Much of the tyrosine that does not get incorporated into proteins is catabolized for energy production. Another significant fate of tyrosine is conversion to the catecholamines. The catecholamines are dopamine, norepinephrine, and epinephrine. All three catecholamines exert effects in numerous locations in the body as either a neurotransmitter or as a hormone. Within the brain the catecholamines exert their effects as neurotransmitters, in the periphery they do so as hormones. The details of catecholamine action exerted via activation of specific receptors are only covered briefly in this section. For greater detail go to the Biochemistry of Nerve Transmission page.
Tyrosine is transported into catecholamine-secreting neurons and adrenal medullary cells where catecholamine synthesis takes place. The first step in the process requires tyrosine hydroxylase which, like phenylalanine hydroxylase (of tyrosine synthesis), requires tetrahydrobiopterin (H4B, or written as BH4) as cofactor. The tyrosine hydroxylase reaction represents the rate-limiting reaction of catecholamine biosynthesis. The dependence of tyrosine hydroxylase on H4B necessitates the coupling to the action of dihydropteridine reductase (DHPR) as is the situation for phenylalanine hydroxylase and tryptophan hydroxylase (see below). The product of the tyrosine hydroxylase reaction is 3,4-dihydrophenylalanine (L-DOPA; more commonly just DOPA). The enzyme DOPA decarboxylase then converts DOPA to dopamine. The enzyme dopamine β-hydroxylase then converts dopamine to norepinephrine. Dopamine β-hydroxylase is a major vitamin C and copper (Cu2+)-dependent enzyme whose activity is negatively affected in Menkes disease. The last step of catecholamine biosynthesis is the conversion of norepinephrine to epinephrine which involves a methylation reaction. The enzyme phenylethanolamine N-methyltransferase catalyzes this methylation reaction utilizing SAM as a methyl donor. In addition to epinephrine synthesis, the last reaction generates S-adenosylhomocysteine. Within the substantia nigra locus of the brain, and some other regions of the brain, synthesis proceeds only to dopamine. Within the locus coeruleus region of the brain the end product of the pathway is norepinephrine. Within adrenal medulla chromaffin cells, tyrosine is converted to norepinephrine and epinephrine.
The tyrosine hydroxylase gene (symbol: TH) is located on chromosome 11p15.5 and is composed of 14 exons that generate three alternatively spliced mRNAs that encode tyrosine hydroxylase isoforms a, b, and c, Mutations in the TH gene are associated with the develpoment of Segawa syndrome. Segawa syndrome is an autosomal recessive disorder that manifests in early infancy as a DOPA-responsive dystonia. Two distinct phenotypes are associated with Segawa syndrome, one that manifests very early and presents with symptoms of greater severity, and a later onset less severe type that responds better to L-DOPA therapy. DOPA decarboxylase is encoded by the aromatic L-amino acid decarboxylase gene (symbol: DDC) which is located on chromosome7p12.2 and is composed of 18 exons that generate multiple alternatively spliced mRNAs. The DDC encoded enzyme is also responsible for the conversion of 5-hydroxytryptophan to serotonin (see next section) and tryptophan to tryptamine. The dopamine β-hydroxylase gene (symbol: DBH) is located on chromosome 9q34 and is composed of 12 exons that encode a protein of 617 amino acids. Dopamine β-hydroxylase is found as both a soluble and a membrane-bound enzyme dependent upon whether or not the signal peptide from the precursor protein is present. The phenylethanolamine N-methyltransferase gene (symbol: PNMT) is located on chromosome 17q12 and is composed of 5 exons that generate two alternatively spliced mRNAs, one of which is non-coding, the other encodes a protein of 282 amino acids.
