Highlights Lecture #12 Spring 2017

Highlights

1. Coenzymes are non-amino acid molecules that help an enzyme to perform catalysis. Common ones include biotin (used in carboxylation reactions) and coenzyme A (used to hold fatty acids). Other commonly used ones include NAD+/NADH (used in oxidation/reduction reactions) and the flavin nucleotides (FAD/FADH2), which are also used in oxidation/reduction reactions. . Many coenzymes are also vitamins.

Highlights Lipids/Membranes

1. Lipids are cellular compounds that have at least some part of them that is very hydrophobic. The category includes fatty acids, triacylglycerols, sphingolipids, glycerophospholipids, steroids, and others.

2. Fatty acids are amphipathic compounds (part polar, part non-polar), consisting of a long non-polar carbon chain at one end and a polar carboxyl group at the other end.

3. Fatty acids can have all single bonds (saturated), one double bond (unsaturated) or more than one double bond (polyunsaturated). Double bonds of biological origin are almost all cis in nature. Trans double bonds arise from chemical modification for use in food (typically by partial hydrogenation of vegetable oil).

4. Fats (and oils) consist of glycerol esterified to three fatty acids. The resulting compound is very non-polar. Fats are the primary energy storage forms of animals. The different between a fat and an oil is simply that a fat is solid at room temperature, whereas an oil is liquid at room temperature. Oils will tend to have shorter fatty acids and/or less saturated fatty acids, since these features favor lowering of the melting point.

5. Fats/oils can have any combination of saturated and unsaturated fatty acids in them. The more unsaturated the fatty acids are in a fat/oil, the more likely it will be an oil – that is, liquid at room temperature.

6. Essential fatty acids are those that an organism cannot synthesize and must be in the diet.

7. Phosphoacylglycerols (glycerophospholipids or phosphoglycerides) are important components of cellular membranes (part of the lipid bilayer). They differ from fats in having a phosphate replace one of the fatty acids on the end carbon of glycerol. Addition of only a phosphate creates phosphatidic acid. In fact, phosphatidic acid is a branch point in the synthesis of fats and glycerophospholipids.

8. When a compound is esterified to the phosphate it creates a phosphatidyl compound. Example compounds attached to the phosphate on phosphatidic acid are ethanolamine (creating phosphatidylethanolamine), choline (creating phosphatidylcholine), serine (creating phosphatidylserine), and others.

9. Sphingolipids are derived from sphingosine. Cerebrosides and gangliosides are sphingolipids that are common in the membranes of brain tissue. Sphingomyelin is a sphingolipid common in the membranes of nerve tissue. Cerebrosides are sphingolipids that have only a simple sugar (usually glucose) attached to them. Gangliosides are sphingolipids that have more complex sugars attached to them.

10. Steroids are compounds with ring structures that are made from cholesterol. Steroids include sex hormones (androgens, estrogens) and in animals are derived from cholesterol. Cholesterol is an important constituent of animal membranes, giving them integrity.

11. Glycerophospholipids and sphingolipids form lipid bilayers, not micelles.

12. Cellular membranes are sensitive to temperature. The higher the temperature, the more fluid the membrane. The fluidity of a membrane is related to the fatty acids in it. Membranes with shorter and more unsaturated fatty acids are fluid at lower temperatures than membranes with longer and more saturated fatty acids.

13. Animals live in widely varying environmental conditions. Fish, for example live in an environment that is cooler than most land animals. Their membranes have more polyunsaturated fatty acids than our cells do. Cells in environments with high temperatures will have membranes with fatty acids that are longer and more saturated than ours are.

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