Why is krebs cycle called a cycle

Before the fuel molecules can be inserted in the Krebs Cycle, they must first all be converted into acetyl-CoA. Looking at the path of a nutrient, such as glucose, the oxidation of the molecule takes place in the glycolysis. The product of the glycolysis is pyruvate. In a further reaction, which is catalyzed by the enzyme complex pyruvate dehydrogenase, acetyl-CoA is formed out of pyruvate, which can be introduced into the citric acid cycle or Krebs Cycle.

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle, is part of a series of chemical reactions that organisms use to break down food into a form of energy that cells can use. The cycle occurs in the mitochondria of cells, using 2 molecules of pyruvic acid from glycolysis to produce the energy molecules. The final product of the Krebs cycle is oxaloacetic acid. It is a cycle because oxaloacetic acid oxaloacetate is the exact molecule needed to accept an acetyl-CoA molecule and start another turn of the cycle.

Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Step 1. The first step is a condensation step, combining the two-carbon acetyl group from acetyl CoA with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate.

CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. Step 2.

Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA.

Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP.

There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP. In particular, protein synthesis primarily uses GTP. Step 6. The name citric acid cycle is derived from the first product generated by the sequence of conversions, i.

The reactions are seen to comprise a cycle inasmuch as citric acid is both the first product and the final reactant, being regenerated at the conclusion of one complete set of chemical rearrangements. Citric acid is a so-called tricarboxylic acid, containing three carboxyl groups COOH. Hence the Krebs cycle is sometimes referred to as the tricarboxylic acid TCA cycle. The Krebs cycle begins with the condensation of one molecule of a compound called oxaloacetic acid and one molecule of acetyl CoA a derivative of coenzyme A; see coenzyme.