Synthesis of the catecholamines from tyrosine. Tyrosine is converted to each of the three catecholamines through a series of four reactions. The tissue from which the neurotransmitter/hormone is derived expresses a specific set, or all, of these enzymes such that only dopamine (substantia nigra) is the result, or only norepinephrine (locus coeruleus), or both norepinephrine and epinephrine (adrenal medulla). DOPA decarboxylase is encoded by the aromatic L-amino acid decarboxylase gene (DDC). Dopamine β-hydroxylase is a critical vitamin C (ascorbate) and copper (Cu2+-dependent enzyme.
Once synthesized, dopamine, norepinephrine and epinephrine are packaged in granulated vesicles for secretion in response to the approriate nerve impulse. Within these vesicles, norepinephrine and epinephrine are bound to ATP and a protein called chromogranin A. Norepinephrine is the principal neurotransmitter of sympathetic postganglionic nerves. Both norepinephrine and epinephrine are stored in synaptic knobs of neurons that secrete it, however, epinephrine is not a mediator at postganglionic sympathetic nerve impulses. The major location, within the brain, for norepinephrine synthesis is the locus coeruleus of the brainstem. The major brain region for the synthesis of dopamine is the substantia nigra which is located below the posterior hypothalamus and next to the ventral tegmetal area. The presence of high concentrations of tyrosine in the locus coeruleus and the substantia nigra leads to increased melanin synthesis which confers on these brain regions a dark bluish coloration observable in brain sections. Indeed, these brain regions are so-called due to the dark bluish-black pigmentation. The Latin term, substantia nigra, means "black substance". The Latin word caeruleus means "dark blue, blue, or blue-green".
Outside the brain, the major site of norepinephrine and epinephrine synthesis is in adrenal medullary chromaffin cells. Outside the brain, dopamine is synthesized in several tissues including the gastrointestinal system where its actions reduce gastrointestinal motility, the pancreas where its actions inhibit insulin synthesis, and in the kidneys where its actions increases sodium excretion and urinary output.
The actions of norepinephrine and epinephrine are exerted via receptor-mediated signal transduction events. The receptors to which epinephrine and norepinephrine bind are referred to as adrenergic receptors. The adrenergic receptors are members of the G-protein coupled receptor (GPCR) family. There are two distinct casses of adrenergic receptor identified as the α (alpha) and β (beta) receptors. There are two distinct subclasses of α adrenergic receptor identified as the α1 and α2 sub-classes. For greater detail on the adrenergic receptors go to the Biochemistry of Nerve Transmission page. Within each class of adrenergic receptor there are several sub-classes. The α1 class contains the α1A, α1B, and α1D receptors. The α1 receptor class are coupled to Gq-type G-proteins that activate PLCβ resulting in increases in IP3 and DAG release from membrane PIP2. The α2 class contains the α2A, α2B, and α2C receptors. The α2 class of adrenergic receptors are coupled to Gi-type G-proteins that inhibit the activation of adenylate cyclase and therefore, receptor activation results in reduced levels of cAMP and consequently reduced levels of active PKA. The β class of receptors is composed of three subtypes: β1, β2, and β3 each of which couple to Gs-type G-proteins resulting in activation of adenylate cyclase and increases in cAMP with concomitant activation of PKA.
Dopamine binds to dopamineric receptors identified as D-type receptors and there are five subclasses identified as D1, D2, D3, D4, and D5. All five dopamine receptors belong the the G-protein coupled receptor (GPCR) family. The D1 and D5 dopamine receptors are coupled to the activation of Gs-type G-proteins and, therefore, receptor activation results in activation of adenylate cyclase. The D2, D3, and D4 dopamine receptors are coupled to Gi-type G-proteins and, therefore, receptor activation results in the inhibition of adenylate cyclase. For more details on the dopamine receptors go to the Biochemistry of Nerve Transmission page.
Epinephrine and norepinephrine are catabolized to inactive compounds through the sequential actions of catecholamine-O-methyltransferase (COMT) and monoamine oxidase (MAO). Compounds that inhibit the action of MAO have been shown to have beneficial effects in the treatment of clinical depression, even when tricyclic antidepressants are ineffective. The utility of MAO inhibitors was discovered serendipitously when patients treated for tuberculosis with isoniazid showed signs of an improvement in mood; isoniazid was subsequently found to work by inhibiting MAO.
