Glycoproteins: Comprehensive Study Notes
Glycoproteins: Detailed Study Notes
Overview
Glycoproteins are proteins with covalently bound oligosaccharide chains (glycans).
Glycosylation, the enzymatic attachment of sugars, is the most frequent posttranslational modification of proteins.
Reversible glycosylation can occur with a single sugar (N-acetylglucosamine, GlcNAc) bound to serine (Ser) or threonine (Thr), which can be a site of reversible phosphorylation; this is a key metabolic regulatory mechanism.
Nonenzymic attachment of sugars is called glycation; it can have serious pathologic consequences (e.g., poorly controlled diabetes mellitus).
Glycoproteins are a major class of glycoconjugates (glycoproteins, proteoglycans, glycolipids, glycosphingolipids).
Almost all plasma proteins and many peptide hormones are glycoproteins; many cell membrane proteins contain substantial carbohydrate and may be anchored by a glycan to the lipid bilayer.
Alterations in glycoprotein and glycoconjugate structures on cancer cell surfaces are increasingly implicated in metastasis.
Biomedical Importance
Glycoproteins play diverse roles in health and disease, with carbohydrate content ranging from 1% to >85% by weight.
The glycan structures are dynamic and respond to signals related to cell differentiation, physiology, and neoplastic transformation due to changes in glycosyltransferase expression patterns.
Glycosylation status influences biological information conveyed by glycans, particularly through interactions with lectins and other molecules, modulating cellular activity.
Glycoconjugates include glycoproteins, proteoglycans, glycolipids, and glycosphingolipids.
They are integral to structural roles, lubrication, transport, immunology, enzymes, hormone action, cell attachment, and development.
Glycoprotein alterations are implicated in cancer metastasis and immune recognition.
Oligosaccharide Chains Encode Biological Information
The information in glycan patterns is secondary information (not the primary genetic code) and depends on:
Expression patterns of glycosyltransferases in the biosynthetic cells
Substrate specificities and affinities of these glycosyltransferases
Availability of carbohydrate substrates
This leads to microheterogeneity: most glycoproteins display multiple glycoforms; not all chains are complete in a given glycoprotein; some chains are truncated.
Glycans influence protein properties via interactions with lectins and other carbohydrate-binding proteins, modulating cellular processes.
Oligosaccharide Chains in Human Glycoproteins
Eight monosaccharides predominate in human glycoproteins (Table 46-3):
Galactose (Gal), Glucose (Glc), Mannose (Man), N-acetylneuraminic acid (NeuAc), Fucose (Fuc), N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc), Xylose (Xyl).
N-acetylneuraminic acid (NeuAc) is usually terminal in oligosaccharide chains, attached to subterminal Gal or GalNAc residues; NeuAc is the major sialic acid species in humans.
Other sugar residues, including sulfates, can be present (often attached to Gal, GlcNAc, or GalNAc).
Sugar donors for glycoprotein synthesis are sugar nucleotides (e.g., UDP- and GDP-activated sugars or CMP-activated sugars), not free sugars:
UDP-Gal, UDP-Glc, GDP-Man, CMP-NeuAc, UDP-GalNAc, UDP-GlcNAc, UDP-Xyl, etc.
The first sugar moiety commonly incorporated in N-linked glycosylation is N-acetylgalactosamine (GalNAc).
The glycan chains range in length from 1–2 residues to very large structures and can be linear or branched.
Some glycoproteins contain both O- and N-linked glycan chains (e.g., glycophorin).
The carbohydrate components contribute to charge, solubility, conformation, proteolysis resistance, and protein–protein interactions, as well as cell surface recognition.
Lectins: Tools and Roles
Lectins are carbohydrate-binding proteins that agglutinate cells or precipitate glycoconjugates; most lectins are themselves glycoproteins.
Lectins require at least two sugar-binding sites to cause agglutination/precipitation; those with a single site do not.
Lectins are used to purify glycoproteins, probe glycoprotein patterns on cell surfaces, and generate mutant cells deficient in glycan biosynthesis enzymes.
Asialoglycoprotein receptor (in liver) binds desialylated glycoproteins with exposed galactose, leading to endocytosis and catabolism.
Plant lectins (phytohemagglutinins) can cause injury if ingested in undercooked legumes due to intestinal mucosa stripping.
Three Major Classes of Glycoproteins
Based on the linkage between the polypeptide and oligosaccharide chains:
1) O-linked glycoproteins: O-glycosidic linkage to Ser/Thr (and sometimes Tyr) with sugars like GalNAc-Ser/Thr.
