Return to The Medical Biochemistry Page
Secreted and Membrane-Associated Proteins
Proteins that are membrane bound or are destined for excretion are synthesized by ribosomes associated with the membranes of the endoplasmic reticulum (ER). The ER associated with ribosomes is termed rough ER (RER). This class of proteins all contain an N-terminus termed a signal sequence or signal peptide. The signal peptide is usually 13-36 predominantly hydrophobic residues. The signal peptide is recognized by a multi-protein complex termed the signal recognition particle (SRP). This signal peptide is removed following passage through the endoplasmic reticulum membrane. The removal of the signal peptide is catalyzed by signal peptidase. Proteins that contain a signal peptide are called preproteins to distinguish them from proproteins. However, some proteins that are destined for secretion are also further proteolyzed following secretion and, therefore contain pro sequences. This class of proteins is termed preproproteins.
|
|
Mechanism of synthesis of membrane bound or secreted proteins. Ribosomes engage the ER membrane through interaction of the signal recognition particle, SRP in the ribosome with the SRP receptor in the ER membrane. As the protein is synthesized the signal sequence is passed through the ER membrane into the lumen of the ER. After sufficient synthesis the signal peptide is removed by the action of signal peptidase. Synthesis will continue and if the protein is secreted it will end up completely in the lumen of the ER. If the protein is membrane associated a stop transfer motif in the protein will stop the transfer of the protein through the ER membrane. This will become the membrane spanning domain of the protein. |
back to the top
Proteolytic Cleavage
Most proteins undergo proteolytic cleavage following translation. The simplest form of this is the removal of the initiation methionine. Many proteins are synthesized as inactive precursors that are activated under proper physiological conditions by limited proteolysis. Pancreatic enzymes and enzymes involved in clotting are examples of the latter. Inactive precursor proteins that are activated by removal of polypeptides are termed proproteins.
A complex example of post-translational processing of a preproprotein is the cleavage of prepro-opiomelanocortin (POMC) synthesized in the pituitary (see the Peptide Hormones page for discussion of POMC). This preproprotein undergoes complex cleavages, the pathway of which differs depending upon the cellular location of POMC synthesis.
Another is example of a preproprotein is insulin. Since insulin is secreted from the pancreas it has a prepeptide. Following cleavage of the 24 amino acid signal peptide the protein folds into proinsulin. Proinsulin is further cleaved yielding active insulin which is composed of two peptide chains linked togehter through disulfide bonds.
Still other proteins (of the enzyme
class) are synthesized as inactive precursors called zymogens. Zymogens are
activated by proteolytic cleavage such as is the situation for several proteins
of the blood clotting cascade.
back to the top
Glycoproteins
Membrane associated carbohydrate is exclusively in the form of oliogsaccharides covalently attached to proteins forming glycoproteins, and to a lesser extent covalently attached to lipid forming the glycolipids. Glycoproteins consist of proteins covalently linked to carbohydrate. The predominant sugars found in glycoproteins are glucose, galactose, mannose, fucose, GalNAc, GlcNAc and NANA. The distinction between proteoglycans and glycoproteins resides in the level and types of carbohydrate modification. The carbohydrate modifications found in glycoproteins are rarely complex: carbohydrates are linked to the protein component through either O-glycosidic or N-glycosidic bonds. The N-glycosidic linkage is through the amide group of asparagine. The O-glycosidic linkage is to the hydroxyl of serine, threonine or hydroxylysine. The linkage of carbohydrate to hydroxylysine is generally found only in the collagens. The linkage of carbohydrate to 5-hydroxylysine is either the single sugar galactose or the disaccharide glucosylgalactose. In ser- and thr-type O-linked glycoproteins, the carbohydrate directly attached to the protein is GalNAc. In N-linked glycoproteins, it is GlcNAc.