Catabolism of the catecholamine neurotransmitters. Only clinically important enzymes are included in this diagram. The catabolic byproducts of the catecholamines, whose levels in the cerebrospinal fluid are indicative of defects in catabolism, are in blue underlined text. Abbreviations: TH = tyrosine hydroxylase, DHPR = dihydropteridine reductase, H2B = dihydrobiopterin, H4B = tetrahydrobiopterin, MAO = monoamine oxidase, COMT = catecholamine-O-methyltransferase, MHPG = 3-methoxy-4-hydroxyphenylglycol, DOPAC = dihydroxyphenylacetic acid.
Tryptophan serves as the precursor for the synthesis of serotonin (5-hydroxytryptamine, 5-HT, see also Biochemistry of Nerve Transmission) and melatonin (N-acetyl-5-methoxytryptamine).
Pathway for serotonin and melatonin synthesis from tryptophan. Abbreviations: THP = tryptophan hydroxylase, DHPR = dihydropteridine reductase, H2B = dihydrobiopterin, H4B = tetrahyrobiopterin, 5-HTP = 5-hydroxytryptophan, AADC = aromatic L-amino acid decarboxylase, SNA = serotonin N-acetylase, HOMT = hydroxyindole-O-methyltransferase.
Serotonin is synthesized through a two-step process involving a tetrahydrobiopterin-dependent hydroxylation reaction (catalyzed by tryptophan-5-monooxygenase, also called tryptophan hydroxylase) and then a decarboxylation catalyzed by aromatic L-amino acid decarboxylase. Tryptophan hydroxylase is normally not saturated and as a result, an increase in dietary uptake of tryptophan will lead to increased brain serotonin content. Tryptophan hydroxylase represents the rate-limiting step in serotonin and melatonin synthesis. Humans possess two tryptophan hydroxylase genes identified as TPH1 and TPH2. The TPH1 gene is located on chromosome 11p15.3–p14 and is composed of 10 exons that encode a protein (tryptophan 5-hydroxylase 1) of 444 amino acids. The TPH2 gene is located on chromosome 12q21.1 and is composed of 14 exons that encode a protein (tryptophan 5-hydroxylase 2) of 490 amino acids. The aromatic L-amino acid oxidase enzyme is also called DOPA decarboxylase and is encoded by the DDC gene as detailed in the previous section.
Serotonin is present at highest concentrations in platelets and in the gastrointestinal tract. Lesser amounts are found in the brain and the retina. Serotonin containing neurons(serotonergic neurons) have their cell bodies in the midline raphe nuclei of the brain stem and project to portions of the hypothalamus, the limbic system, the neocortex and the spinal cord. After release from serotonergic neurons, most of the released serotonin is recaptured by an active reuptake mechanism. The re-uptake of serotonin is catalyzed by the SLC family transporter, SCL6A4. The function of the antidepressant, Prozac®, and related drugs called selective serotonin re-uptake inhibitors (SSRIs), is to inhibit this reuptake process, thereby, resulting in prolonged serotonin presence in the synaptic cleft. Mutations in the SLC6A4 gene affect the rate of serotonin re-uptake have been shown to be correlated with obsessive-compulsive disorder, anxiety-related traits and are suspected to be involved in sudden infant death syndrome, aggressive behavior in Alzheimer disease patients, and depression-susceptibility in persons experiencing emotional trauma.
The function of serotonin is exerted upon its interaction with specific receptors. Several serotonin receptors have been cloned and are identified as 5HT1, 5HT2, 5HT3, 5HT4, 5HT5, 5HT6, and 5HT7. Within the 5HT1 group there are subtypes 5HT1A, 5HT1B, 5HT1D, 5HT1E, and 5HT1F. There are three 5HT2 subtypes, 5HT2A, 5HT2B, and 5HT2C as well as two 5HT5 subtypes, 5HT5a and 5HT5B. Most of these receptors are coupled to G-proteins that affect the activities of either adenylate cyclase or phospholipase Cβ (PLCβ). The 5HT3 class of receptors are ion channels, referred to as ionotropic receptors.
