Question

In Krebs cycle, OAA accepts acetyl CoA to form
A. Citric acid
B. Oxalosuccinate
C. Fumarate
D. Succinyl CoA

Answer 1

The citric acid cycle or the tricarboxylic acid cycle or the Krebs cycle is a series of chemical reactions that occurs in the mitochondria of the eukaryotic cell during aerobic respiration and generates or releases stored energy of the cell.

Complete answer:
Kreb’s cycle is the main source of energy for the cell and is a part of the cellular respiration of the cell. It was discovered by the scientist Hans Krebs. It releases energy by the oxidation of the Acetyl Co-A derived from carbohydrates, fats, and proteins, which is further converted into many products and releases ATP, NADH, carbon dioxide, $FADH_2$, and precursors of various amino acids. It consists of 8 steps that form a closed loop and involves redox, dehydration, hydration, and decarboxylation reactions. The main steps of the cycle are-

Step 1: Joining Oxaloacetate and Acetyl Co-A to form Citrate.
Step 2: Conversion of Citrate to Isocitrate.
Step 3: Oxidation of Isocitrate to alpha-ketoglutarate to release carbon dioxide. As a result, one molecule of NADH is formed.
Step 4: Oxidation of alpha-ketoglutarate to form Succinyl Co-A and one molecule of NADH. Carbon dioxide is again released in this process.
Step 5: Conversion of Succinyl Co-A to Succinate to form one molecule of GTP.
Step 6: Conversion of Succinate to Fumarate to produce one molecule of $FADH_2$.
Step 7: Conversion of Fumarate to Malate.
Step 8: Conversion of Malate to Oxaloacetate, the starting product of the cycle. This step produces one NADH molecule.
The enzymes used in the Kreb cycle stepwise are- Citrate synthase, Aconitase, Isocitrate dehydrogenase, Alpha-ketoglutarate dehydrogenase, Succinyl Co-A synthase, Succinate dehydrogenase, Fumarase, and Malate dehydrogenase.
As a result of this 8 steps cycle reaction, each molecule of Acetyl CoA produces- two molecules of carbon dioxide, three molecules of NADH, one molecule of $FADH_2$, one molecule of GTP, and three hydrogen ions.
GTP can be converted to ATP by the enzyme Nucleoside diphosphate kinase.

Hence, the correct answer is option (A)

Note: In prokaryotic cells, this cycle occurs in the cytoplasm of the cell, whereas, in eukaryotes, it occurs in the matrix of the mitochondria. Glycolysis results in the oxidation of glucose to pyruvic acid that is further oxidized to form the first product of the Krebs cycle- Acetyl Co-A. This cycle can be used for the metabolism of lipids, glucose, amino acids, proteins, etc.