Question

Which of the following is an example of irreversible reaction?

• A. Inhibition of methanol by ethanol
• B. Inhibition of succinate by malonate
• C. Cyanide poisoning
• D. All of these

Answer 1

The correct option is C Cyanide poisoning

A competitive inhibitor resembles the substrate very closely structurally and competes with the substrate for binding to the active site of the enzyme. It binds to the active site and forms the enzyme-inhibitor complex which doesn’t give products. The inhibition is reversible. It can be reversed by increasing the substrate concentration. Increasing the substrate concentration increases the probability of substrate to bind with the enzymes and form products. Eg:- methanol gets converted to formaldehyde in the presence of alcohol dehydrogenase. Formaldehyde is poisonous to us. Ethanol acts as a competitive inhibitor of methanol and stops the reaction. So, in case of methanol poisoning, ethanol is administered to counteract the effect. Malonate acts as a competitive inhibitor of succinate and binds to succinate dehydrogenase.
A non competitive inhibitor binds at a site other than the active site. It can bind to the free enzyme or the ES complex. It changes the shape of the active site such that substrate is not able to bind to it. Eg:- Heavy metal poisoning (Hg poisoning). Cyanide poisoning- cyanide inhibits mitochondrial enzyme cytochrome oxidase. It results in the failure of cellular respiration and leads to the death of the organism.