Term
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Definition
| Any protein with one or more covalently bound carbohydrate units that do not contain a serial repeat unit. |
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Term
| T or F: Most of the soluble and membrane proteins synthesized in the ER, including those destined for other locations, are glycoproteins. |
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Definition
| True. In contrast, very few cytosolic proteins are glycosylated. |
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Term
| T or F: Glycoproteins are very homogeneous in their structure. |
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Definition
False.
- mass of carbohydrate in each glycoprotein can vary from a couple percent to 80%.
- number of covalently linked carbohydrates can vary from one to hundreds.
- number of sugar residues in each carbohydrate typically ranges from a single sugar residue to an oligosaccharide containing 15 sugar residues.
- structure of the carbohydrates (specific sugars and their specific linkages) can vary considerably from one carbohydrate to the next on the same glycoprotein resulting in extensive microheterogeneity |
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Term
| How are carbohydrates bound to glycoproteins? |
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Definition
Via N- or O- glycosyl bonds:
1. N-Glycosyl linkage to asparagine (found in 90% of all glycoproteins)
2. O-Glycosyl linkage to serine or threonine
3. O-Glycosyl linkage to hydroxylysine (mostly found in collagens)
Glycoproteins may contain a single type of glycosidic linkage or may contain both N- and O- linked oligosaccharides in the same protein. |
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Term
| What are the 3 types of glycopeptide bonds? |
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Definition
1. N-Glycosyl linkage to asparagine (found in 90% of all glycoproteins)
2. O-Glycosyl linkage to serine or threonine
3. O-Glycosyl linkage to hydroxylysine (mostly found in collagens) |
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Term
| Which oligosaccharides are the most common in glycoproteins? |
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Definition
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Term
| What is the process by which N-linked glycoproteins are synthesized? |
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Definition
1. Sugar residues are added to the lipid dolichol phosphate in the ER membrane.
2. Specific glycosyltransferases transfer sugars one at a time in a specific sequence from nucleotide-sugars (UDP-GlcNAc and GDP-Man) or dolichol-sugars to dolichol.
3. Initially, the oligosaccharide is built on the cytosolic side of the ER membrane, but halfway through the process, the dolichol flips its orientation in the membrane and the remaining sugars are added from the luminal side, using sugar-dolichol phosphate as donor. (Note that sugars are added to dolichol phosphate via nucleotide-sugars.)
4. Preformed oligosaccharide is transferred from dolichol to target protein as the protein is synthesized by ER membrane bound ribosomes (cotranslationally; MBC5 Fig 12-51).
5. Oligosaccharyl transferase associated with each translocon in the ER membrane scans the incoming polypeptide and transfers the oligosaccharide from dolichol to the appropriate asparagine residues. |
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Term
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Definition
| Lipid in the ER membrane to which sugar residues are added one at a time to synthesize N-linked glycoproteins. |
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Term
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Definition
| Enzyme that adds sugars to dolichol phosphate. |
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Term
| Oligosaccharyl transferase |
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Definition
| Associated with each translocon in the ER membrane scans the incoming polypeptide and transfers the oligosaccharide from dolichol to the appropriate asparagine residues. |
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Term
| In synthesis of N-linked glycoproteins, what serves as the sugar donor when the oligosaccharide is being built on the cytosolic side of the ER membrane? The luminal side? |
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Definition
1. Nucleotide-sugars (UDP-GlcNAc and GDP-Man) or dolichol-sugars.
2. Sugar-dolichol phosphate |
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Term
| Which enzymes are involved in modifying the N-linked oligosaccharides after it's been attached to protein? |
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Definition
| Following attachment to protein, N-linked oligosaccharides are modified by an ordered series of sugar removals and additions carried out by specific glycosidases and glycosyl transferases as the protein moves from the ER through the Golgi (MBC Fig 13-31). The initial trimming occurs in the ER, while additional trimming and addition of sugars occurs in the Golgi using nucleotide-sugars as the sugar donors. Each step in the processing pathway is dependent on completion of the previous step; each enzyme will only recognize the product of the previous reaction as substrate. |
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Term
| In modification of N-linked glycoproteins, what serves as the sugar donor in the Golgi? |
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Definition
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Term
| What are the three major classes of N-linked oligosaccharides? |
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Definition
1) High mannose (oligomannose)
2) Complex
3) Hybrid |
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Term
| How does synthesis of O-linked oligosaccharides differ from that of N-linked oligosaccharides? |
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Definition
1) Sugars are added to proteins one at a time to build the oligosaccharide directly on the protein. In N-linked oligosaccharides, sugars are built on dolichol phosphate in the ER membrane transferred at once onto the protein by oligosaccharyl transferase.
2. Sugars are added in the Golgi post-translationally. In N-linked oligosaccharides, sugars are added co-translationally to the appropriate asparagine residues.
3. Synthesis requires serial action of specific glycosyltransferases, located in the Golgi
membranes, that use nucleotide-sugars as the sugar donors. |
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Term
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Definition
| The most abundant macromolecules in mucus are mucins, which are typically synthesized and secreted by specialized cells within the epithelium. Mucins are viscous glycoproteins composed of ~80% carbohydrate by mass. The vast majority of this carbohydrate consists of O-linked oligosaccharides of varying structure, although some N-linked oligosaccharides are also present (see Fig below from J Clin Invest 1991 88:1012). Mucins are rod shaped proteins with a central region enriched in Ser and Thr to allow for the high degree of O-glycosylation. Other regions are enriched in Cys which allows polymerization of the molecules via disulfide bonds. The polymerization and high density of carbohydrate allows these glycoproteins to generate a hydrated gel and makes them resistant to proteases. |
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Term
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Definition
The A, B and O antigens are oligosaccharide components of glycoproteins and glycolipids found on the suface of red blood cells. They are determined by a single genetic locus, the ABO locus on chromosome 9, that encodes a glycosyltransferase responsible for transfering a terminal sugar to a galactose in the O antigen.
