Chapter 7 Carbohydrates and Glycoconjugates of the Cell Surfaces PDF

Chapter 7 Carbohydrates and Glycoconjugates of the Cell Surfaces Chapter Summary ™ Carbohydrates (CH2O)n, n ≥3 ™ Nomenclature Ç Monosaccharides (simple sugars) Ç Oligo- and polysaccharides: Polymers of simple sugars ™ Classification Ç Aldose (aldehyde) and ketos...

Chapter 7 Carbohydrates and Glycoconjugates of the Cell Surfaces Chapter Summary ™ Carbohydrates (CH2O)n, n ≥3 ™ Nomenclature Ç Monosaccharides (simple sugars) Ç Oligo- and polysaccharides: Polymers of simple sugars ™ Classification Ç Aldose (aldehyde) and ketose (ketone) Ç Triose, tetrose, pentose, hexose, etc. ™ Stereochemistry Ç Aldose n ≥3, ketose n ≥4 have at asymmetric carbons (chiral centers) Ç D- and L- Configuration: Refer to configuration of highest numbered asymmetric carbon Ç D- and L- forms: Mirror images: Enantiomers Ç With >1 asymmetric carbon: Diastereomers: Configurations that differ at 1 or more chiral carbons but not mirror image molecules Ç Epimers: Two molecules that differ in configuration about 1 asymmetric carbon ™ Ring structures Ç Pyranoses: Six-membered, oxygen-containing ring Ç Furanose: Five-membered, oxygen-containing ring Ç Anomeric carbon: Ketone or aldehyde carbon that becomes chiral upon ring formation Ç Anomers: α, β differ in configuration about anomeric carbon  α-Configuration: In Fischer projection, OH of anomeric carbon on same side as OH of highest numbered asymmetric carbon  β-Configuration: In Fischer projection, OH of anomeric carbon on opposite side as OH of highest numbered asymmetric carbon Ç Haworth projections: Three-dimensional representation: Groups to right in Fischer projection draw down in Haworth projection Ç Conformations  Chair and boat conformations due to ring pucker  Axial and equatorial orientation of groups attached to ring ™ Sugar derivatives Ç Acids: Oxidation of free anomeric carbon to carboxylate Ç Alcohols Ç Deoxysugars Ç Esters (ATP and GTP) Ç Amino sugars: Glucosamine Ç Muramic acid Ç Glycosides: Anomeric carbon reacted with alcoholic function. ™ Sugars with free anomeric carbon are reducing sugars Ç End of sugar polymer with free anomeric carbon: Reducing end  Ç Opposite end: Nonreducing end ™ Oligosaccharides Ç Disaccharides  Maltose: Diglucose  Lactose: Galactose and glucose  Fructose: Fructose and glucose: Nonreducing sugar ™ Storage polysaccharides Ç Starch: α Amylose and amylopectin  α Amylose: Linear chains of α(1→4)D-glucose  Amylopectin: Linear chains of α(1→4)D-glucose with α(1→6)D- glucose branches every 12 to 30 residues  Animal digestion of starch α Amylase: Animals: Hydrolysis of internal α(1→4) glycosidic linkage α Amylase: Microorganisms: Exoamylase: releases disaccharide maltose Ç Glycogen: α(1→4) D-glucose chains with α(1→6) D-glucose branches every 8 to 12 residues Ç Dextrans: Bacteria: α(1→6) D-glucose polymers ™ Structural polymers Ç Cellulose: Plant cell wall: Linear polymer of β(1→4) D-glucose Ç Chitin: Exoskeleton and fungi cell wall: β(1→4) N-acetyl-D-glucosamine ™ Glycosaminoglycans: Polymers with disaccharide repeat: Negatively charged ™ Peptidoglycan of bacterial cell walls Ç Gram-negative bacterial cell wall  NAM-NAG repeats crosslinked with tetrapeptide  Attached to outer cell wall via 57-amino acid protein  Protein attached to aminopimelic acid (replaces 10% of D- alanines)  Outer cell wall: Lipopolysaccharides: Antigenic Ç Gram-positive bacterial cell walls  NAM-NAG repeats crosslinked with tetrapeptide and pentaglycine  No outer cell wall  Teichoic acid: Ribitol phosphate or glycerol phosphate polymers ™ Glycoproteins Ç Functions: Structural, enzymatic, receptors, transport, immunoglobins Ç Linkage  O-linked Amino acids are serine, threonine or hydroxylysine Sugars: N-Acetylgalactosamine  N-linked Amino acid is asparagines Functions: Protein folding and turn-over ™ Proteoglycans: Glycoproteins Ç Sugar: Glycosaminoglycans Ç Interact with variety of molecules