MDCAT Biology chapter 6: Biology · Enzymes MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
28 exam-style questions on Biology · Enzymes, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A human enzyme is kept at 70 °C for ten minutes and then cooled to 37 °C. Its activity at 37 °C will most likely be
A
higher than before, because heating activated it
B
very low, as heat has denatured it permanently
C
normal, because the enzyme recovers on cooling
D
normal, because 70 °C is its optimum temperature
Show answer
Answer: B. High temperature breaks the bonds holding the enzyme's three-dimensional shape, so the active site is lost; for most enzymes this denaturation is irreversible and activity does not return on cooling. Most human enzymes work best near 37 °C.
2. The protein part of an enzyme, which cannot work until its cofactor is attached, is called the
A
holoenzyme
B
coenzyme
C
apoenzyme
D
prosthetic group
Show answer
Answer: C. Apoenzyme (protein) + cofactor = holoenzyme, the complete active enzyme. Coenzymes and prosthetic groups are kinds of cofactor, not the protein part.
3. An enzyme has a maximum rate of 60 units. Inhibitor X lowers the rate at low substrate concentrations, but at a very high substrate concentration the rate again reaches 60 units. Inhibitor Y lowers the maximum rate to 25 units, however much substrate is added. Which conclusion is correct?
A
X is non-competitive and Y is competitive
B
X is competitive and Y is non-competitive
C
both X and Y are competitive inhibitors
D
both X and Y are non-competitive inhibitors
Show answer
Answer: B. Inhibition that can be swamped by excess substrate is competitive, so X competes for the active site. Y still lowers the maximum rate when substrate is in great excess, so it must act away from the active site: it is non-competitive.
4. An enzyme solution kept in a refrigerator at 4 °C shows very little activity, but works normally again when warmed to 37 °C. This is because at low temperature
A
the enzyme is denatured, but it renatures fully on warming
B
the active site is destroyed and then made again
C
the substrate changes into an inhibitor of the enzyme
D
molecules move slowly, but the enzyme is not denatured
Show answer
Answer: D. Cold lowers the kinetic energy of the enzyme and substrate molecules, so they collide less often, but the enzyme's shape is unharmed and activity returns on warming. Denaturation by heat, in contrast, is usually permanent.
5. Which of these digestive enzymes works best in an alkaline medium?
A
salivary amylase
B
pepsin in the stomach
C
sucrase
D
pancreatic lipase
Show answer
Answer: D. Pancreatic juice is alkaline, and pancreatic lipase has an alkaline optimum pH. Pepsin needs strong acid, and salivary amylase works best close to neutral pH.
6. In a pathway A → B → C → D, each step catalysed by a different enzyme, a build-up of D slows the whole pathway because D binds to the first enzyme at a site away from its active site. This is
A
competitive inhibition by a substrate analogue
B
denaturation of the first enzyme by excess product
C
positive feedback, which speeds up the first enzyme
D
feedback inhibition by a non-competitive inhibitor
Show answer
Answer: D. The end product switching off the first enzyme of its own pathway is feedback (end-product) inhibition; because D binds away from the active site, it acts as a non-competitive (allosteric) inhibitor. The effect is reversible, so the pathway restarts when D is used up.
7. In the lock-and-key model of enzyme action, proposed by Emil Fischer, the 'key' stands for the
A
enzyme
B
substrate
C
active site
D
product
Show answer
Answer: B. The substrate (key) fits exactly into the rigid active site of the enzyme (lock). The product is released after the reaction and no longer fits.
8. An inhibitor forms strong covalent bonds with an amino acid in the active site of an enzyme. Which result is expected?
A
The inhibitor is changed into product and released.
B
The inhibition is overcome by adding much more substrate.
C
The enzyme recovers fully once the free inhibitor is washed away.
D
The enzyme stays inactive even after free inhibitor is removed.
Show answer
Answer: D. A covalently bound inhibitor is not released, so the enzyme is permanently inactivated: this is irreversible inhibition, seen with some poisons and drugs. Recovery on removing the inhibitor, or on adding more substrate, happens only with reversible inhibitors.
9. Detachable inorganic ions, such as Mg2+ and Cl− (the ion that salivary amylase needs), that some enzymes need for their activity are called
A
activators
B
coenzymes
C
prosthetic groups
D
apoenzymes
Show answer
Answer: A. Inorganic ion cofactors are called activators. Coenzymes and prosthetic groups are organic molecules, and the apoenzyme is the protein part of the enzyme itself.
10. A change of pH away from the optimum reduces the activity of an enzyme mainly because it
A
breaks the peptide bonds that link the enzyme's amino acids together
B
reduces the kinetic energy of the enzyme and substrate
C
alters the charges on amino acids, changing the active site
D
raises the concentration of the substrate too far
Show answer
Answer: C. pH changes the ionisation of acidic and basic R groups, which disturbs the ionic and hydrogen bonds that hold the active site in shape, so the substrate binds poorly. The peptide bonds of the chain are not broken.
28, each with the answer and a short explanation, across 2 topics: Enzyme action, Factors and inhibition. Every chapter test draws ten of them, so you can take it many times.
Is the MDCAT Biology chapter 6 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