|
|
O-linkage to GalNAc |
N-linkage to GlcNAc |
The predominant carbohydrate attachment in glycoproteins of mammalian cells is via N-glycosidic linkage. The site of carbohydrate attachment to N-linked glycoproteins is found within a consensus sequence of amino acids, N-X-S(T), where X is any amino acid except proline. When an analysis of proteins in the public databases is carried out, it can be shown that approximately 65% of all the proteins contain at least one occurrence of the Asn-X-Ser/Thr consensus. N-linked glycoproteins all contain a common core of carbohydrate attached to the polypeptide. This core consists of three mannose residues and two GlcNAc. A variety of other sugars are attached to this core and comprise three major N-linked families:
1. High-mannose type contains all mannose outside the core in varying amounts.
2. Hybrid type contains various sugars and amino sugars.
3. Complex type is similar to the hybrid type, but in addition, contains sialic acids to varying degrees.
|
Structures of oligosaccharides of the 3 major classes of glycoprotein.Open squares: GlcNAc; open circles: mannose; open diamonds: galactose; filled squares: fucose; filled triangles: sialic acid the greek symbols α and β followed by numbers refers to the type of linkage. |
Most proteins that are secreted, or bound to the
plasma membrane, are modified by carbohydrate attachment. The part that is
modified, in plasma membrane-bound proteins, is the extracellular portion of the
protein that is modified. Intracellular proteins are less frequently modified
by carbohydrate attachment. However, the attachment of carbohydrate to
intracellular proteins confers unique functional activities on these proteins.
Linkage of carbohydrate to cytosolic and/or nuclear proteins occurs via
O-linkage and involves attachment of GlcNAc to serine or threonine residues.
The linkage is catalyzed by the enzyme O-GlcNAc transferase, OGT. Several
transcription factors and RNA polymerase II have been shown to be modified by
O-GlcNAc linkage.
back to the top
Mechanism of Carbohydrate Linkage to Protein
The protein component of all glycoproteins is synthesized from polyribosomes that are bound to the endoplasmic reticulum (ER). The processing of the sugar groups occurs cotranslationally in the lumen of the ER and continues in the Golgi apparatus for N-linked glycoproteins. Attachment of sugars in O-linked glycoproteins occurs post-translationally in the Golgi apparatus. Sugars used for glycoprotein synthesis (both N-linked and O-linked) are activated by coupling to nucleotides. Glucose and GlcNAc are coupled to UDP and mannose is coupled to GDP.
O-linked sugars: The synthesis of O-linked glycoproteins occurs via the stepwise addition of nucleotide-activated sugars directly onto the polypeptide. The nucleotide-activated sugars are coupled to either UDP, GDP (as with mannose) or CMP (for instance, NANA). The attachment of sugars is catalyzed by specific glycoprotein glycosyltransferases. Evidence indicates that each specific type of carbohydrate linkage in O-linked glycoproteins is the result of a different glycosyltransferase.
N-linked sugars: As indicated earlier, the three major classes of N-linked carbohydrate modifications are high-mannose, hybrid and complex. The major distinguishing feature of the complex class is the presence of sialic acid, whereas the hybrid class contains no sialic acid.
In contrast to the step-wise addition of sugar groups to the O-linked class of glycoproteins, N-linked glycoprotein synthesis requires a lipid intermediate: dolichol phosphate. Dolichols are polyprenols (C80-C100) containing 17 to 21 isoprene units, in which the terminal unit is saturated. In the image below the black bracket denotes the isoprene unit which can be repeated numerous times. The phosphate moiety of dolichol phosphate is attached to the -OH group.
As indicated, the formation of the GlcNAc-β-Asn linkage in proteins occurs in the endoplasmic reticulum (ER) through cotranslational addition of a preassembled carbohydrate core structure that is delivered via the carbohydrate-dolichol lipid intermediate. The preassembled carbohydrate core structure comprises three terminal residues of glucose attached to a branched cluster of nine mannose residues that are in turn attached to two GlcNAc residues attached to dolicholpyrophosphate. The structure is abbreviated Glc3Man9GlcNAc2-P-P-dolichol. This structure is commonly referred to as the lipid-linked oligosaccharide (LLO) whereas the oligosaccharide structure itself is termed the en bloc oligosaccharide. In mammalian cells the importance of the terminal glucose residues is evident from the fact that transfer of Man9GlcNAc2-P-P-dolichol is some 25-times less efficient than the complete structure. In addition, structures that contain three terminal glucose residues, but not the complete Man9GlcNAc2 structure, are efficiently transferred to protein by oligosaccharyltransferase. Synthesis of the en bloc dolichol-P-P-oligosaccharide unit begins on the cytoplasmic face of the ER membrane and prior to transfer to the protein, the structure “flips” to the luminal side.
Pathway of Synthesis and Transfer of the LLO unit in the ER.