Some serotonin receptors are presynaptic and others postsynaptic. The 5HT2A receptors mediate platelet aggregation and smooth muscle contraction. The 5HT2C receptors are suspected in control of food intake as mice lacking this gene become obese from increased food intake and are also subject to fatal seizures. The 5HT3 receptors are present in the gastrointestinal tract and are involved in the regulation of emesis (vomiting). Also present in the gastrointestinal tract are 5HT4 receptors where they function in secretion and peristalsis. The 5HT6 and 5HT7 receptors are distributed throughout the limbic system of the brain and the 5HT6 receptors have high affinity for antidepressant drugs.
Melatonin is derived from serotonin within the pineal gland and the retina, where the necessary N-acetyltransferase enzyme is found. The pineal parenchymal cells secrete melatonin into the blood and cerebrospinal fluid. Synthesis and secretion of melatonin increases during the dark period of the day and is maintained at a low level during daylight hours. This diurnal variation in melatonin synthesis is brought about by norepinephrine secreted by the postganglionic sympathetic nerves that innervate the pineal gland. The effects of norepinephrine are exerted through interaction with β-adrenergic receptors. This leads to increased levels of cAMP, which in turn activate the N-acetyltransferase required for melatonin synthesis. Melatonin functions by inhibiting the synthesis and secretion of other neurotransmitters such as dopamine and GABA.back to the top
Vasodilators, such as acetylcholine and bradykinin, do not exert their effects upon the vascular smooth muscle cell in the absence of the overlying endothelium. When acetylcholine (or bradykinin) binds its receptor on the surface of endothelial cells, a signal cascade, coupled to the activation phospholipase C-β (PLCβ), is initiated. The PLCβ-mediated release of inositol trisphosphate, IP3 (from membrane associated phosphatidylinositol-4,5-bisphosphate, PIP2), leads to the release of intracellular stores of Ca2+. In turn, the elevation in Ca2+ leads to the production of endothelium-derived relaxing factor (EDRF) which then diffuses into the adjacent smooth muscle. Within smooth muscle cells, EDRF reacts with the heme moiety of a soluble guanylate cyclase, resulting in activation of the latter and a consequent elevation of intracellular levels of cGMP. The net effect is the activation of cGMP-dependent protein kinase (PKG) and the phosphorylation of substrates leading to smooth muscle cell relaxation. The coronary artery vasodilator drugs, such as nitroprusside and nitroglycerin, act to increase intracellular release of EDRF and thus, the activation of the cGMP signal cascade.
Quite unexpectedly, EDRF was found to be the free radical diatomic gas, nitric oxide, NO. So stunning was the elucidation of the pathway to, and actions of, NO that Drs. Murad, Ignarro and Furchgott were awarded the Nobel Prize in 1998 for their work on this system. NO is formed by the action of NO synthase, (NOS) on the amino acid arginine. The role of Ca2+ ions in this process is related to the calmodulin subunits of NOS enzymes (see below).
Neuronal NOS (nNOS), also called NOS-1. The NOS1 gene is located on chromosome 12q24.22 and is composed of 35 exons the generate multiple alternatively spliced mRNAs that encode three characterized isoforms.
Inducible or macrophage NOS (iNOS), also called NOS-2. The NOS2 gene is located on chromosome 17q11.2 and is composed of 29 exons that encode a protein of 1153 amino acids.
Endothelial NOS (eNOS), also called NOS-3. The NOS3 gene is located on chromosome 7q36 and is composed of 29 exons that generate multiple alternatively spliced mRNAs that encode four characterized isoforms.