2) N-linked glycoproteins: N-glycosidic linkage to the amide nitrogen of Asn with GlcNAc (Asn–GlcNAc linkage).
3) GPI-anchored glycoproteins: linked via a glycosylphosphatidylinositol (GPI) moiety to the C-terminus; attaches to phosphatidylinositol in the membrane and terminates in a protein-linked glycan.O-glycosylation often occurs post-translationally in the Golgi; N-glycosylation occurs cotranslationally in the endoplasmic reticulum (ER).
The number of oligosaccharide chains per protein varies from 1 to >30, with chains ranging from 1–2 residues to large complex structures; chains can be linear or branched; some proteins contain multiple types of glycan chains.
Mucins are a special class of O-glycosylated glycoproteins with high O-glycan content and VNTRs (variable-number tandem repeats).
O-Linked Glycoproteins: Subclasses and Mucins
Four O-glycosidic linkage subclasses in humans:
1) Predominant N-acetylgalactosamine (GalNAc) linked to Ser/Thr; often followed by Gal or NeuAc, with varied chain lengths.
2) Proteoglycans: contain a galactose–galactose–xylose (the “link trisaccharide”) attached to Ser.
3) Collagens: contain a galactose–hydroxylysine linkage.
4) Nuclear/cytosolic proteins: a single GlcNAc attached to Ser/Thr.Mucins:
Highly O-glycosylated; carbohydrate content >50% by weight.
Contain VNTRs rich in Ser, Thr, and Pro; O-glycans cluster on these repeats.
Mucins provide lubrication and form protective barriers on epithelial surfaces; secretory mucins are oligomeric and linked by disulfide bonds, contributing to high molecular mass and gel-like viscosity.
Negative charge due to abundant NeuAc and sulfate groups.
Membrane-bound mucins participate in cell–cell interactions and can mask cell-surface antigens; cancer cells often overexpress mucins and present cancer-specific epitopes.
O-glycoprotein biosynthesis in the Golgi:
Chains are built by sequential donation of sugars from sugar nucleotides catalyzed by glycoprotein glycosyltransferases.
Golgi contains carrier systems (transporters/antiporters) to move sugar nucleotides (e.g., UDP-Gal, GDP-Man, CMP-Neu5Ac) across membranes; antiport mechanism balances influx with efflux of nucleotide monophosphates (UMP, GMP, CMP).
Humans have ~41 glycoprotein glycosyltransferases; enzyme families are named for the sugar nucleotide donor; subfamilies are defined by the glycosidic linkage formed; some reactions proceed with retention, others with inversion of configuration at C-1.
Enzyme binding to sugar nucleotides induces conformational changes enabling donor sugar transfer to the acceptor substrate; glycosyltransferases are highly substrate-specific and often act sequentially on products of prior steps.
Spatial organization in the Golgi ensures ordered processing of glycans; incomplete glycan chains lead to microheterogeneity.
First sugar moiety usually GalNAc; glycosylation can be incomplete; many glycoproteins show truncated glycans.
N-Linked Glycoproteins: Structure and Biosynthesis
N-linked glycoproteins feature an asparagine–N-acetylglucosamine (Asn–GlcNAc) linkage.
Three major classes of N-linked oligosaccharides: complex, high-mannose, and hybrid.
All share a common core pentasaccharide: linked to the Asn; outer branches differ among classes.
Complex structures have 2–5 outer branches (antennae); often terminate with NeuAc, with underlying Gal and GlcNAc residues (disaccharide: N-acetyllactosamine, i.e., ext{Gal}-eta 1-4- ext{GlcNAc} repeating units [ ext{Gal}-eta 1-3/eta 1-4- ext{GlcNAc}-eta 1-]n); blood group antigens (I/A/B) belong to this class.
High-mannose: core plus 2–6 extra mannose residues.
Hybrid: features of both complex and high-mannose structures.
Core structure and core processing:
The biosynthesis begins on the cytosolic side of the ER with dolichol pyrophosphate-linked oligosaccharide assembly, then translocation into the ER lumen for further processing before transfer to the nascent polypeptide.
Consensus glycosylation site: where ; some proteins lack a clear consensus sequence for glycosylation.
