The doctor suspects mitochondrial disease which can occur at multiple levels in different mitochondrial processes. To help the doctor determine where the defect might have occurred:
1. Explain what would happen if the interconversions of the Cori cycle occurred and remained within a single cell.
The cori cycle refers to a pathway of carbohydrate metabolism which links the processes of anaerobic glycolysis (glucose breakdown) in skeletal muscle tissue and glucose synthesis that is gluconeogenesis in liver cells. Anaerobic glycolysis is the major source of energy in form of ATP for skeletal muscle cells when they are under intense activity; it generates 2 moles of ATP for every molecule of glucose. This is due to the fact that these cells have very few mitochondria hence there is rapid depletion of the small stores of oxygen and ATP stored in them during such activities. As a result, the muscle cells are forced to resort to anaerobic respiration. The product of anaerobic glycolysis in the active muscle cells is lactate which is secreted into blood and transported to the liver (Newsholme & Leech, 2010, p.103).
Upon reaching the liver, the lactate is synthesized back to glucose via the process of gluconegenesis following which it is released back to the bloodstream and transported to the muscle to serve either as fuel for more contractions or to replenish glycogen stores and thus effectively completing the cycle. Gluconeogenesis is highly energy intensive; it consumes 6ATPs for every single molecule of glucose that is formed. Aerobic metabolism in the liver is the primary source of ATP required for gluconeogenesis. In essence therefore, restriction of the interconversions of the Cori cycle to a single cell would result in total wastage of effort in which glucose would be consumed and then resythesized at the expense of two things the first one being ATP whereby a net loss of 4 ATP molecules per cycle would occur and secondly, GTP hydrolysis (Nesholme & leach, 2010, p.103).
2. Construct a dynamic model to show the doctor why the citric acid cycle is central to aerobic metabolism.
Diagram 1: Summary of metabolism
As illustrated in the above diagram, the citric acid cycle occupies a central position in aerobic metabolism serving as the link between the various types of metabolism to include that of fats, proteins and carbohydrates with the electron transport/transfer chain. The importance of the electron transport chain cannot be undermined because most of the ATP required for energy reactions in the cell is produced in that chain.
3. Explain where in the citric acid cycle a defect can occur that prevents an increased conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) in response to an increased energy need and how the products of the citric acid cycle are converted into ATP.
The citric acid cycle comprises of eight reactions. In the first reaction, acetyl CoA formed from oxidative decarboxylation of pyruvate, from either the amino acids, carbohydrates or fatty acids metabolic chains is condensed with oxaloacetate formed in the eighth and final step of the citric acid cycle to form a 6-carbon molecule of citrate. This reaction is catalyzed by the enzyme citrate synthetase. In the second reaction, citrate is isomerized by the enzyme aconitase to form isocitrate. The third reaction is characterized by dehydrogenation of isocitrate to form α-ketoglutarate with CO2 and NADH+H2 being produced as by-products. The reaction is catalyzed by the enzyme isocitrate dehydrogenase. In the fourth reaction, α-ketoglutatarate in the presence of NAD++CoA-SH undergoes oxidative decarboxylation to form succcinyl-CoA. The latter reaction is catalyzed by the enzyme α-ketoglutarate dehydrogenase. CO2 and NADH+H+ are formed as by-products in this reaction. During the fifth reaction, the enzyme succinate thiokinase catalyzes GTP formation from GDP +Pi at the expense of the thioester bond. In the sixth reaction, succinate undergoes dehydrogenation to form fumarate. This reaction is catalyzed by succinate dehydrogenase. The hydrogen released in this specific reaction is accepted by FAD to form FADH2. In the seventh reaction, fumarate is hydrated in the presence of the enzyme fumarase resulting in the formation of malate. During the last reaction which is catalyzed by the enzyme malate dehydrogenase, malate is dehydrogenated to form oxaloacetate which combines with acetyl-CoA in the first reaction and the cycle begins all over again. The hydrogen released in the eighth reaction is accepted by NAD+ (Seager & Slabaugh, 2010, p.367).
In summary therefore, the citric acid cycle oxidizes acetyl-CoA to two CO2 molecules with the concomitant reduction of 3 NAD+ molecules to form NADH and 1 FAD molecule to form FADH2 and the formation of 1 ATP molecule via GTP. The products of the citric acid cycle that is, FADH2 and NADH are then re-oxidized via a number of electron carriers in the electron transfer chain in a sequential manner. Molecular oxygen is then hydrogenated by the protons and electrons released by the two to form water. The re-oxidation process is coupled with the phosphorylation of ADP to form molecular energy in form of ATP, a process termed oxidative phosphorylation (Seager & Slabaugh, 2010, p.367).
In essence therefore, any defects in the citric acid cycle that results in the decline in production of any intermediate and subsequently the other intermediates as well as the 3 molecules of NADH and 1 molecule of FADH2 may cause a decline in the formation of ATP from ADP during oxidative phosphorlyation. These defects mainly involve the enzymes at the three regulatory points in the citric acid cycle that is steps one, three and four. The three enzymes are citrate synthetase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase respectively. Abnormalities in any of these enzymes would cause them to be impervious to ADP stimulation resulting in a decrease in ATP production when energy needs are high. (Seager & Slabaugh, 2010, p.367).
4. Explain the role of coenzyme Q10 in ATP synthase.
Coenzyme Q10 is basically a small benzoquinone electron carrier which is in addition lipid-soluble and found within the inner mitochondrial membrane. Its major function in the cell is to carry protons and electrons by a redox cycle. Being lipid soluble implies that it is hydrophobic in nature and thus can diffuse freely within the membrane. It is reduced to its ubiquinol form (QH2) once it accepts two 2 electrons and 2 protons from either FADH2 or FMNH2 (flavin mononucletide).The latter is formed from the transfer of hydrogen atoms from NADH2 to FMN. The two electrons are then transferred to a series of cytochrommes in the electron transfer chain. Subsequent release of the protons and electrons oxidizes it back to its ubiquinone form. The two protons on the other hand are released into the intermembrane space thus creating a proton gradient across the inner membrane of the mitochondria. The reaction finally culminates with the formation of water at the end of the cytochrome series from the combination of protons, electrons and oxygen. This formation of water molecules is driven by the energy which is released by the act of protons passing down the proton gradient that was created across the inner membrane of the mitochondrion as previously mentioned. In essence therefore, the role of coenzyme Q10 during the generation of ATP can be termed as a pivotal role (Webb, 2006, p.157).
Newsholme, E. & Leech, A. (2010). Functional biochemistry in health and disease. Oxford, UK: John, Wiley & Sons, Ltd.
Seager, S.L. & Slabaugh, M.R. (2010). Organic and biochemistry for today. Belmont, CA: BROOKS/COLE Cengage Learning.
Webb, G.P. (2006). Dietary supplements and functional foods. Oxford, UK: Blackwell Publishing Ltd.