Among the tightest known protein–protein complexes is that between the enzyme angiogenin and ribonuclease inhibitor; the dissociation constant for the human proteins is 5x10−16 mol/L.[3][4] Another biological example is the binding protein streptavidin, which has extraordinarily high affinity for biotin (vitamin B7/H, dissociation constant, Kd ≈10−14 mol/L).[5] In such cases, if the reaction conditions change (e.g., the protein moves into an environment where biotin concentrations are very low, or pH or ionic conditions are altered), the reverse reaction can be promoted.