Dolichol pyrophosphate oligosaccharide assembly (early steps):
Step 1: UDP-N-acetylglucosamine + dolichol phosphate → GlcNAc-PP-Dol
Step 2: another GlcNAc from UDP-GlcNAc
Step 3: five mannose residues added from GDP-mannose to form the dolichol pyrophosphate oligosaccharide core
Step 4: translocation to ER lumen; further mannose and glucose added using dolichol phosphate mannose and dolichol phosphate glucose donors
Step 5: transfer of the dolichol pyrophosphate oligosaccharide to the nascent protein by oligosaccharyItransferase (OST) onto the Asn residue during co-translational translocation
Processing to mature glycans:
In ER: removal of glucose residues and some mannose to form high-mannose glycans
In Golgi: trimming of glucose and mannose and addition of GlcNAc, Gal, and NeuAc to form complex glycans; sometimes partial processing yields hybrid glycans
Role of calnexin/calreticulin folding cycle:
Calnexin (ER membrane lectin) binds monoglucosylated glycoproteins (core glycan lacking the outermost glucose) to prevent aggregation; complex with ERp57 catalyzes disulfide rearrangements for correct folding
Glucosyltransferase reglucosylates misfolded glycoproteins to rebind calnexin-ERp57; if properly folded, deglucosylation occurs and protein proceeds to secretion; if not, retrotranslocation to cytosol for degradation
Calreticulin performs a similar folding assistance in the ER lumen
Regulation and diversity:
About 1% of the human genome encodes glycosylation-related enzymes; there are numerous N-acetylglucosamine transferases and other glycosyltransferases with multiple isoforms
Cancer cells often express altered glycosyltransferase patterns leading to greater glycan branching and altered adhesion, potentially affecting metastasis
Gpi-anchored glycoproteins (see next section) are a distinct class that uses a separate targeting/anchoring mechanism
Glycosylphosphatidylinositol (GPI) Anchored Glycoproteins
GPI-anchored glycoproteins are tethered to the outer leaflet of the plasma membrane by a GPI tail.
Structure:
GPI anchor linked to the protein via an amide bond between the carboxyl-terminus of the protein and a phosphoethanolamine moiety on the glycan linked to phosphatidylinositol (PI).
The GPI lipid is inserted into the outer leaflet of the membrane; the glycan chain contains Gal and GlcNAc, with potential mannose and other sugars; some GPIs have additional modifications (e.g., extra fatty acids, extra phosphorylethanolamine).
Functions of the GPI anchor:
1) Increases lateral mobility of the protein in the plasma membrane compared with transmembrane proteins (GPI-anchored proteins are on the outer leaflet and not spanning both bilayer leaflets).
2) Some GPIs participate in signal transduction, allowing GPI-anchored proteins to act as receptors without a transmembrane domain.
3) GPI structures can direct proteins to apical or basolateral membranes in polarized epithelial cells.Biosynthesis and attachment:
The GPI anchor is preformed in the ER
The nascent protein contains a C-terminal hydrophobic domain that signals GPI anchor attachment; the anchor is attached via a transamidation reaction that replaces the C-terminus hydrophobic domain with the GPI anchor
Initial steps: insertion of fatty acids into the luminal ER membrane; concomitant glycosylation starting with N-acetylglucosamine linked to the PI phosphate
A terminal phosphoethanolamine is added to complete the GPI glycan
Rapidly Reversible O-Linked Glycosylation (O-GlcNAc)
Some proteins (nuclear pore proteins, cytoskeletal proteins, transcription factors, chromatin-associated proteins, and certain oncogenes/tumor suppressors) undergo rapid O-linked glycosylation with a single GlcNAc on Ser/Thr.
This O-GlcNAc cycling is reciprocal with phosphorylation:
The same Ser/Thr sites can be glycosylated or phosphorylated depending on cellular signaling
Enzyme: O-linked N-acetylglucosamine transferase (OGT) uses UDP-GlcNAc as donor and has phosphatase activity; it can replace a phosphate with GlcNAc on Ser/Thr
Site preferences: about half of the reciprocal sites are Pro-Val-Ser (PVS) motifs; activity is influenced by UDP-GlcNAc concentration
Metabolic sensing:
Flux through the hexosamine pathway yields UDP-GlcNAc; 2–5% of glucose metabolism can flow through this pathway, linking nutrient status to protein modification
O-GlcNAc modification is implicated in insulin resistance, glucose toxicity in diabetes, and other neurodegenerative diseases
Advanced Glycation End-Products (AGEs) in Diabetes Mellitus
Glycation is nonenzymatic attachment of sugars (primarily glucose) to amino groups of proteins, DNA, and lipids.
