Michaelis-Menten Calculator

The reaction velocity is v = Vmax × [S] ÷ (Km + [S]). At [S] = Km the enzyme runs at exactly half of Vmax; far above Km it approaches Vmax and more substrate stops helping. Km and [S] must share one concentration unit — the velocity comes out in the unit of Vmax.

An enzyme has no single speed. It runs at whatever rate the substrate on offer allows, and this page gives that rate for one particular offer. The equation splits cleanly in two: Vmax sets the scale, and the ratio of [S] to Km decides what fraction of that scale you get. The defaults show it. With Km = 2 mM and [S] = 5 mM the substrate sits at 2.5 times Km, so the enzyme runs at 2.5 ÷ 3.5 = 0.7143 of full speed — against a Vmax of 100 µmol/min, 71.4286 µmol/min. Any other pair with the same ratio yields the same fraction, which is why Km and [S] only ever have to match each other and never Vmax. What the result does not contain is the two numbers it was built from. Km and Vmax are measured, and measured afresh for every enzyme, substrate, temperature and pH; the page stores neither, because there is no single correct value to store. You bring them, usually from a fitted curve of your own, and the velocity is only as good as that fit. The equation then assumes a steady state: one substrate, a complex forming and breaking down at a constant level, nothing inhibiting. Two substrates, an allosteric enzyme or a competitive inhibitor all bend the same curve, and the arithmetic here is then the wrong tool rather than an imprecise one.