kreb cycle steps and regulation

kreb cycle or citric acid cycle;

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of the mitochondria. Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2. This is considered an aerobic pathway because the NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.

Steps of the TCA Cycle

The TCA cycle takes place over eight different steps:

• Step 1: First the acetyl CoA (a two carbon molecule) joins with oxaloacetate (4 carbon molecule) to form citrate (6 carbon molecule).
• Step 2: The citrate is then converted to isocitrate (isomer of citrate)
• Step 3: Isocitrate is then oxidised to alpha-ketoglutarate (a five carbon molecule) which results in the release of carbon dioxide. One NADH molecule is also formed in this step.

The enzyme responsible for catalysing this step is isocitrate dehydrogenase. This is a rate limiting step as isocitrate dehydrogenase is an allosterically controlled enzyme.

• Step 4: Here alpha-ketoglutarate is oxidised to form a 4 carbon molecule which picks up coenzyme A forming succinyl CoA. This conversion also forms a NADH molecule.
• Step 5: Succinyl CoA is then converted to succinate (4 carbon molecule) and one GTP molecule is produced.
• Step 6: Succinate is converted into fumarate (4 carbon molecule) and a molecule of FADH₂ is produced.
• Step 7: Fumarate is converted to malate (another 4 carbon molecule).
• Step 8: Malate is then converted into oxaloacetate and NADH is also produced here.
• It is important to be aware that whilst the primary role of the TCA cycle is production of NADH and FADH₂ it also produces molecules that supply various biosynthetic processes, which can enter or exit the cycle at various points depending on the demand on different reactions. For example, alpha-ketoglutarate can leave the cycle to be converted into amino acids or succinate can be converted to haem.
  Each cycle produces two molecules of carbon dioxide, three molecules of NADH, three hydrogen ions, one molecule of FADH₂ and one molecule of GTP. As such each molecule of glucose produces double this (2 carbon dioxide, 6 NADH, 6 hydrogen ions, 2 FADH₂ and 2 GTP).

• Regulation of enzymes in the citric acid cycle;

• Three reactions of the cycle are catalyzed respectively by the enzymes:

  1. Citrate synthase.
  2. Isocitrate dehydrogenase.
  3. α-ketoglutarate dehydrogenase

  Citrate synthase is responsible for the rate of reaction in the first step of the cycle when the acetyl-CoA is combined with oxaloacetic acid to form citrate. It is inhibited by high concentrations of ATP, acetyl-CoA, and NADH which indicates an already high level of energy supply. The molecule produced in the reaction, citrate, can also act as an inhibitor of the reaction.

  Because citrate synthase is inhibited by the final product of the citric acid cycle as ATP, ADP (adenosine diphosphate) works as an allosteric activator of the enzyme as ATP is formed from ADP. Therefore, the rate of the cycle is reduced when the cell has a high level of ATP.

  The enzyme isocitrate dehydrogenase is an important catalyst in the third step of the reaction. It regulates the speed at which the citrate isomer isocitrate loses a carbon to form the five-carbon molecule α-ketoglutarate. The coenzyme NADH is a product of the reaction and, at high levels, acts as an inhibitor by directly displacing the NAD+ molecules it is formed from.

  The enzyme α-ketoglutarate dehydrogenase is another important catalyst in the fourth step of the cycle where α-ketoglutarate also loses a carbon and combines with Coenzyme A to form succinyl CoA. The two products of the reaction, succinyl CoA and NADH, both work as inhibitors at large concentrations.
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