Biochemistry 2280 Topic 9.docx

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Biochemistry 2280A
Christopher Brandl

Topic 9 Carbohydrates Readings: p.50-54, 68-69, 380-381, 384 By the end of this topic, you should be able to: • Apply appropriate terminology to describe carbohydrates, without memorizing the structres of individual sugars • Convert a cyclized monosaccharide from a Fischer projection to a Haworth projection, and vice versa • Describe how monosaccharides are joined to make polysaccharides • Compare the structures of polysaccharides used for energy storage • Describe the difference between dietary fibre and carbohydrates used for energy Carbohydrates, or sugars, are one of the four main classes of compounds in living cells (Fig 2-15, p.52). Monosaccharides are the simplest sugars and have the molecular formula (CH2On , where n = 3 to 7.Amonosaccharide with three carbon atoms is called a triose; with four, a tetrose; with five, a pentose; with six, a hexose; with seven, a heptose. Monosaccharides are often represented using Fischer projections, which depict the stereochemistry at each carbon atom (p.68, top panel, and structures below).At each carbon atom, the horizontal bonds are meant to come out of the page, toward the viewer, while the vertical bonds go into the page, away from the viewer. Each monosaccharide has a carbonyl group (an oxygen atom double-bonded to carbon). If this group is at the end of the molecule (i.e., if it is an aldehyde group), the sugar is called an aldose. If the group is not at the end of the molecule (i.e., if it is a ketone group), the sugar is called a ketose. Each carbon in a monosaccharide is numbered, starting from the end closest to the carbonyl group. 1 Because the H C O H C O H C O CH OH position of the 2 2 carbonyl group and H C OHH H C OH HO C H C O the orientation of HO C H HOO C H HO C H HO C H the hydroxyl group at interior carbon 4 H C OHH HOO C H H C OH H C OH atoms may vary, H C OHH H C OH H C OH H C OH many isomers are possible for most 6 CH2HH CH 2H CH2OH CH2OH monosaccharides. By convention, Glucose Galactose Mannose Fructose monosaccharides are called D or L based on the orientation of the hydroxyl group on the chiral carbon furthest from the carbonyl group. Biological sugars are usually D isomers, with the hydroxyl group on the right side in a Fischer projection. In cells, pentoses and hexoses are 1 1 predominantly found in ring or cyclic CHOO CHO structures, which are commonly depicted 2 2 using Haworth projections (p.68, bottom left H C OHH H C H panel and structures at right). To form the ring, 3 3 a hydroxyl group reacts with the carbon of the H C OHH H C OH 4 4 carbonyl group; this atom is called the H C OHH H C OH anomeric carbon. Monosaccharide rings are 5 5 said to be in either the α or β configuration, CH 2H CH2OH depending on the stereochemistry at the rbose deoxyrbos
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