CARBOHYDRATESPOLYSACCHARIDES

Dr Sundus Tewfik s.tewfik@londonmet.ac.uk

MS1005N General Biochemistry

32. Divided into homopolysaccharides - only one type of monomer eg all glucose or heteropolysaccharides - with 2 or more different types of monomereg starch has only D-glucose thus it is a homo-polysaccharide,

whereas hyaluronic acid has alternating units of D-glucuronic acid and N-acetyl D-glucosamine.

Homopolysaccharides may be given class names, if one contains glucose it is called a glucan, if one contains mannose it is called a mannan.

33. Storage polysaccs. are deposited as large granules in cytoplasm: starch in plants and glycogen in animals

34. Starch occurs in 2 forms, α-amylose and amylopectin.

-amylose amylopectin

Amylose not truly soluble in water but forms hydrated micelles that give a blue colour with iodine

α-amylose consists of long unbranched chains in which all D-glucose units are in α(14) linkages. The chains vary in Mr from a few thousand to ~500,000.

amylopectin is highly branched with branch points occurring every 12 glucose units. The chains have α(14) linkages but the branch points are α(16) linkages. Its Mr may be as high as 1,000,000.

Amylopectin gives micellar solutions which produce a red/violet colour with iodine

35. >50% carbohydrate ingested by humans is starch. Both amylose and amylopectin are rapidly hydrolysed by α-amylase. This enzyme hydrolyses internal α(14) linkages to yield maltose, maltotriose and α-dextrin.

maltotriose

36. Glycogen is abundant in muscle and liver, it is a polysacc. of D-glucose in α(14) linkages. It is branched every 8-10 glucose units (ie more branched than amylopectin)

37. Structural polysaccs. Cellulose is the most abundant organic compound in the biosphere. Wood ~50% cellulose, cotton ~100% cellulose.All the linkages in cellulose are β(14). Most animals do not have enzymes capable of hydrolysing these linkages, so cellulose not used as nutrient.Some animals such as cows which have cellulases in the rumen capable of hydrolysing cellulose to D-glucose.

cellulose

Every other glucose is flipped over, due to β linkages. This promotes intra-chain and inter-chain H-bonds andvan der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles - microfibrils