Protein portion of an enzyme when it is combined with organic molecule is termed as
Answer: A
Correct answer: apoenzyme
The protein part alone is the apoenzyme, and it is catalytically inactive. Combined with its coenzyme or cofactor it becomes the complete, working holoenzyme.
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Which of the following choices describes a way to graphically determine the type of inhibition being displayed by an inhibitor?
I. Plot initial reaction rate versus the concentration of substrate for the uninhibited enzyme, and then compare to the inhibited enzyme
II. Plot the inverse of the initial reaction rate versus the inverse of the substrate concentration for the uninhibited enzyme, and then compare to the inhibited enzyme
III. Plot the concentration of the inhibitor versus the concentration of substrate
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.
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Which of the following enzymes catalyze the ADP-ribosylation of key cellular enzymes or proteins?
Answer: A
Correct answer: Diphtheria toxin and cholera toxin
Both toxins work by ADP-ribosylating a host protein. Diphtheria toxin modifies elongation factor 2 and so halts protein synthesis; cholera toxin modifies a G protein so that adenylate cyclase is locked on, cyclic AMP floods the cell, and the gut pours out water. In each case one covalent modification of a single protein produces the whole disease.
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Normal blood pH is
Answer: C
Correct answer: 7.4
Arterial blood is held at pH 7.4, within a very narrow range of about 7.35 to 7.45. It is defended by the bicarbonate buffer system, by the lungs adjusting CO2, and by the kidneys excreting acid. Below 7.35 is acidosis and above 7.45 alkalosis; outside roughly 6.8 to 7.8, life cannot continue.
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Feedback inhibition means
Answer: B
Correct answer: End Product inhibition
Feedback inhibition is the inhibition of the first enzyme of a pathway by its end product. As product accumulates it switches the pathway off; as it is consumed the inhibition lifts. Acting on the first, committed step means nothing is wasted on intermediates that would only pile up.
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The active form of enzyme is
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.
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How is pepsinogen activated in the stomach?
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.
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The allosteric inhibitor of an enzyme ___________
Answer: C
Correct answer: Participates in feedback regulation
An allosteric inhibitor binds at a regulatory site, not the active site, and changes the enzyme’s shape so that catalysis slows. Typically the inhibitor is the end product of the pathway, so that when enough has accumulated the first enzyme is switched off - which is exactly what feedback regulation means. Nothing is denatured, and the effect is reversible.
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Which of the following is not a coenzyme?
Answer: D
Correct answer: Mn++
Mn2+ is a metal ion, and metal ions are cofactors, not coenzymes - a coenzyme is by definition an organic molecule. NAD, NADP and FAD are all organic, all derived from vitamins, and all coenzymes.
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An enzyme acts best at a particular temperature, called the
Answer: C
Correct answer: Optimum temperature
The optimum temperature is where activity is greatest - the balance between two opposing effects. Warming speeds molecular motion and so the reaction, but beyond a point it begins to denature the protein, and activity falls away sharply. For most human enzymes the optimum is near 37°C.
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