Immediately following transfer of the en bloc oligosaccharide
unit to the protein, processing and alteration of the composition of the
oligosaccharide ensues and continues as the protein passes through the ER then
into and through the Golgi apparatus. Initially, the terminal glucose is
removed through the action of glucosidase I (GI), a membrane bound enzyme
recognizing α-1,2-linked glucose. The remaining two glucose
residues are then removed by glucosidase II (GII), a soluble enzyme recognizing
α-1,3-inked glucose. After removal of the glucose
residues, the action of a-mannosidases removes several mannose residues as the
protein progresses to the Golgi. The action of the various glucosidases and
mannosidases leaves N-linked glycoproteins containing a common core of
carbohydrate consisting of three mannose residues and two GlcNAc. Through the
action of a wide range of glycosyltransferases and glycosidases a variety of
other sugars are attached to this core as the protein progresses through the
Golgi. These latter reactions generate the three major families of N-linked
glycoproteins described above.
back to the top
Lysosomal Targeting of Enzymes
Enzymes that are destined for the lysosomes (lysosomal enzymes) are directed there by a specific carbohydrate modification. During transit through the Golgi apparatus a residue of GlcNAc-1-phosphate (GlcNAc-1-P) is added to the carbon-6 hydroxyl group of one or more specific mannose residues that have been added to these enzymes. The GlcNAc is activated by coupling to UDP and is transferred by UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase (GlcNAc-phosphotransferase), yielding a phosphodiester intermediate: GlcNAc-1-P-6-Man-protein. A second reaction (catalyzed by GlcNAc 1-phosphodiester-N-acetylglucosaminidase) removes the GlcNAc leaving mannose residues phosphorylated in the 6 position: Man-6-P-protein. A specific Man-6-P receptor (MPR) is present in the membranes of the Golgi apparatus. Binding of Man-6-P to this receptor targets proteins to the lysosomes.
Two distinct MPRs have been identified and both are members of the P-type lectin family. Both are type I integral membrane glycoproteins that contain an N-terminal extracellular domain, a single transmembrane domain and a C-terminal cytoplasmic domain. One receptor is large with a molecular weight of approximately 300kDa, the other receptor is smaller with a molecular weight of approximately 46kDa. Structural similarities between these two receptors indicates they are derived from a single ancestral gene with the larger receptor arising through multiple gene duplications. The extracellular portion of the larger receptor contains 15 repeating elements, each of which is highly similar to the extracellular domain of the smaller receptor. Both receptors exist as dimers embedded in the membrane.
The large receptor binds two moles of Man-6-P and the
smaller binds one mole of Man-6-P per subunit, thus 4 and 2 moles of Man-6-P
per dimer, respectively. The bovine and murine versions of the smaller
receptors require divalent cations for ligand binding and thus the receptor has
been termed the cation-dependent Man-6-P receptor (CD-MPR). However, the human counterpart may not require
cations for its activity. The larger receptor does not require divalent cations
for ligand binding and is therefore, commonly referred to as the cation-independent
Man-6-P receptor (CI-MPR). However, the CI-MPR has been shown to bind the
nonglycosylated polypeptide hormone, insulin-like growth factor II (IGF-II) and
as such the larger MPR is more frequently identified as IGF-II/MPR. The
IGF-II/MPR is available at the cell surface and its role in binding IGF-II is
to target this hormone for degradation in the lysosomes. In addition to IGF-II,
the IGF-II/MPR has been shown to bind a diverse array of Man-6-P-containing
proteins as well as several nonglucosylated proteins. Although IGF-II/MPR and
CD-MPR exhibit distinct activities, both receptors function to target newly
synthesized lysosomal enzymes to the lysosomes.
back to the top
Clinical Significances of Glycoproteins
Glycoproteins on cell surfaces are important for communication between cells, for maintaining cell structure and for self-recognition by the immune system. The alteration of cell-surface glycoproteins can, therefore, produce profound physiological effects, of which several are listed below.>
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. For more information of blood group antigens, including ABO visit the blood groups page at the SCARF site.
|
Structure of the ABO blood group carbohydratesR represents the linkage to protein in the secreted forms, sphingolipid in the cell-surface bound form, open square = GlcNAc, open diamond = galactose, filled square = fucose, filled diamond = GalNAc, filled diamond = sialic acid (NANA) |
2. The truncation of erythrocyte surface glycoproteins leads to cell clumping, as in congenital dyserythropoietic anemia type II. Also referred to as HEMPAS (hereditary erythroblastic multinuclearity with positive acidified-serum test).
3. Several viruses, bacteria and parasites have exploited the presence of cell-surface carbohydrates, principally associated with protein (glycoproteins), using them as portals of entry into the cell.