Nitric oxide synthases are very complex enzymes, employing five redox cofactors: NADPH, FAD, FMN, heme and tetrahydrobiopterin (H4B). NO can also be formed from nitrite, derived from vasodilators such as glycerin trinitrate (nitroglycerin) during their metabolism. The half-life of NO is extremely short, lasting only 2-4 seconds. This is because it is a highly reactive free radical and interacts with oxygen and superoxide. NO is inhibited by hemoglobin and other heme proteins which bind it tightly.
Both eNOS and nNOS are constitutively expressed and regulated by Ca2+. The calcium regulation is imparted be the associated calmodulin subunits, thus explaining how vasodilators such as acetylcholine effect smooth muscle relaxation as a consequence of increasing intracellular endothelial cell calcium levels. Although iNOS contains calmodulin subunits, its activity is unaffected by changes in Ca2+ concentration. iNOS is transcriptionally activated in macrophages, neutrophils, and smooth muscle cells.
The major functions of NO production through activation of iNOS are associated with the bactericidal and tumoricidal actions of macrophages. Overproduction of NO via iNOS is associated with cytokine-induced septic shock such as occurs post-operatively in patients with bacterial infections. Bacteria produce endotoxins such as lipopolysaccharide (LPS) that activate iNOS in macrophages.
Nitric oxide is involved in a number of other important cellular processes in addition to its impact on vascular smooth muscle cells. Events initiated by NO that are important for blood coagulation include inhibition of platelet aggregation and adhesion and inhibition of neutrophil adhesion to platelets and to the vascular endothelium. NO is also generated by cells of the immune system and as such is involved in non-specific host defense mechanisms and macrophage-mediated killing. NO also inhibits the proliferation of tumor cells and microorganisms. Additional cellular responses to NO include induction of apoptosis (programmed cell death), DNA breakage and mutation.
Chemical inhibitors of NOS are available and can markedly decrease production of NO. The effect is a dramatic increase in blood pressure due to vasoconstriction. Another important cardiovascular effect of NO is exerted through the production of cGMP, which acts to inhibit platelet aggregation.back to the top
Creatine synthesis begins in the kidneys using the amino acids arginine and glycine. The formation of guanidinoacetate from these two amino acids is catalyzed by the enzyme glycine amidinotransferase, also called L-arginine:glycine amidinotransferase. Glycine amidinotransfersae is encoded by the GAMT gene locatred on chromosome 15q21.1 and is composed of 12 exons that encode a 423 amino acid protein that localizes to the mitochondria. Guanidinoacetate it transported to the blood and picked up by heptocytes where it is methylated forming creatine. The methyl donor for this reation is S-adenosylmethionine (SAM) and the reaction is catalyzed by the enzyme guanidinoacetate N-methyltransferase. This latter enzyme is encoded by the GAMT gene located on chromosome 19p13.3 and is composed of 6 exons that generate two alternatively spliced mRNAs that encode isoform a (236 amino acids) and isoform b (269 amino acids). Following synthesis creatine is released to the blood where is is picked up by the brain and skeletal muscle cells through the action of the SLC family member transporter, SLC6A8. Within these cells creatine is phosphorylated by creatine kinases (CK; also called creatine phosphokinase, CPK) that generate the high-energy storage compound, creatine phosphate. There are two creatine kinase genes in humans identified as the muscle creatine kinase gene (symbol: CKM) and the brain creatine kinase gene (symbol: CKB). The CKM gene is located on chromosome 19q13.32 and is composed of 9 exons that encode a protein of 381 amino acids. The CKB gene is located on chromosome 14q32 and is composed of 8 exons that encode a protein of 381 amino acids. Different combinations of the proteins encoded by these two genes generate tissue-specific isoforms of the functional enzymes. For details on the different forms of CK go to the Enzyme Kinetics page.
