Book : Molecular Biology
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Posted by: CHELSEA
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Writer CHELSEA

Carbohydrate

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Overview

Monosaccharides polymerize to form polysaccharides. Glucose is a typical monosaccharide. It has two important types of functional group: 1. A carbonyl group (aldehydes in glucose, some other sugars have a ketone group instead, 2. Hydroxyl groups on the other carbons. Glucose exists mostly in ring structures. 5-OH adds across the carbonyl oxygen double bond. This is a socalled internal hemi-acetal. The ring can close in either of two ways, giving rise to anomeric forms, -OH down (the alpha-form) and -OH up (the beta-form) The anomeric carbon (the carbon to which this -OH is attached) differs significantly from the other carbons. Free anomeric carbons have the chemical reactivity of carbonyl carbons because they spend part of their time in the open chain form. They can reduce alkaline solutions of cupric salts. Sugars with free anomeric carbons are therefore called reducing sugars. The rest of the carbohydrate consists of ordinary carbons and ordinary -OH groups. The point is, a monosaccharide can therefore be thought of as having polarity, with one end consisting of the anomeric carbon, and the other end consisting of the rest of the molecule. Monosaccharide can polymerize by elimination of the elements of water between the anomeric hydroxyl and a hydroxyl of another sugar. This is called a glycosidic bond. If two anomeric hydroxyl groups react (head to head condensation) the product has no reducing end (no free anomeric carbon). This is the case with sucrose. If the anomeric hydroxyl reacts with a non- anomeric hydroxyl of another sugar, the product has ends with different properties. • A reducing end (with a free anomeric carbon). • A non-reducing end. This is the case with maltose. Since most monosaccharide has more than one hydroxyl, branches are possible, and are common. Branches result in a more compact molecule. If the branch ends are the reactive sites, more branches provide more reactive sites per molecule.



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