A allele - encodes a transferase that transfers an N-acetyl galactosamine (GalNAc)
B allele - encodes a transferase that transfers a galactose (Gal); differs from the A allele by only 4 amino acids
O allele – encodes a non-functional protein due to a single nucleotide deletion that causes a frameshift |
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Term
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Definition
| Type O individuals (genotype OO) do not make either transferase, so their red blood cells contain only the O antigen. Their red blood cells can be transfused into an individual with any blood type (universal donor) because there are no A or B antigens to be recognized by the recipient’s immune system. |
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Term
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Definition
| Type A individuals (genotype AA or AO) make only the GalNAc transferase, so their red blood cells have the A antigen and can be transfused into individuals with type A or AB blood. These recipients do not make antibodies that recognize the A antigen (self). |
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Term
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Definition
| Type B individuals (genotype BB or BO) make only the Gal transferase, so their red blood cells have the B antigen and can be transfused into an individual with type B or AB blood. These recipients do not make antibodies that recognize the B antigen (self). |
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Term
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Definition
| Type AB individuals (genotype AB) make both transferases, so their red blood cells have both antigens and can be transfused only into individuals with type AB blood. In contrast, they can receive transfusions from an individual with any blood type (universal acceptor) because they do not make antibodies that recognize either the A or B antigens (self). |
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Term
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Definition
Typically contain ≥ 95% carbohydrate by mass and a small polypeptide core. Carbohydrate portion consists of glycosaminoglycan chains:
- linear polymer with repeating disaccharide unit: (uronic acid – hexosamine)n
- six classes that differ in disaccharide structure (Devlin Fig 16.14)
- carboxyl groups of uronic acids and sulfate groups make them highly negatively charged
- covalently attached to core protein via a few linker sugars attached to Ser or Thr as in O-linked glycoproteins (except hyaluronate which is bound via a linker protein)
attract water to form lubricants (mucous secretions) and gels that can spring back when compressed (synovial fluid in joints and vitreous humor in eye).
found in extracellular matrix and on cell surfaces |
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Term
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Definition
| Most abundant glycosaminoglycan; found in cartilage, tendons, ligaments and aorta. |
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Term
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Definition
| Found in skin, blood vessels, and heart valves. |
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Term
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Definition
| Found in cornea and loose connective tissue. |
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Term
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Definition
| Intracellular component of mast cells; used as a pharmaceutical anticoagulant. |
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Term
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Definition
| Similar structure to heparin; found in basement membranes and on cell surfaces where it serves as a ligand for various proteins. |
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Term
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Definition
| Not sulfated; not covalently bound to core protein; found in synovial fluid of joints, vitrous humor of eye, umbilical cord, loose connective tissue and cartilage; serves as shock absorber and lubricant. Hyaluronate is bound to the core protein via a linker protein and often serves to aggregate large numbers of proteoglycans. |
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Term
| Synthesis of proteoglycans |
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Definition
Proteoglycans are synthesized similarly to the O-linked glycoproteins (Devlin Fig 16.15):
- the core protein is synthesized in the ER and then transported to the Golgi
- a series of different glycosyltransferases act sequentially to build a linker region, consisting of 4 sugars, on a serine of the core protein.
- repeated action of two specific glycosyltransferases add the sugars of the repeating disaccharide unit one at a time to build the glycosaminoglycan; the sugar donors are nucleotide-sugars.
- additional enzymes are required for sulfation, acetylation and epimerization (conversion of glucuronic acid to iduronic acid); they act while the chain is elongating. |
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Term
| Linker region (proteoglycans) |
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Definition
| 4 sugars, on a serine of the core protein. |
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Term
| What are the 3 possible types of glycosylation? |
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Definition
| N-linked oligosaccharides, O-linked oligosaccharides and O-linked glycosaminoglycans. These proteoglycans may also interact non-covalently with hyaluronate to form large proteoglycan aggregates (as shown above). |
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Term
| Degradation of glycoproteins and proteoglycans |
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Definition
The degradation of glycoproteins and proteoglycans occurs in lysosomes by the sequential action of lysosomal hydrolases (proteases and glycosidases):
- endoglycosidases first remove the carbohydrates from the proteins
- specific glycosidases act on each glycosidic bond in a serial fashion but in reverse order to that in which they were added; this dismantles the oligosaccharides and glycosamino- glycans into their component monosaccharides which can be reused by the cell.
- proteases cleave the protein component into amino acids which can be reused by the cell. - for glycosaminoglycans, sulfatases and acetylases are required to remove sulfates and
acetyl groups. |
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Term
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Definition
| Genetic diseases caused by defects in the lysosomal hydrolases required for the degradation of glycosaminoglycans. Over time, partially digested glycosaminoglycans accumulate in lysosomes and undigested glycosaminoglycans accumulate in blood and connective tissues. This results in permanent progressive cellular damage. After cell death, these materials appear in the urine. There are 7 distinct classes of these diseases which differ clinically. Patients typically appear normal at birth but then show a progressive decline in physical and/or mental function due to effects on bones, connective tissues, neurons, and organs. |
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Term
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Definition
| Genetic diseases caused by defects in the lysosomal hydrolases required for degradation of oligosaccharides. As above, partially digested products accumulate in lysosomes and appear in urine after cell death. |
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