Enzyme kinetics — If one enzyme has Km = 0.50 M and another has Km = 0.05 M for the same substrate, the enzyme with Km = 0.50 M has __________ affinity for the substrate.

Introduction / Context:
Michaelis–Menten kinetics relate reaction velocity to substrate concentration via two parameters: Vmax and Km. The Km is the substrate concentration at which the reaction rate is half of Vmax and is inversely related to enzyme–substrate affinity under classic assumptions. Comparing Km values helps infer which enzyme binds substrate more tightly.

Given Data / Assumptions:

• Both enzymes act on the same substrate and follow Michaelis–Menten behavior.
• Lower Km implies higher apparent affinity (ES complex forms effectively at lower [S]).
• We compare 0.50 M versus 0.05 M.

Concept / Approach:
At lower Km, the enzyme reaches half-maximal velocity at a lower substrate concentration, indicating stronger binding (higher affinity). Conversely, a higher Km reflects weaker binding. Therefore, the enzyme with Km = 0.50 M has lesser affinity than the one with Km = 0.05 M.

Step-by-Step Solution:

Verification / Alternative check:
Plotting v vs. [S] for both Km values shows the 0.05 M enzyme achieving higher fractional velocity at any given low [S], indicating greater affinity.

Why Other Options Are Wrong:

• Greater/approximately equal/half: contradict the inverse Km–affinity relationship.

Common Pitfalls:
Assuming Km is a constant measure of binding affinity in all cases; while often correlated, complex mechanisms can decouple Km from true Kd. For simple cases, the inverse relationship is a useful heuristic.

Final Answer:
lesser