Enzyme Active Sites & Thermal Stability — NEET Biology

Correct answer: A — S1, S3, S4 and S5

The core concept is the factors affecting enzyme activity, including temperature, pH, and substrate concentration. Let's evaluate each statement based on NCERT text: S1: NCERT states, "enzymes isolated from organisms who normally live under extremely high temperatures (e.g., hot vents and sulphur springs), are stable and retain their catalytic power even at high temperatures (upto 80°-90°C)." This contradicts the statement that they lose activity above 40°C. Thus, statement S1 is incorrect. S2: NCERT states, "Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat." This statement is correct. S3: NCERT states, "Enzymes generally function in a narrow range of temperature and pH (Figure 9.5). Each enzyme shows its highest activity at a particular temperature and pH called the optimum temperature and optimum pH." This contradicts the statement that they function over a broad range. Thus, statement S3 is incorrect. S4: NCERT states, "The reaction ultimately reaches a maximum velocity (Vmax) which is not exceeded by any further rise in concentration of the substrate. This is because the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecules." The velocity reaches Vmax and does not decline rapidly; the reason is enzyme saturation, not necessarily substrate inhibition as a general rule here. Thus, statement S4 is incorrect. S5: NCERT states, "The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary structure of the protein. These include temperature, pH..." This directly contradicts the statement that tertiary structure is unaffected. Thus, statement S5 is incorrect. Therefore, S1, S3, S4, and S5 are NOT correct statements. The final answer is A, as it correctly identifies all the incorrect statements regarding factors affecting enzyme activity. Concept Recap: Enzyme activity is sensitive to temperature and pH, showing optimum activity within a narrow range. High temperatures denature enzymes, while low temperatures cause temporary inactivity. Substrate concentration affects reaction velocity up to saturation, but generally does not cause a decline.