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den69056_app.qxd 1/8/03 7:17 AM Page 795 Appendix G Energy Yields from Aerobic Respiration: Some Alternatives In Chapter 21 we described catabolic processes as occurring in three stages. In stage I, dietary protein, carbohydrate, and lipid are hydrolyzed into small subunits that can cross the membranes of the cells of the intestine and are transported to the cells of the body. In stage II, these monomers enter cells of the body and are converted into a form that can be completely oxidized. For carbohydrates, glucose is used as a substrate for the glycolysis pathway, the first stage of carbohydrate metabolism. In this pathway, glucose is converted into two pyruvate molecules. In the process, two ATP, net, are produced by substrate level phosphorylation and two NADH are formed by oxidation of glyceraldehyde. Under aerobic conditions, that is, when oxygen is present, the pyruvate is transported into organelles called mitochondria. Here the pyruvate dehydrogenase complex is involved in the reaction pyruvate coenzyme A NAD acetyl CoA CO2 NADH In stage III, the two-carbon acetyl group is completely oxidized in the reactions of the citric acid cycle. When glycolysis occurs under anaerobic conditions, it is followed by fermentation reactions, such as the lactate and alcohol fermentations. These reactions reduce pyruvate—or a molecule produced from pyruvate—and simultaneously oxidize the NADH produced in glycolysis. As a result, the net energy yield from glycolysis under anaerobic conditions is only two ATP. No further ATP energy is harvested from the oxidation of the NADH. It is simply reoxidized in the fermentation reactions. Under aerobic conditions, the energy yield of glycolysis is much greater because the high-energy electrons carried by NADH are shuttled into mitochondria and used in oxidative phosphorylation to produce more ATP. There are two shuttle systems: the glycerol-3-phosphate shuttle found in skeletal muscle and nerve cells and the oxaloacetate-malate shuttle found in heart and liver cells. Because skeletal muscle produces the majority of the ATP for the body, it is the glycerol-3-phosphate shuttle that is used most commonly when discussing metabolic energy yields. In Example 22.1, calculation of the ATP harvest of glycolysis is based on this shuttle. Let’s consider the reactions involved in the glycerol-3-phosphate shuttle. In this shuttle, the NADH produced in the cytoplasm is oxidized in a reaction that reduces dihydroxyacetone phosphate to glycerol-3-phosphate. CH2OH A NADH CPO A CH2OPO32 CH2OH A NAD CHOH A CH2OPO32 Dihydroxyacetone phosphate Glycerol-3-phosphate The glycerol-3-phosphate then passes through the outer mitochondrial membrane and is oxidized to dihydroxyacetone phosphate by the enzyme glycerol-3phosphate dehydrogenase. This enzyme, which is located in the inner mitochondrial 795 den69056_app.qxd 796 1/8/03 7:17 AM Page 796 Appendix G Energy Yields from Aerobic Respiration: Some Alternatives membrane, simultaneously reduces FAD to FADH2. The electrons from each FADH2 are then used during oxidative phosphorylation to produce two ATP. The dihydroxyacetone phosphate returns to the cytoplasm to continue the shuttle process. The main drawback of the glycerol-3-phosphate shuttle is that only two ATP are produced for each cytoplasmic NADH. The reason is that the electrons are shuttled to FADH2, which yields only two ATP by oxidative phosphorylation. (The energy yield of the oxidation of mitochondrial NADH is three ATP.) Thus the total energy yield from glycolysis under aerobic conditions in muscle and nerve cells is two ATP, produced by substrate level phosphorylation, plus four ATP (two ATP per NADH), produced by oxidative phosphorylation. This provides an energy yield of six ATP per glucose. The oxaloacetate-malate shuttle system is more efficient. In this system, cytoplasmic NADH reduces oxaloacetate to malate. COO A CPO NADH A CH2 A COO COO A CHOH NAD A CH2 A COO Oxaloacetate Malate Malate is then transported into the mitochondrion where it is reoxidized to oxaloacetate. Mitochondrial NAD is reduced in the process. These electrons are then used in oxidative phosphorylation to produce three ATP per NADH. Thus the energy yield of glycolysis in heart and liver cells is two ATP, produced by substrate level phosphorylation, plus six ATP (three ATP per NADH), produced by oxidative phosphorylation. This gives an energy yield of eight ATP per glucose. The oxaloacetate cannot cross the mitochondrial membrane to return to the cytoplasm to continue the cycle. It is able to return only after a series of reactions involving the amino acids glutamate and aspartate. In summary, the energy yield of glycolysis depends on the conditions present (aerobic versus anaerobic) and the type of cell. The following table summarizes the energy gains from glycolysis under various conditions. Condition Anaerobic Aerobic, muscle (glycerol-3-phosphate shuttle) Aerobic, heart (oxaloacetate-malate shuttle) ATP by Substrate Level Phosphorylation ATP by Oxidative Phosphorylation 2 0 2 4 2 6