Enzymes - NEET Previous Year Questions with Complete Solutions

NTA typically asks 2 to 4 questions directly from the Enzymes sub-topic within Biomolecules. Some years, an additional question on enzyme-related metabolism (like the role of NAD⁺ in respiration) appears under other chapters. Over the last 10 years, enzymes have contributed 6–12 marks in total across all NEET papers.

Competitive inhibition and the active site are the most frequently tested topics — they have appeared in 9 and 8 questions respectively across 15 years of NEET papers. Cofactors (coenzymes, prosthetic groups, metal ion activators) are the third-highest priority. Prepare these three before anything else.

Both models appear in NEET, but the induced fit model (Koshland, 1958) is more frequently tested in recent years (2019 onwards). NEET questions often ask students to distinguish which model explains flexibility in the active site. The induced fit model is the correct, modern explanation — the lock and key model is the historical one.

A coenzyme is an organic, non-protein molecule that is loosely attached to the enzyme and can be removed, such as NAD⁺ or FAD. A prosthetic group is also organic and non-protein but is tightly or covalently bound to the enzyme — for example, haem in catalase. Both are types of cofactors, and NEET distinguishes them by their binding strength.

In competitive inhibition, the inhibitor molecule resembles the substrate and competes for the same active site. Adding more substrate reverses this inhibition. Km increases but Vmax remains unchanged. In non-competitive inhibition, the inhibitor binds at a different (allosteric) site, permanently reducing Vmax without changing Km. NEET most frequently tests competitive inhibition.