Free Enzymes MCQs with Answers

31. An enzyme that is secreted and works outside the cell that made it is called an extracellular enzyme. An example is

• A. trypsin
• B. DNA polymerase
• C. catalase
• D. RNA polymerase

Explanation: The pancreas exports trypsin into the duodenum, where it digests food outside any cell. DNA and RNA polymerases and catalase all work inside the cells that make them, which is what makes them intracellular enzymes.

32. The protein part of a conjugated enzyme, which on its own is catalytically inactive, is called the

• A. prosthetic group
• B. apoenzyme
• C. holoenzyme
• D. co-substrate

Explanation: The apoenzyme needs its non-protein partner, the cofactor, to become the active holoenzyme. Calling the whole active complex the apoenzyme is a common slip, so remember that holoenzyme equals apoenzyme plus cofactor.

33. Ribozymes are unusual biological catalysts because they are

• A. lipid molecules with enzyme activity
• B. synthetic polymers made in the laboratory
• C. RNA molecules that catalyse reactions
• D. proteins permanently fixed to ribosomes

Explanation: The discovery that certain RNA molecules can catalyse reactions, such as self splicing, showed that not all enzymes are proteins. This evidence supported the idea of an early RNA world in which RNA stored information and catalysed reactions.

34. The ending most commonly used when naming an enzyme is

• A. -ose
• B. -in
• C. -ol
• D. -ase

Explanation: Enzyme names end in -ase added to the substrate or reaction type, giving urease, lipase and dehydrogenase. The ending -ose belongs to sugars such as glucose and maltose, which is the mix-up the examiner is testing.

35. One molecule of carbonic anhydrase can hydrate up to a million molecules of carbon dioxide every second. This demonstrates the enzyme's

• A. instability at body temperature
• B. gradual consumption during the reaction
• C. very high turnover number
• D. resistance to all inhibitors

Explanation: The turnover number is how many substrate molecules one enzyme molecule converts per second, and carbonic anhydrase has one of the highest known. The enzyme itself is unchanged and reused, so the huge rate reflects catalytic power, not consumption.

36. Enzymes that catalyse the splitting of bonds with the addition of water belong to the class called

• A. hydrolases
• B. ligases
• C. transferases
• D. isomerases

Explanation: Hydrolases, such as amylase, lipase and peptidase, break bonds by inserting water. Transferases move groups between molecules, ligases join molecules using ATP, and isomerases rearrange a molecule without changing its formula.

37. Which statement about enzymes as biological catalysts is correct?

• A. They are consumed during the reaction
• B. They allow energetically impossible reactions to occur
• C. They increase the energy content of the products
• D. They are regenerated unchanged at the end of the reaction

Explanation: An enzyme leaves the reaction exactly as it entered, which is why tiny amounts can process huge quantities of substrate. It only speeds a reaction that could happen anyway by lowering the activation energy; it never changes the energy of substrates or products.

38. Storing food in a refrigerator slows its decay mainly because low temperature

• A. permanently denatures the enzymes of microbes
• B. greatly reduces the rate of enzyme controlled reactions
• C. kills all bacteria and fungi at once
• D. changes the pH of the food

Explanation: Cold reduces the kinetic energy of molecules so enzyme catalysed spoilage reactions slow dramatically, but the enzymes are not destroyed and work again on warming. This reversibility is exactly why freezing preserves food without sterilising it.

39. The specific region of an enzyme to which the substrate binds and where catalysis occurs is called the

• A. allosteric site
• B. active site
• C. cofactor site
• D. regulatory site

Explanation: The active site is a cleft formed by the folding of the chain, whose shape and chemistry fit the substrate. Allosteric sites are separate locations where regulators bind, so a question about the substrate's own binding place points only to the active site.

40. Koshland's induced fit model improved on the lock and key hypothesis by proposing that

• A. the active site is completely rigid
• B. the substrate changes its shape before binding
• C. enzymes have no binding site at all
• D. the active site moulds itself around the substrate as it binds

Explanation: Koshland argued in 1966 that binding itself induces a conformational change that tightens the fit and stresses bonds in the substrate, whereas the older lock and key model pictured the active site as a fixed shape. Both models agree that the fit is specific.