#31 Biochemistry Lecture (Lipid/Fat Metabolism I) from Kevin Ahern's BB 350

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Lecture Highlights
Respiration

1. Making ATP requires ADP. If there is no ADP, ATP Synthase CANNOT rotate and NO PROTONS CAN PASS THROUGH. In metabolic control, the inner mitochondrial membrane must remain impermeable to protons. When this occurs, the electron transport won’t run unless oxidative phosphorylation is occurring and oxidative phosphorylation won’t occur unless there is a proton gradient. When electron transport is not running, oxygen consumption decreases. When it is running, oxygen consumption increases.
2. At rest, your body’s stores of ATP are high and ADP are low. Under these conditions, oxidative phosphorylation is not occurring much, so the proton gradient remains high, thus stopping electron transport. When electron transport stops, NADH accumumlates, stopping the citric acid cycle.
3. When you exercise, your ATP levels fall and ADP levels rise. Thus, oxidative phosphorylation starts and the proton gradient falls. This starts electron transport, which causes NAD and FAD concentrations to rise, thus starting the citric acid cycle.
4. 2,4 dinitrophenol was used as a “miracle” diet drug about a century ago with disastrous consequences. It acted to poke a hole in the mitochondrial inner membrane, allowing protons to leak back into the matrix without passing through complex V. As a result, when 2,4 DNP is present, electrons move through the electron transport system, but NO ATP is made. Consequently, cells burn everything they can to try to make ATP, but they can’t. Oxygen consumption goes way up and the body temperature goes way up. Death was a frequent result of 2,4DNP use.
5. Uncoupling of mitochondria occurs when anything permeabilizes the inner wall of the mitochondrion to protons.
6. 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.
7. 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.
8. 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. 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 by CoA inside the mitochondrial matrix.
4. Fatty acid oxidation occurs inside the mitochondrion. The process is referred to as beta oxidation. One cycle of beta oxidation cleaves one 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.

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