Serpin mechanism of action. Fate of the serpin and proteinase complex via the branched pathway. The serpin (I) inhibition of proteinase (E) proceeds via an initial noncovalent, Michaelis-like complex (EI) that involves no conformational change within the proteinase or the body of the serpin. Subsequent peptide bond hydrolysis results in an acyl-enzyme intermediate (EI^#) that progresses to either a kinetically trapped loop-inserted covalent complex (EI⁺, inhibitory pathway) or a cleaved serpin (I*) and free proteinase (noninhibitory or substrate pathway). The serpin body is in yellow. Free serine proteinase is in green and covalently bound proteinase is in red. Reprinted with permission from Cold Spring Harbor Laboratory.