Transcript 041607
Sugars • Polyhydroxy AND aldehyde or ketone • Fischer projection: – Linear structure, depicts stereochemistry – D- or L- from stereochemistry of distal carbon – Name from stereochem of other carbons • Sugars typically cyclic in solution – Usually last hydroxyl attacks carbonyl carbon – Generates additional stereocenter • Reversible attack: Mutarotation – Cyclic form not flat: chair form • Glycosidic bond – Polymerization of sugars – Through epimeric hydroxyl: hemiacetal → acetal • Lose tendency for mutarotation Polysaccharides • Branching: – Donor hydroxyl forming the acetal need not be the 4 carbon Polysaccharides • Branching: two (or more) hydroxyls form acetals with other monosaccharides Polysaccharides • Energy storage – Store glucose as a polymer • Starch: plant cells • Glycogen: animal cells – Can have lots of glucose with minimal osmolarity – No defined molecular weight, no ‘template’ (à la DNA/protein synthesis) • Characteristics a consequence of chemistry/enzymology Polysaccharides • Energy sources: – Compact storage, but end monosacch are accessible to enzymatic removal (a1→4) • Structural polysaccharides – Cellulose (plants), chitin (bugs) • Highly hydrogen bonded: compact, strong b1→4 linkages: mammalian cells lack the enzyme to degrade Cellulose Chitin: b1→4 linkages a1→4 linkages Sugar/peptide conjugates • Peptidoglycans – eg. bacterial cell wall Sugar/peptide conjugates • Proteoglycans – Multicellular organisms: usually extracellular biomolecules • connective tissue • cellular adhesion molecules • lots of hydrogen/non-covalent bonding – Dominated by carbohydrate • Mass • Biological activity – Sugar joined to the peptide through a hydroxyl on the protein Sugar/peptide conjugates • Glycoproteins – Proteins have the biological activity: modified or protected by the carbohydrates – Typically extracellular proteins (secreted) or extracellular regions of integral membrane proteins – Saccharides added in ER and Golgi – Sugars protect the protein component from extracellular proteases – Sugars influence binding by other proteins/molecules • Lectins (eg. P-selectin) recognize/bind specific sugars & mediate cell-cell interactions