Enzyme-Substrate Complex Formation — NEET Biology

Correct answer: B — S2 and S4 only

The core concept here focuses on the fundamental features and mechanisms of enzyme action. Let's evaluate each statement based on NCERT text: S1: NCERT describes the enzyme as "unchanged enzyme" after releasing products and being "ready to bind to another molecule of the substrate and run through the catalytic cycle once again." This statement is correct. S2: Enzymes increase reaction rates by lowering activation energy, but they do not alter the overall difference in potential energy between substrates and products (ΔG). This difference determines the spontaneity of a reaction, not changed by enzymes. Thus, statement S2 is incorrect. S3: NCERT explicitly states, "There is thus, an obligatory formation of an ‘ES’ complex. This complex formation is a transient phenomenon." This statement is correct. S4: Figure 9.4 and the accompanying text state that enzymes "bring down this energy barrier" (activation energy). The transition state is a high-energy state, and enzymes lower this energy peak, not elevate it. Thus, statement S4 is incorrect. S5: This is a fundamental principle of catalysis. Enzymes accelerate the rate at which equilibrium is reached but do not change the position of equilibrium (equilibrium constant) or the overall direction of a reversible reaction. This is implied by NCERT showing S and P energy levels unchanged. This statement is correct. Therefore, S2 and S4 are NOT correct statements. The final answer is B, as S2 and S4 inaccurately describe features of enzyme action. Concept Recap: Enzymes are biological catalysts that lower activation energy, increasing reaction rates without being consumed or altering the reaction's overall free energy change or equilibrium constant. They form transient enzyme-substrate complexes.