The catalytic efficiency and capacity of an enzyme are described by different kinetic constants. Which parameter represents the number of substrate molecules converted to product per enzyme active site per unit time when the enzyme is fully saturated?

• A kcat (turnover number) ✓
• B Km
• C kcat/Km
• D Hill coefficient

Explanation

The turnover number kcat equals Vmax divided by the total enzyme concentration, giving molecules of substrate transformed per active site per second at saturating substrate. Km reflects the substrate concentration at half-maximal velocity, not a molecular throughput. The ratio kcat/Km measures catalytic efficiency at low substrate concentrations and is the best way to compare two substrates acting on the same enzyme, but it is not the saturated-site rate. The Hill coefficient describes cooperativity of oligomeric enzymes.

Reference: Lehninger Principles of Biochemistry, 8th ed.

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