Citric Acid Cycle - Biochemistry

The citric acid cycle is amphibolic—that is, both anabolic and catabolic. Anabolism occurs when the citric acid cycle generates reduced factors, such as NADH and FADH2. Catabolism occurs when the citric acid cycle oxidizes the two carbon atoms of acetyl CoA to carbon dioxide (CO2).

The enzyme alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate (5 carbons) to succinyl-CoA (4 carbons). During this step, one carbon is lost as carbon dioxide and one molecule of is produced. This step is the second, and last step in the Krebs cycle in which carbon dioxide is formed. Recall that the starting material, oxaloacetate, is also 4 carbons long.

During the step in which one of the carbons of isocitrate is lost as carbon dioxide, one molecule is also produced. This reaction is catalyzed by isocitrate dehydrogenase. This leaves a five-carbon molecule known as alpha-ketoglutarate. In the next step, alpha ketoglutarate dehydrogenase acts upon alpha-ketoglutarate and a carbon is lost as carbon dioxide and another molecule of is produced. Later in the cycle, succinate dehydrogenase catalyzes the conversion of succinate to fumarate. This reaction produces one molecule of . The enzyme fumarase then converts fumarate to malate. The final step in the citric acid cycle is the regeneration of oxaloacetate from malate. Malate dehydrogenase catalyzes this reaction, which produces the third molecule of . Note that this is for one turn of the citric acid cycle i.e., for one molecule of acetyl-CoA. Each molecule of glucose yields two molecules of acetyl-CoA via glycolysis and pyruvate dehydrogenase complex.

Glycolysis involves the conversion of glucose into pyruvate. Recall that glucose is a six-carbon molecule, while pyruvate is a three-carbon molecule. Thus for each molecule of glucose that undergoes glycolysis, two molecules of pyruvate are yielded. Next, pyruvate is converted into acetyl-CoA via the pyruvate dehydrogenase complex. Finally, acetyl-CoA enters the citric acid cycle, combining with oxaloacetate as the first step.

Enolase is the enzyme responsible for catalyzing the conversion of 2-phosphoglycerate into phosphoenolpyruvate. This reaction takes place during glycolysis. All other enzymes are involved in the sequence of reactions known as the citric acid cycle.

The citric acid cycle starts with the combination of a four-carbon molecule (oxaloacetate) and a two-carbon molecule (acetyl-CoA) to form a six-carbon molecule (citrate). Since the citric acid cycle is indeed a cycle, oxaloacetate must be regenerated. Thus, two molecules of carbon dioxide are produced throughout the citric acid cycle. The first molecule of carbon dioxide is produced during the conversion of isocitrate into alpha-ketoglutarate. This reaction is catalyzed by isocitrate dehydrogenase. Alpha-ketoglutarate, a five-carbon molecule, is then converted into the four-carbon molecule succinyl-CoA via alpha-ketoglutarate dehydrogenase, yielding another molecule of carbon dioxide. The remaining steps of the citric acid cycle do not involve any more production of carbon dioxide since both succinyl-CoA and oxaloacetate are both four-carbon molecules.

This is the correct sequence of intermediates in the citric acid cycle. Note that both citrate and cis-aconitase are substrates for the same enzyme, aconitase. The net yield of one turn of the citric acid cycle is: , and . The electron carriers then participate in the electron transport system along the inner mitochondrial membrane.

Some catabolic pathways do indeed make citric acid cycle intermediates; for example, plants and bacteria use phospoenolpyruvate carboxylase to create oxaloacetate from phosphoenolypyruvate. Anaplerotic reactions refill the citric acid cycle with intermediates, rather than remove them. Some archaea have a complete citric acid cycle; it is the bacteria that mostly do not have a complete cycle. In eukaryotes, citrate cleavage does indeed take place in the cytosol; that citrate is transported to the cytosol from mitochondria, and the acetyl-CoA can be used for fatty acid synthesis.

The citric acid cycle is a series of reactions that occur within the matrix of the mitochondria. With every turn of the cycle, one acetyl-CoA molecule enters, and a variety of molecules leave. These leaving molecules include , , , and ATP. The acetyl-CoA that enters the cycle is derived from other cellular pathways, such as beta-oxidation and glycolysis. In this way, the citric acid cycle serves as a conduit by which metabolites from other pathways can be broken down to ultimately provide energy for the cell.

