Question

How many NADH + $H^{+}$ are formed in per turn of Krebs cycle

• A. 2
• B. 3
• C. 4
• D. 5

Answer 1

The correct option is B 3

Krebs cycle is a series of chemical reactions that occurs inside the mitochondria of aerobic organisms that aims at releasing the stored energy of organic food substances through the stepwise oxidation of acetyl CoA obtained from pyruvate during glycolysis.

In the 3rd step of Krebs cycle there is formation of 𝝰-ketoglutarate. Isocitrate undergoes oxidative decarboxylation (removal of carboxyl group in the form of evolution of ${\mathrm{CO}}_2$) 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 (removal of carboxyl group in the form of evolution of ${\mathrm{CO}}_2$) 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).