Mechanism:
Glucose forms a Schiff base with amino termini, which undergoes the Amadori rearrangement to yield ketoamines (Fig. 46-5)
Further reactions produce advanced glycation end-products (AGEs) via the Maillard reaction
Pathophysiology:
Hyperglycemia increases protein glycation, altering extracellular matrix properties (e.g., cross-linking of collagen) and contributing to tissue stiffness and vascular damage
-AGEs accumulate in vessel walls and can contribute to atherogenesis via LDL cross-linkingAGE uptake by endothelial cells and macrophages activates NF-κB signaling, increasing proinflammatory cytokines
HbA1c: a nonenzymatic glycation product on hemoglobin A used clinically to monitor long-term glycemic control in diabetes
Glycoproteins in Biology and Disease
Glycoproteins serve numerous roles: structural components (collagens, mucins), transport (transferrin, ceruloplasmin), immune function (immunoglobulins, MHC), hormones (hCG, TSH), enzymes, receptors, and cell–cell interactions.
Glycoproteins participate in fertilization and inflammation; glycoconjugate defects can cause disease.
Glycoproteins in Fertilization
Zona pellucida glycoprotein ZP3 is an O-linked glycoprotein acting as a sperm receptor.
Interaction between sperm surface proteins and ZP3 oligosaccharides triggers the acrosomal reaction, releasing proteases and hyaluronidase to enable sperm penetration.
PH-30 is another glycoprotein important for binding and fusion of sperm and oocyte membranes.
Selectins, Inflammation, and Immune Surveillance
Selectins are Ca2+-binding, cell-surface lectins that mediate leukocyte rolling on endothelium via interactions with sialylated/fucosylated glycans on endothelial ligands.
Rolling permits integrin-mediated firm adhesion, diapedesis, and migration into tissues.
Blocking selectin–ligand interactions can therapeutically reduce inflammatory responses.
Cancer cells often express selectin ligands, potentially contributing to metastasis.
Pathologies Related to Glycoprotein Biosynthesis and Structure
Leukocyte adhesion deficiency II: caused by mutations in Golgi GDP-fucose transporter; leads to defective fucosylation and poor selectin ligand formation; results in severe bacterial infections and mental retardation; oral fucose can be beneficial.
Paroxysmal nocturnal hemoglobinuria: acquired somatic mutation in the enzyme that links GlcNac to PI in GPI anchors; leads to loss of GPI-anchored proteins such as decay-accelerating factor (DAF) and CD59; increased hemolysis during sleep due to complement activation.
Congenital muscular dystrophies: caused by defective glycosylation of a-dystroglycan; impaired interaction with laminin-2 (merosin) affects muscle cell–basement membrane adhesion.
Rheumatoid arthritis: altered IgG glycosylation (less galactose, more N-acetylglucosamine) may enhance inflammation; MBL (mannose-binding lectin) can bind agalactosyl IgG and activate complement, contributing to synovial inflammation.
Mannose-binding protein deficiency: predisposes to infections in infants; MBL binds sugars on pathogens to aid opsonization and complement activation (innate immunity).
I-cell disease: due to failure to target lysosomal hydrolytic enzymes to lysosomes; cells lack N-acetylglucosamine phosphotransferase; lysosomes accumulate undegraded material; plasma contains high lysosomal enzyme activities due to secretion.
Defects in glycoprotein lysosomal hydrolases lead to diseases such as mannosidosis, fucosidosis, sialidosis, aspartylglucosaminuria, and Schindler disease due to enzyme deficiencies like a-mannosidase, a-fucosidase, a-neuraminidase, aspartylglucosaminidase, and a-N-acetylgalactosaminidase respectively.
Viruses, Bacteria, and Parasites Bind Glycans on Human Cells
Influenza A virus binds host cell glycoprotein receptors containing N-acetylneuraminic acid (NeuAc) via hemagglutinin; it also has neuraminidase which cleaves sialic acid to allow virion release.
Neuraminidase inhibitors (e.g., zanamivir, oseltamivir) inhibit viral spread by preventing virion egress.
Influenza classification uses Hemagglutinin (H) and Neuraminidase (N) types; at least 16 H types and 9 N types exist; avian influenza often classified as H5N1.
HIV-1 attaches via gp120 to CD4 and uses gp41 to fuse with host cell; neutralizing gp120-based vaccines are challenged by rapid antigenic variation of gp120.
Helicobacter pylori binds to gastric epithelial cell glycans, enabling stable attachment and infection; many enteric bacteria bind surface glycans to colonize mucosal surfaces.
Plasmodium falciparum (malaria parasite) attachment to human cells is mediated by a parasite surface GPI
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