Free Enzymes MCQs with Answers

21. Activation energy is best defined as the energy

• A. released when the products are formed
• B. stored in the bonds of the substrate
• C. needed to bring the reactants to the transition state so the reaction can proceed
• D. used by the enzyme to bind the substrate

Explanation: Even an energetically favourable reaction needs an initial input to break existing bonds and reach the unstable transition state, and that barrier is what keeps sugars from spontaneously oxidising in the air. An enzyme lowers the barrier by stabilising the transition state, so far more molecules can cross it at body temperature. The energy difference between reactants and products is a separate quantity that the enzyme does not change.

22. Enzymes such as pepsin and trypsin are secreted in an inactive form known as a

• A. coenzyme
• B. cofactor
• C. prosthetic group
• D. zymogen

Explanation: A zymogen, or proenzyme, carries an extra length of polypeptide that blocks the active site, and removing it activates the enzyme once it has reached the place where it is needed. This prevents a protease from digesting the very cells that made it, and failure of the mechanism in the pancreas causes the self digestion seen in acute pancreatitis. Coenzymes and cofactors are helper molecules, not inactive precursors.

23. An enzyme that catalyses the removal of hydrogen from a substrate is classified as a

• A. dehydrogenase
• B. hydrolase
• C. isomerase
• D. ligase

Explanation: Enzymes are usually named from the substrate or the reaction plus the suffix ase, so a dehydrogenase removes hydrogen, as succinate dehydrogenase does in the Krebs cycle, passing it to a coenzyme such as NAD. Hydrolases split bonds by adding water, isomerases rearrange a molecule into its isomer and ligases join two molecules together. Knowing the suffix system lets an unfamiliar enzyme name be interpreted on sight.

24. When substrate is present in excess, the rate of an enzyme catalysed reaction is

• A. independent of enzyme concentration
• B. directly proportional to the enzyme concentration
• C. inversely proportional to the enzyme concentration
• D. zero

Explanation: With substrate always available, every additional enzyme molecule adds its own turnover to the total, so doubling the enzyme doubles the rate and the graph is a straight line through the origin. The line only levels off if the substrate begins to run short and becomes limiting instead. This is the reverse of the saturation curve obtained when enzyme is fixed and substrate is increased.

25. The essential difference between denaturation and inhibition of an enzyme is that denaturation

• A. always occurs at the active site only
• B. can be reversed by adding more substrate
• C. involves the loss of the enzyme's three dimensional shape and is usually permanent
• D. increases the rate of the reaction

Explanation: Denaturation unfolds the whole protein, so the enzyme is destroyed as a catalyst and no amount of substrate will restore it, whereas most inhibition leaves the enzyme intact and is reversible when the inhibitor is removed or, for competitive inhibitors, outcompeted. High temperature and extreme pH denature, while specific molecules inhibit. Only competitive inhibition is relieved by extra substrate.

26. Catalase, one of the fastest known enzymes, catalyses the breakdown of

• A. starch into maltose
• B. protein into amino acids
• C. fat into fatty acids and glycerol
• D. hydrogen peroxide into water and oxygen

Explanation: A single catalase molecule can decompose millions of molecules of hydrogen peroxide per minute, which is necessary because the peroxide produced during metabolism would otherwise damage the cell within seconds. This is why fresh liver put into hydrogen peroxide froths violently, a standard classroom demonstration. Amylase, protease and lipase are the enzymes for the other three substrates listed.

27. A metal ion such as zinc or magnesium that an enzyme needs in order to work is called

• A. a cofactor
• B. a coenzyme
• C. a prosthetic protein
• D. a substrate

Explanation: Cofactor is the term for an inorganic helper, and zinc in carbonic anhydrase and magnesium in the kinases of respiration are standard examples. Coenzymes are organic helpers such as NAD and coenzyme A, usually derived from vitamins, so the distinction is simply inorganic against organic. This is why trace elements are needed in the diet in tiny but non negotiable amounts.

28. Many coenzymes are derived from

• A. minerals absorbed from the soil
• B. vitamins of the B group
• C. fatty acids stored in adipose tissue
• D. the amino acids of the enzyme itself

Explanation: NAD is made from niacin, FAD from riboflavin and coenzyme A from pantothenic acid, so a deficiency of these vitamins cripples respiration and produces widespread symptoms rather than one local effect. This is the biochemical reason a vitamin is needed in very small quantities but cannot be omitted. Minerals supply cofactors instead, which is the parallel case.

29. Enzymes are used in industry in an immobilised form, fixed to an inert support, mainly because

• A. immobilised enzymes work at a much lower temperature
• B. immobilisation changes their specificity
• C. the enzyme can be recovered and reused and does not contaminate the product
• D. immobilised enzymes need no substrate

Explanation: Fixing the enzyme to beads or a membrane lets the product flow past and be collected pure, while the expensive enzyme stays behind for the next batch, and immobilised enzymes also tolerate heat and pH changes better. Lactose free milk is produced this way, by passing milk over immobilised lactase. The specificity of the enzyme is unchanged by immobilisation.

30. Compared with the uninhibited enzyme, a non competitive inhibitor

• A. raises Vmax and leaves Km unchanged
• B. raises both Vmax and Km
• C. lowers Km and leaves Vmax unchanged
• D. lowers Vmax and leaves Km unchanged

Explanation: Because the inhibitor binds elsewhere and cannot be displaced by substrate, a fraction of the enzyme is permanently out of action, so the maximum achievable rate falls while the affinity of the remaining active sites for the substrate is unaltered. A competitive inhibitor shows the opposite pattern, an unchanged Vmax with an apparently raised Km. This pair of effects is the standard way of telling the two kinds apart in the laboratory.