Synthesis of creatine and creatine phosphate: The synthesis of creatine begins from arginine and glycine via the renal enzyme glycine amidinotransferase (GATM). The product, guanidinoacetate, is transported to the liver where it is methylated via the action of guanidinoacetate N-methyltransferase (GAMT) forming creatine. The methyl group is donated from S-adenosylmethione (SAM). Creatine is released to the blood and picked up by the brain and skeletal muscle cells via the action of the SLC6A8 transporter. Creatine kinase (creatine phosphokinase) transfers a phosphate from ATP generating the high-energy intermediate, creatine phosphate.
Creatine is used as a storage form of high energy phosphate. The phosphate of ATP is transferred to creatine, generating creatine phosphate, through the action of creatine phosphokinase. The reaction is reversible such that when energy demand is high (e.g. during muscle exertion) creatine phosphate donates its phosphate to ADP to yield ATP.
Both creatine and creatine phosphate are found in muscle, brain and blood. Creatinine is formed in muscle from creatine phosphate by a nonenzymatic dehydration and loss of phosphate. The amount of creatinine produced is related to muscle mass and remains remarkably constant from day to day. Creatinine is excreted by the kidneys and the level of excretion (creatinine clearance rate) is a measure of renal function.back to the top
Glutathione (abbreviated GSH) is a tripeptide composed of glutamate, cysteine and glycine that has numerous important functions within cells. Glutathione serves as a potent reductant eliminating hydroxy radicals, peroxynitrites, and hydroperoxides; it is conjugated to drugs to make them more water soluble; it is involved in amino acid transport across cell membranes (the γ-glutamyl cycle); it is a substrate for the peptidoleukotrienes; it serves as a cofactor for some enzymatic reactions; and it serves as an aid in the rearrangement of protein disulfide bonds.
GSH is synthesized in the cytosol of all mammalian cells via the two-step reaction shown in the Figure. The rate of GSH synthesis is dependent upon the availability of cysteine and the activity of the rate-limiting enzyme, γ-glutamylcysteine synthetase (also called glutamate-cysteine ligase, GCL). The second reaction of GSH synthesis involves the enzyme, glutathione synthetase, which condenses γ-glutamylcysteine with glycine. Both reactions of GSH synthesis require ATP. Functional GCL is a heterodimer composed of the catalytic subunit and a regulatory subunit called the modifier subunit. The GCL catalytic subunit is encoded by the GCLC gene and the modifier subunit is encoded by the GCLM gene. The GCLC gene is located on chromosome 6p12 and is composed of 16 exons that generate two alterntively spliced mRNA that encode GCL isoform a (637 amino acids) and GCL isoform b (599 amino acids). The GCLM gene is located on chromosome 1p22.1 and is composed of 8 exons that encode a protein of 274 amino acids. The glutathione synthetase gene (symbol: GSS) is located on chromosome 20q11.2 and is composed of 15 exons that encode a protein of 474 amino acids.
In the liver, major factors that determine the availability of cysteine are diet, membrane transport activities of the three sulfur amino acids cysteine, cystine and methionine, and the conversion of methionine to cysteine (see the Amino Acid Metabolism page). Numerous conditions can alter the level of GSH synthesis via changes in glutamate-cysteine ligase activity and expression of the GCLC gene such as oxidative stress, antioxidant levels, hormones, cell proliferation, and diabetes mellitus.
Synthesis of Glutathione. The synthesis of glutathione, which occurs in the cytosol of all cells, proceeds via a two-step reaction involving the ATP-requiring enzymes glutamate-cysteine ligase (GCL) and glutathione synthetase (GSS). The common designation for reduced glutathione (monomeric) is GSH. When glutathione reduces reactive oxygen or nitrogen species, or upon its involvement in many other reduction reactions, it becomes oxidized to the dimeric form which is abbreviated GSSG.
