Figure 1. Diagram of the ubiquitin–proteasome pathway of proteolysis. In an adenosine 5¢ triphosphate (ATP) dependent reaction, ubiquitin (Ub) forms a thiol ester with the E1 ubiquitin-activating enzyme. It is then transferred to an E2 ubiquitin-conjugating enzyme, and then typically to an E3 ubiquitin protein ligase. This enzyme transfers ubiquitin to multiple lysine residues on the target protein and links ubiquitin in polymeric chains. The multi- and polyubiquitinylated target protein enters the center of the proteasome, a multisubunit protease complex, where it is hydrolyzed to oligopeptides and regenerated ubiquitin monomers in an ATP-dependent reaction.

Figure 2. Activation and inactivation of the serine protease factor V. Factor V circulates in the blood as an inactive proenzyme. Upon its cleavage by the serine protease thrombin, it becomes the active serine protease factor Va. This form is in turn inactivated by proteolytic cleavage by the serine protease activated protein C (APC) and its cofactor protein S. The mutation in factor V Leiden allows its activation but slows its inactivation, resulting in an accumulation of factor Va, which promotes a tendency toward excessive blood clotting.