Question

The activity of succinate dehydrogenase is inhibited by
(a) Malonate
(b) Pyruvate
(c) Glycolate
(d) Phosphoglycerate

Answer 1

Hint: These compounds include salts and esters like diethyl (C₂H₅)₂C₃H₂O₄, dimethyl (CH₃)₂C₃H₂O₄, and disodium Na₂ C₃H₂O₄. These compounds are competitive inhibitors.

Complete answer:
An example of competitive inhibition is the enzyme succinate dehydrogenase with succinic acid as its substrate. A substance similar to the substrate takes part and competes with the substrate for the active site and is known as the inhibitor. The presence of such molecules decreases the availability of the active site for the substrate. This also decreases the rate of reaction and this kind of inhibition is known as competitive inhibition. The compounds like malonic acid, glutaric acid, and oxalic have structural similarities with succinic acid. These thus compete with the substrate for binding at the active site of succinate dehydrogenase enzyme.
So, the correct answer is ‘Malonate’.

Note: In the inner mitochondrial membrane, an enzyme known as succinate dehydrogenase is present. It is also known as succinate-coenzyme Q reductase and is found in many bacterial cells. It takes part in the citric acid cycle and the electron transport chain. In the Krebs cycle, it catalyzes the oxidation of succinate into fumarate. The derived electrons are fed to the respiratory chain complex III to reduce oxygen and form water. This allows for the synthesis of ATP by an electrochemical gradient across the mitochondrial inner membrane. At the same time, electrons can be diverted to reduce the ubiquinone pool. This provides reducing substances that are necessary to reduce superoxide anions. These anions originate from an exogenous origin or from the respiratory chain itself. If succinate dehydrogenase is not present there will be a hamper electron flow to respiratory chain complex III and the quinone pool that results in major oxidative stress that leads to the formation of a tumor.