Question

The step in which NADH is not produced is

• A. Succinyl-CoA → Succinate
• B. Pyruvate → Acetyl-CoA
• C. 𝝰.-ketoglutarate → Succinyl-CoA
• D. Malate → OAA

Answer 1

The correct option is A Succinyl-CoA → Succinate

The end product of glycolysis is pyruvate. Glycolysis occurs in cytoplasm. Krebs cycle occurs in the matrix of mitochondria. Pyruvate enters mitochondria through a specific transport protein pyruvate translocase. It undergoes oxidative decarboxylation to produce ${\mathrm{CO}}_2$ and NADH (reduced nicotinamide adenine dinucleotide). The product combines with sulphur containing coenzyme A to form acetyl CoA or activated acetate. The reaction occurs in the presence of an enzyme complex pyruvate dehydrogenase (made up of a decarboxylase, lipoic acid, TPP, transacetylase and ${\mathrm{Mg}}^{2+}$).
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.
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. No NADH (reduced nicotinamide adenine dinucleotide) is produced.