Question

Substrate level phosphorylation occurs during which step of Krebs' cycle?
A)Succinyl CoA - Succinic acid
B)Isocitric acid - Oxalosuccinic acid
C)Oxalosuccinic acid - a-ketoglutaric acid
D)Malic acid - OAA

Answer 1

Hint: Krebs's cycle is defined as the process that takes place after the glycolysis or EMP pathway. This process is made up of many chemical reactions that convert glucose into the energy. This process is also termed as citric acid cycle or tricarboxylic acid.

Complete answer:
Step that are present in the Krebs cycle are

1.The condensation takes place at the very first step. Acetyl group is first transformed into the citric acid by the reaction with water and fatty acids. The enzyme citrate synthase is utilized for the catalyzation.
2.The citric acid that is formed from the previous step is converted into isocitrate
3.Then the next two process is termed as the decarboxylation which produce ketoglutaric acid and succinyl-CoA
4.The succinyl-CoA now gets converted into succinic acid in which one molecule of GTP is produced. In the next reaction ATP is synthesised from the ADP, so this is termed as substrate level phosphorylation and NAD is reduced to NADH and FAD is reduced to FADH.
5.Then at the next step the acetic acid is converted to oxaloacetate which is again converted into citrate and hence the process is repeated.

6.The enzymes that are used in the Krebs cycle are citrate synthase, Aconitase, isocitrate dehydrogenase, ketoglutarate dehydrogenase, succinyl coenzyme A synthetase, succinate succinyl coenzyme A synthetase, succinate dehydrogenase, fumarase and malate dehydrogenase.

In the third step of the citric acid cycle in which the citrate is transformed to ketoglutarate, NAD is reduced to NADH. And in the fourth step where ketoglutarate is transformed into succinyl-coA in which FAD is reduced to FADH. In the last step of the Krebs's cycle NAD is transformed into NADH.

Hence, the correct answer is option (A)

Note: This Krebs's cycle or citric acid cycle occurs two times for each molecule of the glucose and it is the cycle of closed loop. In eukaryotes it occurs in mitochondria where in prokaryotes it takes place in the cytoplasm of the cell.