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BCH2011: Textbook summary - Lecture 22

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Monash University

LECTURE 22 Carbohydrates: Carbohydrates are polyhydroxyl aldehydes or ketones, or substances that yield such compounds on hydrolysis. Many, but not all, carbohydrates have the empirical formula (CH O)n: some also contain nitrogen, phosphorus, or sulfur. 2 There are 3 major size classes of carbohydrates: - Monosaccharides - Oligosaccharides - Polysaccharides Monosaccharides: Also known as simple sugars, consist of a single polyhydroxyl aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar D- glucose, sometimes referred to as dextrose. Monosaccharides of four or more carbons tend to have cyclic structures. Oligosaccharides: Consists of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. The most abundant are the dissacharides, with two monosaccharide units. Typical is sucrose, which consists of the six-carbon sugars D-glucose and D-fructose. In cells, most oligosaccharides consisting of three or more units do not occur as free entities but are joined to non-sugar molecules (lipids and proteins) in glycoconjugates. Polysaccharides: Sugar polymers containing more than 20 or so monosaccharide units. Some polysaccharides, such as cellulose, are linear chains; others, such as glycogen, are branched. Both glycogen and cellulose consist of recurring units of D-glucose, but they differ in the type of glycosidic linkage and consequently have strikingly different properties and biological roles. Monosaccharides and disaccharides: The simplest of the carbohydrates, the monosaccharides, are either aldehydes or ketones with two or more hydroxyl groups; the six-carbon monosaccharide glucose and fructose have five hydroxyl groups. Many of the carbon atoms to which hydroxyl groups are attached are chiral centers, which give rise to the many sugar stereoisomers found in nature. Stereoisomerism in sugars is biologically significant because the enzymes that act on sugars are strictly stereospecific, typically preferring one stereoisomer to another by three or more orders of magnitude, as reflected in Km values or binding constants. The Two Families of Monosaccharides are Aldoses and Ketoses: Monosaccharises are colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. The backbones of common monosaccharides are unbranched carbon chains in which all the carbon atom
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