Question

Which among the following statement is wrong for Krebs’ cycle?

• A. There is one point in the cycle where FAD is reduced to ${\mathrm{FADH}}_2$.
• B. During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised.
• C. The cycle starts with condensation of acetyl group (acetyl CoA) with pyruvic acid to yield citric acid.
• D. There are three points in the cycle where ${\mathrm{NAD}}^{+}$ is reduced to NADH (reduced nicotinamide adenine dinucleotide) + $H^{+}$

Answer 1

The correct option is C The cycle starts with condensation of acetyl group (acetyl CoA) with pyruvic acid to yield citric acid.

Krebs cycle is the stepwise oxidative and cyclic degradation of acetyl CoA derived from pyruvate, the end product of glycolysis.

Krebs’ cycle starts with condensation of acetyl group (acetyl CoA) with oxaloacetate (4- carbon compound) in the presence of a condensing enzyme citrate synthase to form a tricarboxylic, 6- ­carbon compound called citric acid. It is the 1st product of Krebs cycle and CoA is liberated.

In the 5th step of Krebs cycle, conversion of succinyl CoA to succinic acid (also known as succinate) occurs. Succinyl CoA is acted upon by an enzyme synthetase to form succinate (4 carbon compound). The reaction releases sufficient energy to form ATP (in plants) or GTP (in animals). GTP can form ATP through a coupled reaction.

In the 6th step of Krebs cycle, oxidation of succinate occurs. Succinate undergoes dehydrogenation to form fumarate with the help of a membrane based enzyme succinate dehydrogenase. ${\mathrm{FADH}}_2$ is produced in this step.

the 3rd step of Krebs cycle there is formation of 𝝰-ketoglutarate. Isocitrate undergoes oxidative decarboxylation in the presence of enzyme isocitrate dehydrogenase and ${\mathrm{Mn}}^{2+}$. A transient oxalosuccinate is formed as intermediate. It undergoes decarboxylation to form a 5- carbon 𝝰-ketoglutarate also known as 2-oxoglutarate. NADH (reduced nicotinamide adenine dinucleotide) is produced in this step.

In the 4th step of Krebs cycle there is oxidative decarboxylation of 𝝰-ketoglutarate. It is the 3rd oxidative decarboxylation process apart from the linking reaction and step 3 of Krebs cycle. In this step, 𝝰-ketoglutarate is both dehydrogenated (with the help of ${\mathrm{NAD}}^{+}$) and decarboxylated by an enzyme complex 𝝰-ketoglutarate dehydrogenase. The product combines with CoA to form succinyl CoA. NADH (reduced nicotinamide adenine dinucleotide) is produced.

In the 8th step of Krebs cycle, oxidation of malate occurs. Malate is dehydrogenated or oxidised through the agency of malate dehydrogenase to produce oxaloacetate. $H^{+}$ is accepted by ${\mathrm{NAD}}^{+}$. NADH (reduced nicotinamide adenine dinucleotide) is formed in this step.

So, we can see there are 3 steps in Krebs cycle that are associated with the production of NADH (reduced nicotinamide adenine dinucleotide).