into the alveolar spaces to prevent atelectasis following expiration during breathing. Surfactant is comprised primarily of dipalmitoyllecithin; additional lipid components include phosphatidylglycerol and phosphatidylinositol along with proteins of 18 and 36 kDa (termed surfactant proteins). During the third trimester the fetal lung synthesizes primarily sphingomyelin, and type II endothelial cells convert the majority of their stored glycogen to fatty acids and then to dipalmitoyllecithin. Fetal lung maturity can be determined by measuring the ratio of lecithin to sphingomyelin (L/S ratio) in the amniotic fluid. An L/S ratio less than 2.0 indicates a potential risk of RDS. The risk is nearly 75-80% when the L/S ratio is 1.5.
The carbohydrate portion of the ganglioside, GM1, present on the surface of intestinal epithelial cells, is the site of attachment of cholera toxin, the protein secreted by Vibrio cholerae.
These are just a few examples of how sphingolipids and glycosphingolipids are involved in various recognition functions at the surface of cells. As with the complex glycoproteins, an understanding of all of the functions of the glycolipids is far from complete.
back to the topMetabolism of the Eicosanoids
The eicosanoids consist of the prostaglandins (PG), thromboxanes (TX), leukotrienes (LT) and lipoxins (LX). The PGs and TXs are collectively identified as prostanoids. Prostaglandins were originally shown to be synthesized in the prostate gland, thromboxanes from platelets (thrombocytes) and leukotrienes from leukocytes, hence the derivation of their names. The lipoxins are anti-inflammatory eicosanoids synthesized through lipoxygenase interactions (hence the derivation of the name). Lipoxins are potent anti-inflammatory eicosanoid derivatives and their synthesis can be increased in response to ingestion of aspirin (see the Aspirin page for more details on lipoxin functions). An additional class of anti-inflammatory lipid compounds, whose syntheses can also be triggered by aspirin, are the resolvins (Rv) and the protectins (PD) and these are also discussed in the Aspirin page.
Structures of Representative Clinically Relevant Eicosanoids |
|
 |
PGE2 |
 |
TXA2 |
 |
LTB4 |
 |
LXA4 |
 |
LXB4 |
The eicosanoids produce a wide range of biological effects on inflammatory responses (predominantly those of the joints, skin and eyes), on the intensity and duration of pain and fever, and on reproductive function (including the induction of labor). They also play important roles in inhibiting gastric acid secretion, regulating blood pressure through vasodilation or constriction, and inhibiting or activating platelet aggregation and thrombosis.
The principal eicosanoids of biological significance to humans are a group of molecules derived from the C20 fatty acid, arachidonic acid. Minor eicosanoids are derived from eicosapentaenoic acid which is itself derived from α-linolenic acid obtained in the diet. The major source of arachidonic acid is through its release from cellular stores. Within the cell, it resides predominantly at the C–2 position of membrane phospholipids and is released from there upon the activation of PLA2 (see diagram above). The immediate dietary precursor of arachidonate is linoleic acid. Linoleic acid is converted to arachidonic acid through the steps outlined in the figure below. Linoleic acid (arachidonate precursor) and α-linolenic acid (eicosapentaenoate precursor) are essential fatty acids, therefore, their absence from the diet would seriously threaten the body's ability to synthesize eicosanoids.
Arachidonic Acid Synthesis
back to the top
All mammalian cells except erythrocytes synthesize eicosanoids. These molecules are extremely potent, able to cause profound physiological effects at very dilute concentrations. All eicosanoids function locally at the site of synthesis, through receptor-mediated G-protein linked signaling pathways.
Two main pathways are involved in the biosynthesis of eicosanoids. The prostaglandins and thromboxanes are synthesized by the cyclic pathway, the leukotrienes by the linear pathway.
