Highlights Lecture #31 Spring 2017

1. Shuttles are necessary to carry electrons from NADH into the mitochondrion. They include the glycerol phosphate shuttle of insect muscle (reduces DHAP to glycerol phosphate, which moves across inner mitochondrial membrane) and the malate/aspartate shuttle of our cells.

2. The glycerol phosphate shuttle takes electrons from NADH in the cytoplasm, but puts electrons onto FAD in the mitochondrion, resulting in fewer protons being pumped.

3. The malate/aspartate shuttle takes electrons from NADH (onto oxaloacetate to make malate, which is transported) and then reverses the reaction in the mitochondrion, resulting in no reduction of protons pumped.

Lipid Metabolism

1. Fats store considerably more energy per gram than carbohydrates, due to the fact that fats are not water soluble and do not absorb water. They are also more “reduced” than carbohydrates and can be further oxidized.

2. Fatty acids are released from fats or glycerosophospholipids by lipases or phospholipases, respectively. Fats are broken down as a result of a cascade of reactions starting with binding of a hormone to a receptor on the surface of the adipocyte (fat cell). This starts a signaling process, which puts a phosphate onto triacylglycerol lipase, activating it, and causing the fat to be cleaved to fatty acids and glycerol.

3. Fatty acids are “activated” by being joined to Coenzyme A (CoA) in the cytoplasm. (Note that fatty acids are carried in the bloodstream by serum albumin.) To transport fatty acids across the cell’s mitochondrial membrane, however, the CoA is replaced by carnitine. After the acyl-carnitine makes it across the mitochondrial membrane, the carnitine is quickly replaced once again by CoA inside the mitochondrial matrix.

4. Fatty acid oxidation occurs inside the mitochondrion. The process is referred to as beta oxidation because the beta carbon of the fatty acid is the one that gets oxidized. One cycle of beta oxidation cleaves one two carbon unit (acetyl-CoA) from the fatty acid and leaves the fatty acid shorter by two carbons. Thiolase is an enzyme that cleaves the acetyl group from the fatty acid chain in the last step of beta oxidation. The first enzyme of fatty acid oxidation, Acyl-CoA dehydrogenase, has been linked to sudden infant death syndrome.

5. In each cycle of beta oxidation, one NADH and one FADH2 is produced. The acetyl-CoA that is released can be oxidized further in the citric acid cycle.

6. Oxidation of unsaturated fatty acids requires two additional enzymes beyond those of beta oxidation. The two enzymes are enoyl-CoA isomerase and another enzyme you are not responsible for. The first enzyme catalyzes conversion of cis bonds between carbons 3 and 4 to trans bonds between carbons 2 and 3 so it can be oxidized in beta oxidation. The second enzyme catalyzes conversion of two double bonds into one cis double bond between carbons 3 and 4, which is, in turn, converted to a trans between carbons 2 and 3 for oxidation in beta oxidation.

7. Ketone bodies are the body’s means to provide energy to the brain when glucose concentrations are VERY low. This can occur during starvation or with some types of diabetes. Ketone bodies are made by putting together acetyl-CoAs to make four carbon intermediates by reversing the reaction catalyzed by thiolase. The four carbon molecules are converted to a six carbon molecule (called HMG-CoA) and it is broken back down to a four carbon molecule and acetyl-CoA. Ketone bodies are carried in the blood to the brain. One of these intermediates is chemically unstable and breaks down to acetone, which is exhaled from the lungs. Thus, the smell of acetone on a person’s breath may be a sign of a serious problem.

8. Unsaturated fatty acids produced biologically are almost always in the cis configuration. By contrast, partial hydrogenation of vegetable oils (to raise the melting point) causes trans bond to be produced and thus trans fats are made. Trans fats are linked to atherosclerosis. The reason is not completely clear, but the random positioning of the trans bonds may be a factor.

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