inhibition constant (Ki)
The inhibition constant, Ki, measures how tightly an inhibitor binds to its target. It is the counterpart of Kd for a molecule whose job is to get in the way — an antagonist at a receptor or an inhibitor of an enzyme. A low Ki means the inhibitor binds very tightly and is effective at low concentrations; a high Ki means it binds weakly.
Ki is usually determined indirectly, by measuring how much of an inhibitor it takes to displace a labelled reference ligand or to slow an enzyme, and then correcting for the conditions of the experiment. This correction is what makes Ki more fundamental than the raw IC50 (the concentration giving half-maximal inhibition), because IC50 shifts with the amount of competing substrate or ligand present, whereas Ki aims to describe the inhibitor's intrinsic affinity.
Ki lets scientists compare inhibitors on a level playing field and rank candidate drugs by binding strength. The honest caveat is that a tight-binding inhibitor (low Ki) is not automatically a good drug: it must still reach its target in the body, act selectively, and be safe — properties that Ki, a pure binding measure, cannot capture.
When screening enzyme inhibitors, chemists rank candidates by Ki, prioritizing those with the lowest values for the most potent binding.
Ranking inhibitors by Ki.
Ki is the inhibitor analogue of Kd; both express affinity as a concentration, with lower values meaning tighter binding. Ki is derived from, but conceptually cleaner than, IC50.