The role of GSH as a reductant is extremely important particularly in the highly oxidizing environment of the erythrocyte. The sulfhydryl of GSH can be used to reduce peroxides formed during oxygen transport. Endogenously produced hydrogen peroxide (H2O2) is reduced by GSH in the presence of selenium-dependent GSH peroxidase. Hydrogen peroxide can also be reduced by catalase, which is present only in the peroxisomes. In the mitochondria, GSH is particularly important because mitochondria lack catalase. The resulting oxidized form of GSH consists of two molecules disulfide bonded together (abbreviated GSSG). The enzyme glutathione reductase utilizes NADPH as a cofactor to reduce GSSG back to two moles of GSH. Hence, the pentose phosphate pathway is an extremely important pathway of erythrocytes for the continuing production of the NADPH needed by glutathione reductase. In fact as much as 10% of glucose consumption, by erythrocytes, may be mediated by the pentose phosphate pathway. Detoxification of xenobiotics or their metabolites is another major function of GSH. These compounds form conjugates with GSH either spontaneously or enzymatically in reactions catalyzed by members of the glutathione S-transferase (GST) family. The conjugates formed are usually excreted from the cell and, in the case of the liver they are excreted in the bile. Humans contain several GST enzymes that are divided into several classes identified as alpha, mu, pi, theta, zeta, and omega. There are four GST alpha genes, (GSTA1, GSTA2, GSTA3, GSTA4), five mu genes (GSTM1, GSTM2, GSTM3, GSTM4, GSTM5), one pi gene (GSTP1), one theta gene (GSTT1), one zeta gene (GSTZ1), and one omega gene (GSTO1).
Several mechanisms exist for the transport of amino acids across cell membranes. Many are symport or antiport mechanisms that couple amino acid transport to sodium transport. The γ-glutamyl cycle is an example of a group transfer mechanism of amino acid transport. Although this mechanism requires more energy input, it is rapid and has a high capacity. The cycle functions primarily in the kidney, particularly renal epithelial cells. The enzyme γ-glutamyl transpeptidase is located in the cell membrane and shuttles GSH to the cell surface to interact with an amino acid. Reaction with an amino acid liberates cysteinylglycine and generates a γ-glutamyl-amino acid which is transported into the cell and hydrolyzed to release the amino acid. Glutamate is released as 5-oxoproline and the cysteinylglycine is cleaved to its component amino acids. Regeneration of GSH requires an ATP-dependent conversion of 5-oxoproline to glutamate and then the two additional moles of ATP that are required during the normal generation of GSH.back to the top
One of the earliest signals that cells have entered their replication cycle is the appearance of elevated levels of mRNA for ornithine decarboxylase (ODC), and then increased levels of the enzyme, which is the first enzyme in the pathway to synthesis of the polyamines. Because of the latter, and because the polyamines are highly cationic and tend to bind nucleic acids with high affinity, it is believed that the polyamines are important participants in DNA synthesis, or in the regulation of that process.
The key features of the pathway are that it involves putrescine, an ornithine catabolite, and S-adenosylmethionine (SAM) as a donor of 2 propylamine residues. The first propylamine conjugation yields spermidine and addition of another to spermidine yields spermine.
The function of ODC is to produce the 4-carbon saturated diamine, putrescine. At the same time, SAM decarboxylase cleaves the SAM carboxyl residue, producing decarboxylated SAM (S-adenosymethylthiopropylamine), which retains the methyl group usually involved in SAM methyltransferase activity. SAM decarboxylase activity is regulated by product inhibition and allosterically stimulated by putrescine. Spermidine synthase catalyzes the condensation reaction, producing spermidine and 5'-methylthioadenosine. A second propylamine residue is added to spermidine producing spermine.
The signal for regulating ODC activity is unknown, but since the product of its activity, putrescine, regulates SAM decarboxylase activity, it appears that polyamine production is principally regulated by ODC concentration.
The butylamino group of spermidine is used in a posttranslational modification reaction important to the process of translation. A specific lysine residue in the translational initiation factor eIF-4D is modified. Following the modification the residue is hydroxylated yielding a residue in the protein termed hypusine.back to the top