A. Human immunodeficiency virus (HIV), the causative agent of AIDS, gains entry into cells of the immune system by attaching to a class of cellular receptors known as the chemokine receptors, most notably CXCR4 and CCR5. For more information on chemokines and their receptors visit the C.O.P.E site.
B. Members of the poxvirus family of viruses gain entry into cells, most frequently migratory leukocytes, by attaching to chemokine receptors including CCR1, CCR5 and CXCR4 (Science [1999] vol. 286 pp. 1968-1971).
C.Dystroglycan (DG) is a component of the dystrophin-glycoprotein complex. It is a laminin receptor encoded by a single gene and cleaved by postranslational processing into two proteins, peripheral membrane α-DG and transmembrane β-DG. α-DG interacts with laminin-2 in the basal lamina and β-DG binds to dystrophin containing cytoskeletal proteins in muscle and peripheral nerves. DG is involved in agrin- and laminin-induced acetylcholine receptor clustering at neuromuscular junctions, morphogenesis, early development, and the pathogenesis of muscular dystrophies. Evidence has shown that α-DG present on Schwann cell membranes is the receptor for Mycobacterium leprae and also serves as the receptor for the arenavirus class of pathogens (Science (1998) vol. 282 pp. 2076-2079 and 2079-2081). Arenaviruses cause hemorrhagic fever in humans. Lymphocytic choriomeningitis virus (LCMV), Lassa fever virus (LFV), Oliveros and Mobala (all members of the arenavirus family) all bind to α-DG. The specificity of this interaction was demonstrated by the resistance to LCMV infection of cells harboring a null mutation in DG.
D. Rhinoviruses utilize attachment to ICAM-1 (intercellular adhesion molecule-1) to gain entry into cells.
E. The pathogenic human parvovirus, B19, attaches to the erythrocyte-specific cell-surface globoside identified as erythrocyte P antigen to infect erythrocytes.
F. The malarial parasite Plasmodium vivax, binds to the erythrocyte chemokine receptor known as the Duffy blood group antigen (also known as the erythrocyte receptor for interleukin-8) to infect erythrocytes.
G. The MN blood group system is a well-characterized set of erythrocyte surface antigens that represent the variable carbohydrate modifications of the trans-membrane glycoprotein, glycophorin. Glycophorin is the cellular receptor for influenza virus as well as the receptor for erythrocyte invasion by the malarial parasite Plasmodium falciparum.
H. Helicobacter pylori is the bacterium responsible for chronic active gastritis and gastric and duodenal ulcers; it is also the causative agent for one of the most common forms of cancer in humans, adenocarcinoma. This bacterium attaches to the Lewis blood group antigen on the surfaces of gastric mucous cells.
I. Rabies virus binds to cells through interactions with neural cell adhesion molecule (N-CAM).
J. The receptor for fibroblast growth factor (FGF) has been reported to be the portal of entry for human herpes virus Type I. Recent new evidence indicates that the portal of entry for human herpes simplex Type I viruses is 3-O-sulfated heparan sulfate (Cell 99:13-22, 1999).
K. Human herpesvirus 6 (HHV-6) infection occurs in virtually all persons within the first 2 years of life and persists the entire lifetime. In immunocompromised patients HHV-6 causes opportunistic infections and is the causative agent of exanthema subitum. HHV-6 has been linked to multiple sclerosis and to the progression of AIDS. The cellular receptor for HHV-6 is the cell-surface type-I glycoprotein, CD46 (Cell 99:817-827, 1999).
4. Some glycoproteins are tethered to the membrane by a lipid linkage: the protein is attached to the carbohydrate through phosphatidylethanolamine (PE) linkage, and the carbohydrate is in turn attached to the membrane via linkage to phosphatidylinositol (PI), which anchors the structure within the membrane. The linkage is called a glycosylphosphotidylinositol (GPI) anchor, and proteins that are anchored in this way are termed glypiated proteins. The disease, paroxysmal nocturnal hemoglobinuria, results from the loss of the erythrocyte surface glycoprotein, decay-accelerating factor, (DAF). DAF prevents erythrocyte lysis by complement. When this factor is lost from the erythrocyte surface, abnormal hemolysis occurs, with the end result of hemoglobin accumulation in the urine.
