Question

Which among the following is a competitive inhibitor of succinic dehydrogenase?
A. alpha-ketoglutarate
B. Malate
C. Malonate
D. Oxaloacetate

Answer 1

Hint: Succinic dehydrogenase is an enzyme complex that acts as a catalyst during the kreb’s cycle in the oxidation of succinate to fumarate.

Complete answer:
Succinic dehydrogenase is an enzyme complex that is found in eukaryotes in the inner membrane of their mitochondria and in many bacterial cells. It is the only enzyme that participates in both citric acid cycle and electron transport chain. Now let’s understand what a competitive inhibitor is. A competitive inhibitor is a compound that resembles the substrate that is binding to the enzyme,that binds to it at the active site. As these compounds resemble the substrate sometimes this will end up binding with the enzyme instead of the normal substrate which will result in an interruption in the chemical pathway.
The enzyme can only bind either substrate or inhibitor at a time. Let’s analyze the options to find out the competitive inhibitor of succinate dehydrogenase.
Option A) alpha-ketoglutarate is a molecule which regulates the speed of the citric cycle. It is converted into succinyl-CoA and produces NADH thereby providing electrons for the respiratory chain, and also provides protein to muscles by being a source of glutamine. This is not an inhibitor of succinic dehydrogenase. Therefore this is an incorrect option.
Option B) The kreb's cycle malate regenerates oxaloacetate with the help of NAD(plus) and the enzyme malate dehydrogenase and this completes the kreb’s cycle. This does not act as an inhibitor. Therefore this is an incorrect option.
Option C) Malonate is the salt or ester of malonic acid whose organic structure(dicarboxylate) is similar to that of succinic dehydrogenase. This competes with SDH to bind with the active site of the SDH complex and hinders the oxidation of succinic acid to fumaric acid. Therefore this is the correct option.
Option D) Oxaloacetate is an intermediate compound in the Krebs cycle which reacts with acetyl-coA to form citrate. This reaction is catalysed by citrate synthase. This is not an inhibitor. Therefore this is an incorrect option.

Hence, the correct answer is option (C).

Note: In the kreb’s cycle after the oxidation of succinate ,the electrons are fed into the respiratory chain ubiquinone pool. This will reduce oxygen and result in the formation of water. Another function of dehydrogenase is creating an electrochemical gradient across the membrane of mitochondria which allows the synthesis of ATP.