Download Sugars and Polysaccharides: Monosaccharides, Derivatives, and Polysaccharides and more Study notes Biochemistry in PDF only on Docsity! Sugars and Polysaccharides Monosaccharides Polysaccharides Glycoproteins Monosaccharides (simple sugars) Classification monosaccharides are classified according to the chemical nature of the carbonyl group and # of C atoms aldose - aldehyde ketose - ketone triose - 3 tetrose - 4 pentose - 5 hexose - 6 heptose - 7 D-sugars same absolute configuration as D-glyceraldehyde based on asymmetric center farthest removed from carbonyl group Polysaccharides (glycans) homopolysaccharides - glucans heteropolysaccharides May form branched as well as linear chains Carbohydrate analysis Purification by chromatography and electrophoresis Affinity of proteins for carbohydrates - lectins concanavalin A binds α-D-glucose and α-D-mannose residues agglutinin binds β-N-acetylmuramic acid and α-N- acetylneuraminic acid Methylation analysis used to determine monosaccharide linkages - methyl esters not at the anomeric carbon are resistant to acid hydrolysis but glycosidic bonds are not Periodic acid oxidation cleaves C-C bond between diols (forms dialdehydes, releases formate from anomeric carbon) Exoglycosidases - specifically hydrolyze corresponding monosaccharides from nonreducing end of oligosaccharides (i.e., β-galactosidase, α-mannosidase) Endoglycosidases - specifically hydrolyze glycosidic bonds between nonterminal sugar residues Polysaccharides Disaccharides Sucrose - O-α-D-glucopyranosyl-(1 → 2)-β-D- fructofuranoside (nonreducing sugar) Lactose - O-β-D-galactopyranosyl-(1 → 4)-D- glucopyranose (reducing sugar) Maltose - O-α-D-glucopyranosyl-(1 → 4)-D-glucopyranose Isomaltose - O-α-D-glucopyranosyl-(1 → 6)-D- glucopyranose Cellobiose - O-β-D-glucopyranosyl-(1 → 4)-D- glucopyranose Polysaccharides Structural polysaccharides: cellulose and chitin Cellulose - primary structural component of plant cell walls, accounts for over half of the biosphere carbon! up to 15,000 D-glucose residues, β(1 → 4) linkage (specific microorganisms have enzymes to cleave linkage) Chitin - principle structural component of exoskeleton of invertebrate, also present in cell wall of fungi and algae homopolymer of N-acetyl-D-glucosamine, β-(1 → 4) linkage similar structure as cellulose Glycoproteins Protein covalently attached to carbohydrate Variable carbohydrate content Proteoglycans Protein plus covalently and noncovalently associated glycosaminoglycan Basic structure - hyaluronic acid backbone noncovalently linked (stabilized by link protein) to core protein, which is covalently linked to glycosaminoglycans (often keratan sulfate and chondroitin sulfate) Three regions to glycosaminoglycan portion: 1. N-terminal segment, relatively few chains, covalently linked to core protein Asn residues 2. Oligosaccharide rich segment, keratan sulfate chains, covalently linked to core protein Ser and Thr residues 3. C-terminal region, rich in chondroitin sulfate, covalently linked to core protein Ser residues through Gal-Gal-Xyl trisaccharides Glycoproteins Bacterial Cell Walls Gram-positive - ~250 Å Gram-negative - ~30 Å Peptidoglycan (murein) - covalently linked polysaccharide and polypeptide chains linear chains, alternating β(1 → 4)-linked N- acetylglucosamine (NAG or GlcNAc) and N- acetylmuramic acid (NAM or MurNAc) NAM’s lactic acid residue amide bond to D-amino acids (resistant to proteases) Penicillin binds and inactivates cross-linking enzymes Gram positive surfaces have teichoic acids Gram negative have unusual polysaccharides (O-antigens) Glycoproteins Glycoprotein Structure and Function Almost all secreted and membrane-associated eukaryotic proteins are glycosylated N-linked NAG (GlcNAc) β-linked to amide N of Asn in peptide sequence Asn-X-Ser or Asn-X-Thr, where X = any amino acid (save Pro or Asp) Core saccharide sequence = (Man)3-(NAG or GlcNAc)2 O-linked Disaccharide core β-galactosyl-(1 → 3)-α-N- acetylgalactosamine α-linked to OH of Ser or Thr