Other important GPI linked proteins are the enzymes acetylcholinesterase, intestinal and placental alkaline phosphatase and 5'-nucleotidase, the cell adhesion molecule N-CAM (neural cell adhesion molecule) and the T-cell markers Thy-1 and LFA-3 (lymphocyte function associated antigen-3).
|
The GPI linkage of the T-cell marker Thy-1Line represents the outer surface of the membrane. Squiggles represent the lipid portion of the GPI linkage embedded in the membrane. Open circles = mannose, filled diamonds = GalNAc, filled squares = fucose. Filled pentagons = ethanolamine, solid circles with P = phosphates> |
5. The proper degradation of glycoproteins has medical relevance. Degradation occurs within lysosomes and requires specific lysosomal hydrolases, termed glycosidases. Exoglycosidases remove sugars sequentially from the non-reducing end and exhibit restricted substrate specificities. In contrast, endoglycosidases cleave carbohydrate linkages from within and exhibit broader substrate specificities. Several inherited disorders involving the abnormal storage of glycoprotein degradation products have been identified in humans. These disorders result from defects in the genes encoding specific glycosidases, leading to incomplete degradation and subsequent over-accumulation of partially degraded glycoproteins. As a general class, such disorders are known as lysosomal storage diseases and include the diseases known as mucolipidoses that result from incomplete degradation of the carbohydrate portions of glycolipids.
6. Defects in the proper targeting of glycoproteins to the lysosomes can also lead to clinical complications. Deficiencies in the enzyme responsible for the transfer of GlcNAc-1-P to Man residues (GlcNAc phosphotransferase) in lysosomal enzymes leads to the formation of dense inclusion bodies formation in the fibroblasts. Two disorders related to deficiencies in the targeting of lysosomal enzymes are termed I-cell disease (mucolipidosis II) and pseudo-Hurler polydystrophy (mucolipidosis III, also called mucolipidosis-HI). I-cell disease is characterized by severe psychomotor retardation, skeletal abnormalities, coarse facial features, painful restricted joint movement, and early mortality. Pseudo-Hurler polydystrophy is less severe; it progresses more slowly, and afflicted individuals live to adulthood.
Defects in Degradation of Asn-GlcNAc Type Glycoproteins
Disease |
Enzyme Deficiency |
Symptoms/Comments |
| aspartylglycosaminuria | aspartylglycosaminidase | progressive mental retardation, delayed speech and motor development, coarse facial features |
| β-Mannosidosis | β-mannosidase | primarily neurological defects, speech impairment |
| α-Mannosidosis | α-mannosidase | mental retardation, dystosis multiplex, hepatosplenomegaly, hearing loss, delayed speech |
| GM1 Gangliosidosis | β-galactosidase | also identified as a glycosphingolipid storage disease or lysosomal storage disease |
| (Sandhoff disease) | β-hexosaminidases A and B | also identified as a glycosphingolipid storage disease or lysosomal storage disease |
|
Sialidosis (also identified as Mucolipidosis I) |
neuraminidase (sialidase) | myoclonus, congenital ascites, hepatosplenomegaly, coarse facial features, delayed mental and motor development |
| Fucosidosis | α-fucosidase | progressive motor and mental deterioration, growth retardation, coarse facial features, recurrent sinus and pulmonary infections |
back to the top
OMIM links for additional Defects in Glycoprotein Degradation
Acylation
Many proteins are modified at their N-termini following
synthesis. In most cases the initiator methionine is hydrolyzed and an acetyl
group is added to the new N-terminal amino acid. Acetyl-CoA is the acetyl donor
for these reactions. Some proteins have the 14 carbon myristoyl group added to
their N-termini. The donor for this modification is myristoyl-CoA. This latter
modification allows association of the modified protein with membranes. The
catalytic subunit of cyclicAMP-dependent protein kinase (PKA) is myristoylated.
back to the top
Methylation
Post-translational methylation occurs at lysine
residues in some proteins such as calmodulin and cytochrome c. The activated
methyl donor is S-adenosylmethionine.
back to the top
Phosphorylation
Post-translational phosphorylation is one of the most common protein modifications that occurs in animal cells. The vast majority of phosphorylations occur as a mechanism to regulate the biological activity of a protein and as such are transient. In other words a phosphate (or more than one in many cases) is added and later removed.
Physiologically relevant examples are the phosphorylations that occur in glycogen synthase and glycogen phosphorylase in hepatocytes in response to glucagon release from the pancreas. Phosphorylation of synthase inhibits its activity, whereas, the activity of phosphorylase is increased. These two events lead to increased hepatic glucose delivery to the blood.
