Q.13 The catalytic efficiency for an enzyme is defined as
Catalytic efficiency measures how effectively an enzyme turns substrate into product. The correct definition is kcat / Km.
Correct Answer
The catalytic efficiency for an enzyme is kcat / Km. This ratio, known as the specificity constant, indicates how well an enzyme performs at low substrate concentrations by combining turnover rate with substrate affinity.
Option Breakdown
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kcat: Represents the turnover number, or maximum substrate molecules converted per enzyme per second at saturation. It measures catalytic speed but ignores substrate binding efficiency.
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Vmax / kcat: Vmax is maximum reaction velocity, directly proportional to kcat and enzyme concentration (Vmax = kcat × [E]). This ratio equals enzyme concentration, not efficiency.
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kcat / Km: Correct choice. kcat shows reaction speed; Km reflects substrate concentration for half Vmax (lower Km means higher affinity). Their ratio quantifies efficiency, especially under non-saturating conditions.
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kcat / Vmax: Inverts the prior option, yielding 1/[E], which depends on enzyme amount rather than intrinsic performance.