 |
Synthesis of the clinically relevant prostaglandins and thromboxanes from arachidonic acid. Numerous stimuli (e.g. epinephrine, thrombin and bradykinin) activate PLA2 which hydrolyzes arachidonic acid from membrane phospholipids. The prostaglandins are identified as PG and the thromboxanes as TX. Prostaglandin PGI2 is also known as prostacyclin. PGE2 is synthesized from PGH2 via the action of one of several PGE synthases, where PGE synthase-1 (PGES1) appears to be the key enzyme. Two forms of PGD2 synthases have been identified (hematopoietic and lipocalin prostaglandin D synthases, hPGDS and lPGDS) that convert PGD2 from PGH2. Prostacyclin (PGI2) is synthesized from PGH2 via the action of prostacyclin synthase (PGIS). Prostaglandin F synthase (PGFS) converts PGH2 to PGF2α or PGD2 to 9α,11β-PGF2α,β. The subscript 2 in each molecule refers to the number of carbon-carbon double bonds present. Place mouse over structure names to see the structure. |
 |
Synthesis of the clinically relevant leukotrienes from arachidonic acid. The leukotrienes are identified as LT. Numerous stimuli (e.g. epinephrine, thrombin and bradykinin) activate PLA2 which hydrolyzes arachidonic acid from membrane phospholipids. The enzyme 5-lipoxygenase (5-LOX) in association with the protein, 5-LOX activating protein (FLAP), catalyzes the conversion of arachidonic acid, first to 5-hydroperoxyeicosatetraenoic acid (5-HPETE) which spontaneously reduces to 5-hydroxyeicosatetraenoic acid (5-HETE), and then to LTA4. LTA4 is unstable and is converted to LTB4 in neutrophils and monocytes harboring LTA4 hydrolase. In mast cells and eosinophils, which harbor LTC4 synthase, LTA4 is converted to LTC4. The leukotrienes LTC4, LTD4, LTE4 and LTF4 are known as the peptidoleukotrienes or the cysteinyl leukotrienes because of the presence of amino acids. The peptidoleukotrienes, LTC4, LTD4 and LTE4 are components of slow-reacting substance of anaphylaxis (SRSA). SRSA was originally identified as an activity released from sensitized lung after immunologic challenge. The subscript 4 in each molecule refers to the number of carbon-carbon double bonds present. Place mouse over structure names to see the structure. |
 |
Synthesis of the lipoxins from arachidonic acid. Three pathways exist for the synthesis of the lipoxins. The "classic" pathway involves 5-LOX activity in leukocytes followed by 12-LOX action in platelets. The action of 15-LOX in epithelial cell (such as in the airway) followed by 5-LOX action in leukocytes is the second major lipoxin synthesis pathway. The action of aspirin on COX-2 (see the Aspirin page for more details) in epithelial, or endothelial cells as wells as in monocytes results in the eventual production of the 15 epi-lipoxins (also referred to as aspirin triggered lipoxins, ATLs). Place mouse over structure names to see the structure. |
The linear pathway is initiated through the action of lipoxygenases (LOXs) of which there are three forms, 5-LOX, 12-LOX and 15-LOX. It is 5-LOX that gives rise to the leukotrienes. The leukotrienes are synthesized by several different cell types including white blood cells (leukocytes, hence the derivation of the name of the compounds), mast cells, lung, spleen, brain and heart.
The lipoxins are synthesized through the concerted actions of 15-LOX (acting on arachidonic acid in epithelial cells, such as in the airway) followed by 5-LOX in leukocytes or through the actions of 5-LOX in leukocytes followed by 12-LOX action in platelets. Details of the functions of the lipoxins can be found in the Aspirin page.
The cyclic pathway is initiated through the action of prostaglandin G/H synthase, PGS (also called prostaglandin endoperoxide synthetase). This enzyme possesses two activities, cyclooxygenase (COX) and peroxidase. There are 3 forms of the COX activity. COX-1 (PGS-1) is expressed constitutively in gastric mucosa, kidney, platelets, and vascular endothelial cells. COX-2 (PGS-2) is inducible and is expressed in macrophages and monocytes in response to inflammation. The primary triggers for COX-2 induction in monocytes and macrophages are platelet-activating factor, PAF and interleukin-1, IL-1. Both COX-1 and COX-2 catalyze the 2-step conversion of arachidonic acid to PGG2 and then to PGH2. Most recently a splice variant of COX-1 mRNA has been found in many tissues such as heart, kidney and several neuronal tissues. This variant mRNA retains intron 1 from the COX-1 gene and the encoded protein has been termed COX-3. In vitro evidence indicates that the canine COX-3 enzyme is inhibited by acetominophen. However, data also shows that in humans the consequence of inclusion of intron 1 sequences leads to synthesis of a truncated protein with no significant homology to COX-1 or COX-2 and that this protein appears unresponsive to acetominophen action.
A widely used class of drugs, the non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, indomethacin, naproxen, and phenylbutazone all act upon the cyclooxygenase activity, inhibiting both COX-1 and COX-2. Aspirin is unique among the class of NSAIDs in that its actions on relief from pain (analgesia) and as an anti-inflammatory as well as a heart protective drug are not solely due to its ability to inhibit COX activity (see the Aspirin page for details).
