Enzymes Regulation Questions and Answers

Answer: C

Correct answer: holoenzyme

The holoenzyme is the complete, active molecule: apoenzyme plus its cofactor or coenzyme. Neither part works alone - the protein supplies the specificity and the binding site, the cofactor the chemistry the amino acids cannot do, such as carrying electrons.

Answer: A

Correct answer: not pass through semipermeable membrane

Enzymes are large protein molecules, far too big to cross a semipermeable membrane. That is what keeps them inside the cell or the organelle where they belong, and it is why a cell must secrete an enzyme deliberately if it is to act outside.

Answer: D

Correct answer: The non-covalent binding of cAMP somewhere other than the active site

Allosteric regulation has two requirements, and only this option meets both: the regulator binds away from the active site, and it binds non-covalently, so the effect is readily reversible. Phosphorylation is covalent modification, a different mechanism; and anything binding at the active site is competing with the substrate, not acting allosterically.

Answer: C

Correct answer: Competitive inhibition

An inhibitor that occupies the active site competes with the substrate for it, which is competitive inhibition. Because the two compete, the effect is overcome by raising the substrate concentration - so \(K_m\) rises while \(V_{max}\) is unchanged. Such inhibitors usually resemble the substrate in shape.

Answer: A

Correct answer: I and II

Both plots work. Plotting rate against substrate concentration (I) for inhibited and uninhibited enzyme shows the change in \(K_m\) and \(V_{max}\), and the double-reciprocal or Lineweaver-Burk plot (II) shows it more clearly still, as a change in the intercepts: competitive inhibition moves the x-intercept, non-competitive the y-intercept. Plot III relates two concentrations to each other and reveals nothing about the kinetics.

Answer: C

Correct answer: That it is a noncompetitive inhibitor

The diagnostic test is whether extra substrate can overcome the inhibition. A competitive inhibitor competes for the active site, so it can be outcompeted; a noncompetitive inhibitor binds elsewhere and disables the enzyme whatever the substrate concentration. Since more substrate made no difference here, the inhibitor is noncompetitive - and \(V_{max}\) will be found to have fallen while \(K_m\) is unchanged.

Answer: B

Correct answer: Raise the Michaelis constant

A competitive inhibitor makes the enzyme appear to have less affinity for its substrate, since some active sites are occupied by the inhibitor at any moment. More substrate is therefore needed to reach half-maximal velocity, so \(K_m\) rises. \(V_{max}\) is unchanged, because a high enough substrate concentration still displaces the inhibitor entirely.

Answer: C

Correct answer: Lowers the maximum rate of the enzymatic reaction

A noncompetitive inhibitor binds at a separate site and disables whatever enzyme molecules it occupies, whatever the substrate concentration. In effect it removes enzyme from the pool, so \(V_{max}\) falls while \(K_m\) stays the same - the molecules still working have the same affinity as before.

Answer: A

Correct answer: A portion is cleaved, activating the enzyme

Pepsinogen is a zymogen: it is secreted inactive, and the stomach’s acid cleaves off a blocking peptide to expose the active site - after which the pepsin produced activates more pepsinogen itself. Secreting a digestive protease in inactive form is what stops it digesting the cell that made it, and the cleavage is irreversible.

Answer: A

Correct answer: RNA acting as enzymes

A ribozyme is a molecule of RNA that acts as a catalyst. Its discovery by Cech and Altman overturned the assumption that all enzymes are proteins, and it matters for another reason: the peptidyl transferase activity of the ribosome is itself a ribozyme, so protein synthesis is catalysed by RNA. That is one of the strongest arguments for an early RNA world.