The enzymes that phosphorylate proteins are termed kinases and those that remove phosphates are termed phosphatases. Protein kinases catalyze reactions of the following type:
ATP + protein <----> phosphoprotein + ADP
In animal cells serine, threonine and tyrosine are the amino acids subject to phosphorylation. The largest group of kinases are those that phsophorylate either serines or threonines and as such are termed serine/threonine kinases. The ratio of phosphorylation of the three different amino acids is approximately 1000/100/1 for serine/threonine/tyrosine.
Although
the level of tyrosine phosphorylation is minor, the importance of
phosphorylation of this amino acid is profound. As an example, the activity of
numerous growth factor receptors is controlled by tyrosine phosphorylation.
back to the top
Sulfation
Sulfate modification of proteins occurs at tyrosine residues such as in fibrinogen and in some secreted proteins (eg gastrin). The universal sulfate donor is 3'-phosphoadenosyl-5'-phosphosulphate (PAPS).
Since sulfate is added permanently it is necessary
for the biological activity and not used as a regulatory modification like that
of tyrosine phosphorylation.
back to the top
Prenylation
Prenylation refers to the addition of the 15 carbon farnesyl group or the 20 carbon geranylgeranyl group to acceptor proteins, both of which are isoprenoid compounds derived from the cholesterol biosynthetic pathway. The isoprenoid groups are attached to cysteine residues at the carboxy terminus of proteins in a thioether linkage (C-S-C). A common consensus sequence at the C-terminus of prenylated proteins has been identified and is composed of CAAX, where C is cysteine, A is any aliphatic amino acid (except alanine) and X is the C-terminal amino acid. In order for the prenylation reaction to occur the three C-terminal amino acids (AAX) are first removed and the cysteine activated by methylation in a reaction utilizing S-adenosylmethionine as the methyl donor.
Important examples of prenylated proteins include the
oncogenic GTP-binding and hydrolyzing protein RAS and the γ-subunit of the visual protein transducin, both of which are farnesylated. Numerous
GTP-binding and hydrolyzing proteins (termed G-proteins) of signal transduction cascades have γ-subunits modified by geranylgeranylation.
back to the top
Vitamin C-Dependent Modifications
Modifications of proteins that depend upon vitamin C as a cofactor include proline and lysine
hydroxylations and carboxy terminal amidation. The hydroxylating enzymes are
identified as prolyl hydroxylase and lysyl hydroxylase. The donor of the amide
for C-terminal amidation is glycine. The most important hydroxylated proteins are the collagens. Several peptide hormones such as oxytocin and vasopressin have C-terminal
amidation.
back to the top
Vitamin K-Dependent Modifications
Vitamin K is a cofactor in the carboxylation of glutamic acid residues. The result of this type of reaction is the formation of a γ-carboxyglutamate (gamma-carboxyglutamate), referred to as a gla residue.
Structure of a gla Residue
The formation of gla residues within several proteins
of the blood clotting cascade is critical
for their normal function. The presence of gla residues allows the protein to chelate
calcium ions and thereby render an altered conformation and biological activity
to the protein. The coumarin-based anticoagulants, warfarin and dicumarol function by
inhibiting the carboxylation reaction.
back to the top
Selenoproteins
Selenium is a trace element and is found as a component of several prokaryotic and eukaryotic enzymes that are involved in redox reactions. The selenium in these selenoproteins is incorporated as a unique amino acid, selenocysteine, during translation. A particularly important eukaryotic selenoenzyme is glutathione peroxidase. This enzyme is required during the oxidation of glutathione by hydrogen peroxide (H2O2) and organic hydroperoxides.
Structure of the Selenocysteine Residue
Incorporation of selenocysteine by the translational machinery
occurs via an interesting and unique mechanism. The tRNA for selenocysteine is
charged with serine and then enzymatically selenylated to produce the
selenocysteinyl-tRNA. The anticodon of selenocysteinyl-tRNA interacts with a
stop codon in the mRNA (UGA) instead of a serine codon. The
selenocysteinyl-tRNA has a unique structure that is not recognized by the
termination machinery and is brought into the ribosome by a dedicated specific
elongation factor. An element in the 3' non-translated region (UTR) of
selenoprotein mRNAs determines whether UGA is read as a stop codon or as a
selenocysteine codon.
back to the top
Return to The Medical Biochemistry Page
Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu