Free Enzymes MCQs with Answers

41. In the lock and key analogy for enzyme action, the lock represents the

• A. enzyme
• B. substrate
• C. product
• D. coenzyme

Explanation: Emil Fischer's analogy pictures the enzyme's rigid active site as the lock and the substrate as the one key that fits it, explaining specificity. The products simply leave the site once the reaction is complete.

42. Which sequence correctly represents enzyme catalysis?

• A. E + P → EP → ES → E + S
• B. ES → E + S → E + P
• C. E + S → ES → EP → E + P
• D. EP → ES → E + S

Explanation: Enzyme and substrate first form the enzyme-substrate complex, which converts to an enzyme-product complex before releasing product and regenerating the free enzyme. Any sequence that starts from product runs the pathway backwards.

43. An enzyme changes the rate of a reaction without altering its

• A. activation energy
• B. speed of completion
• C. overall energy difference between substrates and products
• D. reaction pathway

Explanation: Catalysts lower the activation energy and speed the reaction, but the energy of substrates and products is fixed by chemistry, so the enzyme cannot change it. This is why an enzyme never makes an unfavourable reaction favourable.

44. Enzymes are needed inside cells in only minute quantities because they

• A. are very large molecules
• B. are made of DNA
• C. work only at very high temperatures
• D. are not consumed and can be reused repeatedly

Explanation: Since the enzyme emerges unchanged from each reaction, one molecule can process thousands of substrate molecules per second. Cells therefore regulate metabolism by controlling enzyme activity rather than by stockpiling enzyme.

45. The activation energy of a reaction could in principle be supplied by heat, but cells use enzymes instead because heating would

• A. slow every reaction down
• B. damage cellular molecules and speed all reactions indiscriminately
• C. destroy the substrates
• D. prevent products from forming

Explanation: Raising temperature speeds every reaction and denatures proteins, so it cannot give the selective control life needs. An enzyme lowers the barrier for one specific reaction at body temperature, which is precisely why metabolism can run fast at 37 degrees.

46. When the substrate is present in excess, doubling the concentration of enzyme will

• A. approximately double the rate of the reaction
• B. halve the rate of the reaction
• C. leave the rate unchanged
• D. denature the substrate

Explanation: With abundant substrate every added enzyme molecule finds work at once, so rate is directly proportional to enzyme concentration. The rate only becomes independent of enzyme amount when substrate runs short and active sites compete for it.

47. Trypsin, working in the small intestine, has an optimum pH of about

• A. 8
• B. 2
• C. 5
• D. 11

Explanation: The alkaline environment of the duodenum, fed by bicarbonate from the pancreas, suits trypsin at pH 8. Pepsin in the acidic stomach works best near pH 2, so each digestive enzyme matches the pH of the compartment where it acts.

48. In an experiment on enzyme activity a buffer solution is used in order to

• A. increase the substrate concentration
• B. supply the necessary coenzyme
• C. keep the pH constant throughout
• D. raise the temperature slowly

Explanation: Because pH changes can alter ionisation of the active site and denature the enzyme, a fair test holds pH steady with a buffer. Without one, the acid or alkali produced by the reaction itself would shift the result.

49. A very high fever above 41 degrees Celsius is medically dangerous mainly because

• A. it makes the blood too alkaline
• B. vital enzymes begin to denature and lose function
• C. it stops all diffusion in the body
• D. it destroys red blood cells instantly

Explanation: Proteins start to unfold irreversibly a few degrees above body temperature, so sustained high fever disrupts the enzyme systems of the brain and other organs. This is the same denaturation that cooks an egg, happening inside the body.

50. Below the optimum, raising the temperature of an enzyme controlled reaction from 15 to 25 degrees Celsius will roughly

• A. halve the rate
• B. stop the reaction
• C. have no effect on the rate
• D. double the rate

Explanation: Each 10 degree rise roughly doubles the rate because molecules collide more often and with more energy, until heat damage overtakes this gain near the optimum. Above the optimum the curve falls steeply as the enzyme denatures.