Turnover number

The turnover number (denoted k_cat) is the maximum number of substrate molecules an enzyme converts to product per unit time when the enzyme is fully saturated with substrate. It measures the intrinsic catalytic rate of an enzyme — how fast the catalytic cycle can complete once substrate is bound — independent of binding affinity. A high turnover number means the enzyme processes information quickly: it recognizes, transforms, and releases substrate at high frequency.

Turnover number is one half of the catalytic efficiency equation. An enzyme with extraordinary k_cat but weak substrate binding may be less effective in vivo than one with moderate k_cat and high affinity. The ratio k_cat/K_m captures this trade-off and serves as a measure of how efficiently an enzyme operates at low substrate concentrations — the regime in which most biological systems actually function.