CARBOHYDRATES

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MS1005N General Biochemistry
CARBOHYDRATES POLYSACCHARIDES
Dr Sundus Tewfik s.tewfik_at_londonmet.ac.uk
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32. Divided into homopolysaccharides - only one
type of monomer eg all glucose or
heteropolysaccharides - with 2 or more different
types of monomer eg starch has only D-glucose
thus it is a homo-polysaccharide,
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whereas hyaluronic acid has alternating units of
D-glucuronic acid and N-acetyl D-glucosamine.
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Homopolysaccharides may be given class names, if
one contains glucose it is called a glucan, if
one contains mannose it is called a mannan.
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33. Storage polysaccs. are deposited as large
granules in cytoplasm starch in plants and
glycogen in animals
34. Starch occurs in 2 forms, a-amylose and
amylopectin.
amylopectin
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a-amylose consists of long unbranched chains in
which all D-glucose units are in a(1?4) linkages.
The chains vary in Mr from a few thousand to
500,000.
Amylose not truly soluble in water but forms hydrated micelles that give a blue colour with iodine
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amylopectin is highly branched with branch points
occurring every 12 glucose units. The chains have
a(1?4) linkages but the branch points are a(1?6)
linkages. Its Mr may be as high as 1,000,000.
Amylopectin gives micellar solutions which produce a red/violet colour with iodine
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35. gt50 carbohydrate ingested by humans is
starch. Both amylose and amylopectin are rapidly
hydrolysed by a-amylase. This enzyme hydrolyses
internal a(1?4) linkages to yield maltose,
maltotriose and a-dextrin.
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(No Transcript)
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36. Glycogen is abundant in muscle and liver, it
is a polysacc. of D-glucose in a(1?4) linkages.
It is branched every 8-10 glucose units (ie more
branched than amylopectin)
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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 ß(1?4). 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.
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cellulose
Every other glucose is flipped over, due to ß
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