Question

In Krebs cycle, how much oxidation (dehydrogenation) occurs?
A. $4$
B. $6$
C. $2$
D. $1$

Answer 1

The Krebs cycle (also known as the Citric Acid Cycle or the Tricarboxylic Acid Cycle) is a step-by-step cyclic process that oxidises the pyruvate generated during the glycolytic breakdown of glucose into $C{O_2}$ and water $\left( {{H_2}O} \right)$. It also oxidises acetyl CoA, which is produced when carbohydrates, lipids, and proteins are broken down. When acetyl - CoA reacts with citric acid to create citric acid, the Krebs cycle begins. Sir Hans Krebs, a British biochemist, developed this cycle. In $1953$, he was given the Nobel Prize for this work.

Complete answer:
Option A: The oxidation or dehydrogenation reaction occurs four times in Krebs cycle:
Alpha-ketoglutarate to isocitrate ($NADH$ is formed).
Succinyl co-A to alpha-ketoglutarate ($NADH$ is formed).
Fumarate to Succinate ($FAD{H_2}$ is formed).
Oxaloacetate to malate ($NADH$ is formed).
Hence option A is correct.
Option B: In the aerobic breakdown of one glucose molecule, decarboxylation occurs six times. Oxidative decarboxylation reactions are oxidation reactions that result in the formation of carbon dioxide when a carboxylate group is removed. They're common in biological systems, as evidenced by the citric acid cycle. This type of reaction most likely began from the beginning of life.
So, option B is correct.
Option C: In Krebs cycle, the oxidation or dehydrogenation reaction occurs four times:
Isocitrate to alpha-ketoglutarate ($NADH$ is formed).
alpha-ketoglutarate succinyl co-A ($NADH$ is formed).
Succinate to Fumarate ($FAD{H_2}$ is formed).
malate to oxaloacetate ($NADH$ is formed).
So, option C is incorrect.
Option D: The citric acid cycle is made up of eight phases that include redox, dehydration, hydration, and decarboxylation processes. Each cycle turn produces one GTP or ATP molecule, three $NADH$ molecules, and one $FAD{H_2}$ molecule, all of which are required to make ATP for the cell in subsequent processes of cellular respiration.
So, option D is also not correct.

So, Option A is the correct answer.

Note:
Eight major reactions occur in the Kreb Cycle and the enzymes involved these reactions are:
Citrate synthase
Aconitase
Isocitrate dehydrogenase
Alpha ketoglutarate dehydrogenase
Succinyl coA synthetase
Succinate dehydrogenase
Fumarase
Malate dehydrogenase