The Citric Acid Cycle is a series of enzyme-catalysed reactions that take place in the mitochondrial matrix of all aerobic organisms. It involves the oxidation of the acetyl group of acetyl CoA to two molecules of carbon dioxide. Each cycle produces one molecule of ATP by substrate-level phosphorylation, and reduces three molecules of NAD and one molecule of FAD for use in Oxidative Phosphorylation. The cycle is preceded by Glycolysis, which also occurs in anaerobic respiration, and the pyruvate dehydrogenase complex, which occur in the cytoplasm and the mitochondrial matrix respectively.
In aerobic respiration, glycolysis breaks down one molecule of glucose and two molecules of pyruvate, and gives a net product of 2 ATP molecules. The molecules of pyruvate are then actively transported into the mitochondia where they enter the pyruvate dehydrogenase complex in the matrix. Once the pyruvate molecules are in the matrix, they are decarboxylated and dehydrogenated to produce one reduced NAD, one molecule of carbon dioxide, and one molecule of acetate for each molecule of pyruvate. The acetate is carried into the citric acid cycle when it combines with coenzyme A to form acetyl coenzyme A.
The overall citric acid cycle is shown in the diagram above; the acetate is offloaded from coenzyme A, and condenses with oxaloacetate to form the six carbon molecule of citrate. The citrate is then decarboxylated and dehydrogenated twice to produce two molecules of carbon dioxide, and two molecules of reduced NAD. The remaining four carbon compound is further processed to regenerate oxaloacetate; this also converts one molecule of ADP into ATP. This conversion is referred to as substrate-level phosphorylation.
One turn of the cycle will produce three molecules of reduced NAD, one molecule of reduced FAD, two molecules of carbon dioxide, and one molecule of ATP. The products of the reaction show that the citric acid cycle does not generate a large amount of ATP. Its primary function is to remove electrons from acetyl coenzyme A and use them to reduce NAD and FAD. These molecules function as electron carriers, and are reoxidised by O2 molecules in oxidative phosphorylation to release their electrons into the electron transport chain, where energy is generated for the phosphorylation of ADP into ATP.
The pyruvate dehydrogenase complex is referred to as being the link reaction between glycolysis and cellular respiration because it produces acetyl coenzyme A, which is the fuel for the citric acid cycle. The cycle is then initiated by the condensation of oxaloacetate with the Acetyl coenzyme A. Acetly CoA can also be produced by the degredation of a small number of amino acids which are defined as being ketogenic. This term can be used to describe leucine, lysine, isoleucine, phenylalanine, tryptophan, and tyrosine.
Amino acid degredation means the carbon skeletons can be used to produce major metabolic intermediates, which can then either be oxidised in the Citric Acid cycle, or converted into glucose. In addition to Acetyl CoA, the carbon skeletons of amino acids can be used to produce pyruvate, a-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate; all of which are able to enter the citric acid cycle at different stages. The various conversion pathways each amino acid can take vary in complexity, the products of which serve as the entry point for the enzymes to enter the citric acid cycle.
The molecular structure of the amino acid will determine the entry point into metabolism it is able to take. The branched chain amino acids such as leucine and isoleucine, for example, will yield acetyl CoA, which may enter metabolsim when it condenses with oxaloacetate. The aromatic amino acids such as phenylalanine and tyrosine are more likely to yield the common intermediates fumarate and pyruvate. When there is an excess of Acetyl COA or other metabolic intermediates due to amino acid degradation, the activity of the pyruvate dehydrogenase complex decreases to prevent the unnecessary build up of these fuel molecules.
While the citric acid cycle produces a very small proportion of ATP compared to the rest of aerobic respiration, it serves as an entry point into metabolism for all 20 naturally occuring amino acids. It is because of this that the cycle could arguably be described as being “the hub of the metabolic wheel”. Entry into the cycle by fuel molecules, and the rate at which reactions progress is monitored at several stages. This is because the cycle is a vital source of building blocks for numerous larger molecules, and provides an integral role in anabolism as a whole in large organisms.
The activity of the pyruvate dehydrogenase complex is controlled because the formation of Acetyl CoA from pyruvate is an irreversible reaction, so it cannot be converted back into glucose should there be an overabundance of it. Once Acetyl CoA is formed, it can only take one of two paths: oxidation to CO2 during the progression of the Citric Acid Cycle, or being converted into a component of a lipid molecule. Because of this, high concentrations of Acetyl CoA and NADH will inhibit the functioning of the enzyme required for the pyruvate dehydrogenase complex to occur.
This means that pyruvate from glycolysis will not be broken down, but can be recycled to form glucose again. The pyruvate will once again be converted to Acetyl CoA and NADH when there is a abundance of pyruvate and ADP, and a low concentration of Acetyl CoA and NADH. The rate at which the citric acid cycle itself progresses is controlled at several points. The primary ways the cycle is controlled is by the concentration of the two enzymes isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase.
The former is controlled by the presence of molecules of ADP and ATP. When ADP is present, the affinity that isocitrate dehydrogenase has for its substrate is enhanced. When ATP is present, its affinity is reduced, meaning the overall cycle should progress at a greater rate when there is not enough ATP in a cell. a-ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH, which are the products of the reaction it catalyses. The enzyme is also inhibited by the presence of ATP, as a high energy charge will reduce the function of the enzyme.
The use of several control points in the citric acid cycle highlights its importance as having a central role in metabolism as a whole. It serves as a precursor to oxidative phosphorylation, which is where the majority of ATP in respiring animals is produced. It is for this reason that the rapid replenishment of the reactants in the cycle is a vital process if any are removed for biosynthesis. Oxaloacetate, for example, is often converted into amino acids for protein synthesis, meaning the energy requirements of the cell will increase.
This means the rate of the citric acid cycle will reduce until a minimum quantity of oxaloacetate is present, as acetyl COA produced in the pyruvate dehydrogenase complex cannot enter the cycle unless it condenses with oxaloacetate. To increase the amount of oxaloacetate present, pyruvate is carboxylated in the presence of the enzyme pyruvate carboxylase. Although oxaloacetate is recycled on subsequent turns of the citric acid cycle, it must be constantly replenished when it is drawn off as the cycle could not occur without it.
Althought the citric acid cycle only produces a small amount of the ATP when compared with the rest of the pathways in aerobic respiration, its other roles in metabolism mean describing it as ‘the hub of the metabolic wheel is accurate. It produces both NADH and FADH2, which are passed onto the electron transport chain in oxidative phosphorylation, where the majority of ATP in aerobic organisms is produced. It is also a sorce of numerous biosynthetic precursors such as nucleotide bases, proteins, and haem groups.
This means the only waste molecule produced by the cycle is CO2, which can be removed with relative ease. The cycle may also act as the entry point into melabolism for any amino acids which may have otherwise gone to waste. These factors mean the citric acid cycle operates at a very high level of efficiency, and must be controlled to a high degree. It acts as the final common pathway for the oxidation of many fuel molecules, and plays a huge role in metabolism as a whole as a result of this.
