1
Question
The following scheme shows the relationship between an enzyme (E) and its substrate (S), product (P) and an inhibitor (I):
In the above scheme, assume that
(i) increasing the concentration of S increases the activity of the enzyme
(ii) at low substrate concentrations, the presence of I reduces reaction velocity, and
(iii) the same maximum velocity of reaction can be reached in the presence or absence of I.
Which of the following terms best describes what is occurring in pathway B?
The following scheme shows the relationship between an enzyme (E) and its substrate (S), product (P) and an inhibitor (I):
In the above scheme, assume that
(i) increasing the concentration of S increases the activity of the enzyme
(ii) at low substrate concentrations, the presence of I reduces reaction velocity, and
(iii) the same maximum velocity of reaction can be reached in the presence or absence of I.
Which of the following terms best describes what is occurring in pathway B?
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Solution
The correct option is C Competitive inhibition
In pathway B, the phenomena which is taking place is competitive inhibition (keeping in mind all the given characteristics). Competitive inhibitors impair reaction progress by binding to an enzyme, often at the active site, and preventing the real substrate from binding. At any given time, only the competitive inhibitor or the substrate can be bound to the enzyme (not both). That is, the inhibitor and substrate compete for the enzyme. Competitive inhibition acts by decreasing the number of enzyme molecules available to bind the substrate.
As indicated in statement I, competitive inhibitors can be considered structural analogues of the substrate, and thus compete for the same active binding sites on the enzyme. Because of this competition, if enough substrate is provided, the effect of the competitive inhibitor can be overcome (i.e., the substrate will ultimately occupy all binding sites).
As indicated in statement (ii) and (iii), with a competitive inhibitor, the reaction can eventually reach its normal Vmax, but it takes a higher concentration of substrate to get it there. In other words, Vmax is unchanged as Vmax depends on enzyme concentration, but the apparent Km is higher. A higher Km corresponds to a lower affinity for the substrate. Hence, the extra substrate makes the substrate molecules abundant enough to consistently “beat” the inhibitor molecules to the enzyme.
Thus, a competitive inhibitor does not change the Vmax (velocity) of an enzyme. It does raise the Km (substrate concentration at which velocity is half the maximum) of an enzyme since higher concentrations of substrate would be required to achieve half-maximal activity.
In pathway B, the phenomena which is taking place is competitive inhibition (keeping in mind all the given characteristics). Competitive inhibitors impair reaction progress by binding to an enzyme, often at the active site, and preventing the real substrate from binding. At any given time, only the competitive inhibitor or the substrate can be bound to the enzyme (not both). That is, the inhibitor and substrate compete for the enzyme. Competitive inhibition acts by decreasing the number of enzyme molecules available to bind the substrate.
As indicated in statement I, competitive inhibitors can be considered structural analogues of the substrate, and thus compete for the same active binding sites on the enzyme. Because of this competition, if enough substrate is provided, the effect of the competitive inhibitor can be overcome (i.e., the substrate will ultimately occupy all binding sites).
As indicated in statement (ii) and (iii), with a competitive inhibitor, the reaction can eventually reach its normal Vmax, but it takes a higher concentration of substrate to get it there. In other words, Vmax is unchanged as Vmax depends on enzyme concentration, but the apparent Km is higher. A higher Km corresponds to a lower affinity for the substrate. Hence, the extra substrate makes the substrate molecules abundant enough to consistently “beat” the inhibitor molecules to the enzyme.
Thus, a competitive inhibitor does not change the Vmax (velocity) of an enzyme. It does raise the Km (substrate concentration at which velocity is half the maximum) of an enzyme since higher concentrations of substrate would be required to achieve half-maximal activity.