Carbohydrates I

This course is part of a series taught by Kevin Ahern at Oregon State University on General Biochemistry. For more information about online courses go to http://ecampus.oregonstate.edu/    • Плейлист  

1. Simple carbohydrates are monosaccharides, also called sugars. These include glucose, galactose, and mannose.

2. The suffix '-ose' is used to designate saccharides. The number-related prefixes 'tri', 'tetr', 'pent', 'hex', 'hept', and 'oct' are used to designate saccharides with 3,4,5,6,7, and 8 carbons, respectively.

3. Monosaccharides with an aldehyde group are called aldoses. Those is a ketone group are called ketoses.

4. Glyceraldehyde and dihydroxyacetone are the simplest saccharides we call carbohydrates.

5. Carbon can have as many as four different molecular groups attached to it. If this happens, the carbon is chiral (or asymmetric) and the groups can be arranged in two different ways. These arrangements are called stereoisomers.

6. The letters 'D' and 'L' are an older nomenclature system used to designate whether a particular stereoisomer rotated polarized light rightwards or leftwared, respectively. We use the convention today that the 'D' isomer corresponds to the stereoisomer in which the lowest asymmetric carbon from the top (closest to the bottom) is written on the right side of the molecule.

7. Most biological sugars are in the D configuration.

8. Stereoisomers, such as D-glyceraldehyde and L-glyceraldehyde, are mirror images of each other.

9. When stereoisomers are mirror images of each other, they are called enantiomers. When they are NOT mirror images of each other, but have the same numbers of carbons, they are called diastereomers. When they differ in configuration of only one carbon, they are called epimers.

10. Cyclization of monosaccharides leads typically to five member or six member rings. These are called furanoses (five member rings) and pyranoses (six member rings). Cyclization arises from fromation of hemiacetals in aldoses and hemiketals in ketoses.

11. Cyclization creates a new asymmetric carbon. This carbon is called the anomeric carbon and it can exist in the alpha (down position) or beta (up position) configurations. If the hydroxyl group on the anomeric carbon is unaltered, the ring and linear forms of the sugar can reversibly form. Thus, a sugar in the beta configuration can, in solution, 'flip' to the alpha form by going to the linear form and then reverting back to the ring structure in the alpha configuration. If the hydroxyl of the anomeric carbon is altered (by methylation, for example), the linear structure cannot form and 'flipping' cannot occur.

12. The names we give to linear and ringed sugar structures are as follows - Fischer = Linear structures and Haworth = ring structure. Students should be able to draw Haworth and Fischer projections of glucose, ribose, fructose, and galactose. I'll also add sucrose and lactose (disaccharides) in the next lecture.

13. Conformational isomers of sugars arise from "bending" of bonds. They do NOT arise from flipping configurations around asymmetric carbons. The easiest to understand conformational isomers are the boat and chair forms of glucose.

14. The aldehyde group of aldoses is easily oxidized. Oxidation of the aldehyde to an acid causes something else to be reduced. Thus, aldoses with a free anomeric carbon (one that has a hydroxide) can be called reducing sugars.

15. To be oxidized, an aldose MUST be in the linear form. Remember that ring structures can reversibly go back to the linear form IF the anomeric hydroxyl group is unaltered. Altering an anomeric hydroxyl group (such as putting a methyl group on it) creates a glycoside and LOCKS the sugar in the ring structure.

16. Glycosides are commonly created during formation of disaccharides and longer carbohydrates.