Since the citric acid cycle occurs in mitochondria, only eukaryotes are capable of performing this process. Also, the citric acid cycle is not responsible for the production of urea from ammonia. Rather, it is the urea cycle that performs this role. It is worth noting, however, that the citric acid cycle and the urea cycle are energetically linked by the aspartate-argininosuccinate shunt. This is because these two cycle share a few common intermediates, and the citric acid cycle can help to offset the demanding energy requirements of the urea cycle.

Flavin mononucleotide (FMN) is not produced by the citric acid cycle. This flavin coenzyme is a reactant, but not a product, since FMN will get reduced to FMNH2.

The rest of the answer choices are products of the citric acid cycle (otherwise known as the Krebs cycle).

During the conversion of succinate into fumarate by succinate dehydrogenase, a single molecule of is reduced to as it accepts the hydrogens from succinate. then feeds its electrons into the electron transport chain in the inner mitochondrial membrane.

Pyruvate is the end product of glycolysis. After glycolysis, the three-carbon molecule pyruvate is converted into the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA also produces one molecule of . Acetyl-CoA has one less carbon than pyruvate. The third carbon from pyruvate is lost as carbon dioxide () during the conversion of pyruvate to acetyl-CoA. Recall that since glucose is a six-carbon molecule, two molecules of pyruvate (three carbons each) are formed via glycolysis.

Pyruvate is produced in the last step of glycolysis, then, it is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one . Acetyl-CoA has one less carbon than pyruvate. The third carbon of pyruvate is lost as carbon dioxide () during the conversion of pyruvate to acetyl-CoA. The citric acid cycle begins when the four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) via an aldol condensation, yielding the six-carbon molecule citrate.

Succinyl-CoA synthetase performs substrate level phosphorylation at this step in the Krebs cycle, such that .

The conversion of pyruvate to acetyl-CoA produces onemolecule of NADH, but remember that each glucose yields two pyruvates, so the total NADH from this first step is two. Within the citric acid cycle, there are three steps in which NADH is a byproduct, but again we must remember that each step occurs to two molecules, therefore three NADH byproducts for two molecules yields sixNADH in the cycle proper. Therefore, the total NADH produced in one turn of the citric acid cycle is eight NADH.

Citric acid cycle inputs are derived from glycolysis outputs. Glycolysis produces pyruvate molecules, , and ATP. The pyruvate molecules undergo reactions that convert the three carbon pyruvate to a two carbon acetyl CoA and an one carbon carbon dioxide. The acetyl-CoA molecules are then used as the initial inputs for the citric acid cycle, as they are combined with oxaloacetate. Note that pyruvate itself does not enter the citric acid cycle. and are electron carriers that are produced in the citric acid cycle and are used in electron transport chain to generate ATP.

A glucose (six carbons) molecule enters glycolysis and produces two three carbon molecules (pyruvate). Each pyruvate is broken down into a two carbon acetyl-CoA molecule that enters the citric acid cycle. Each acetyl-CoA molecule produces three and one in the citric acid cycle. This means that two acetyl-CoA (derived from one glucose molecule) produces six and two molecules in the citric acid cycle.

Citric acid cycle involves a series of reactions that are involved in the production of the necessary molecules for electron transport chain. The cycle starts with a two carbon molecule (acetyl-CoA) binding to a four carbon molecule (oxaloacetate). This creates a six carbon molecule (citrate) that can go through a series of reactions. Most of these reactions involve a six carbon molecule. As mentioned, acetyl-CoA has two carbons; therefore, most of the intermediates in this cycle have six carbons, or four more carbons than acetyl-CoA. One turn of citric acid cycle produces , , (carbon dioxide) and one GTP molecule(s).

The first step in the citric acid cycle is for acetyl-CoA to react with oxaloacetate. This forms citrate, which then continues through the cycle, ultimately reforming the oxaloacetate molecule to redo the cycle.

The Krebs cycle produces the most high energy electron carriers of any process involved in cellular respiration. Per glucose molecule, the Krebs cycle produces and .