#24 BB 350 Energy II / Carbohydrates – Kevin Ahern’s Biochemistry Online

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Lecture Highlights
Energy II

1. Metabolic pathways (metabolism = chemical reactions of cells) are usually either catabolic (large molecules broken down to smaller ones) or anabolic (smaller molecules built up into larger ones).

2. Catabolic pathways usually involve oxidation and release energy. Anabolic pathways usually involve reduction and require energy.

3. NAD+ gains electrons from an oxidation reaction to become NADH. Electron carriers are essential for biological oxidations. FAD gains electrons from an oxidation reaction to become FADH2.

4. For every oxidation (loss of electrons) there is a reduction (gain of electrons). NAD+, NADP+, and FAD are common acceptors of electrons. Biological molecules are common sources of electrons (as well as acceptors of electrons, depending on the reaction).

5. Electron carriers must be recycled in the cell.

Highlights Carbohydrates

1. Carbohydrates are sugar-related compounds (also called saccharides). They are polyhydroxyaldehydes and polyhydroxyketones with the general formula CnH2nOn (though there are exceptions). The suffix ‘ose’ is put at the end of a molecular name to indicate it is a carbohydrate. An aldehyde sugar, such as glucose, is thus an aldose. A ketone sugar, such as fructose, is thus a ketose.

2. The prefixes ‘tri’, ‘tetr’, ‘pent’, ‘hex’, ‘hept’, and ‘oct’ are prefixes for monosaccharides with 3,4,5,6,7, or 8 carbons, respectively. A monosaccharide has only one sugar subunit. Glucose and fructose are monosaccharides. Sucrose (subunits of glucose AND fructose) is a disaccharide. Glycogen, which is a polymer of thousands of subunits of glucose, is a polysaccharide.

3. Combinations of the names are possible – fructose is a ketohexose, glucose is an aldohexose, ribose is an aldopentose. Glyceraldehyde is an aldotriose, etc.

4. Asymmetric carbon centers give rise to stereoismers. D-glyceraldehyde and L-glyceraldehyde are mirror images of each other. Stereoisomers that are mirror images of each other are called enantiomers. Stereoisomers that are NOT mirror images of each other are called diastereomers.

5. A Fischer projection is a stick figure representation of a sugar. By convention, the D isomer of a sugar is indicated by a hydroxyl on the right side on the next to the bottom carbon in a Fischer projection. The L form of a sugar (like L-glucose) differs from the D form of a sugar of the same name (like D-glucose) in that the two are mirror images of each other.

6. The geometry of covalent bonds of carbon allows 5 and 6 membered rings to readily form in aldoses and ketoses. Ring structures of sugars are commonly drawn in the Haworth format.

7. Formation of a ring by a sugar creates a new asymmetric carbon called an anomeric carbon. Note that the anomeric carbon will ALWAYS be the carbon that had the aldehyde of ketone group. Note also that when the hydroxyl group of the anomeric carbon is drawn in the up position, it is called the ‘beta’ form, whereas when the hydroxyl is in the down position, it is called ‘alpha’.

8. You should be able to draw glucose, fructose, galactose, and ribose in Haworth and Fischer projections.

9. Reduction of the aldehye or ketone structure of a sugar can lead to compounds like sorbitol that are sometimes called sugar alcohols.

Post time: Jun-29-2017