Because inhibition of COX-1 activity in the gut is associated with NSAID-induced ulcerations, pharmaceutical companies have developed drugs targeted exclusively against the inducible COX-2 activity [e.g. Celebrex® (celecoxib), Prexige® (lumiracoxib) and the recently removed Vioxx® (rofecoxib) and Bextra® (valdecoxib)]. Unlike the effects of aspirin on the action and synthesis activities of COX-2, this latter class of drug does not induce the synthesis of anti-inflammatory lipids. In fact the cardiac benefits of low-dose aspirin are negated when taken along with COX-2 specific inhibitors such as Celebrex.
Another class of anti-inflammatory drug, the corticosteroidal drugs, act to inhibit PLA2, thereby inhibiting the release of arachidonate from membrane phospholipids and the subsequent synthesis of eicosanoids.
back to the topProperties of Significant Eicosanoids
Each of the eicosanoids function via interactions with cell-surface receptors that are members of the G-protein coupled receptor (GPCR) family. There are at least 9 characterized prostaglandin receptors. Receptors that bind the prostaglandin D family of lipids are called the PGD receptors and those that bind E family prostaglandins are called the PGE receptors. The PGD receptors are coupled to the production of cAMP and activation of PKA. The PGE receptors couple to the activation of PLCγ and as a consequence the production of DAG and IP3 from membrane phospholipids. The receptor for prostacyclin (PGI2) is called the PC receptor and it couples to production of cAMP. There are 2 receptors that bind LTB4 called BLT1 and BLT2. The peptidoleukotrienes (cysteinyl leukotrienes) bind to receptors called CysLT1 and CysLT2. The thromboxane receptor is coupled to the activation of PLCγ.
Eicosanoid |
Major site(s) of synthesis |
Major biological activities |
| LXA4 | platelets, endothelial cells, mucosal epithelial cells and other leukocytes via inteactions with PMNs | reduce PMN and eosinophil infiltration to sites of inflammation, stimulate nonphlogistic (non-inflammatory-inducing) monocyte recruitment, stimulate macrophage phagocytosis of apoptotic PMNs, block IL-8 (chemokine) expression, block TNF-α release and actions, stimulate TGF-β action |
| LXB4 | platelets, endothelial cells, mucosal epithelial cells and other leukocytes via inteactions with PMNs | same as for LXA4 |
| PGD2 | mast cells | inhibits platelet and leukocyte aggregation, decreases T-cell proliferation and lymphocyte migration and secretion of IL-1α and IL-2; induces vasodilation and production of cAMP |
| PGE2 | kidney, spleen, heart | increases vasodilation and cAMP production, enhancement of the effects of bradykinin and histamine, induction of uterine contractions and of platelet aggregation, maintaining the open passageway of the fetal ductus arteriosus; decreases T-cell proliferation and lymphocyte migration and secretion of IL-1α and IL-2 |
| PGF2α | kidney, spleen, heart | increases vasoconstriction, bronchoconstriction and smooth muscle contraction |
| PGH2 | precursor to thromboxanes A2 and B2, induction of platelet aggregation and vasoconstriction | |
| PGI2 | heart, vascular endothelial cells | inhibits platelet and leukocyte aggregation, decreases T-cell proliferation and lymphocyte migration and secretion of IL-1α and IL-2; induces vasodilation and production of cAMP |
| TXA2 | platelets | induces platelet aggregation, vasoconstriction, lymphocyte proliferation and bronchoconstriction |
| TXB2 | platelets | induces vasoconstriction |
| LTB4 | monocytes, basophils, neutrophils, eosinophils, mast cells, epithelial cells | powerful inducer of leukocyte chemotaxis and aggregation, vascular permeability, T-cell proliferation and secretion of INF-γ, IL-1 and IL-2 |
| LTC4 | monocytes and alveolar macrophages, basophils, eosinophils, mast cells, epithelial cells | component of SRS-A, microvascular vasoconstrictor, vascular permeability and bronchoconstriction and secretion of INF-γ, recruitment of leukocytes to sites of inflammation, enhance mucus secretions in gut and airway |
| LTD4 | monocytes and alveolar macrophages, eosinophils, mast cells, epithelial cells | same as LTC4 |
| LTE4 | mast cells and basophils | same as LTC4 |
back to the top
Return to The Medical Biochemistry Page
Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu
