The kinetic mechanism of serpin proteinase complex formation - An intermediate between the Michaelis complex and the inhibited complex

Serine proteinase inhibitors (serpins) form enzymatically inactive, 1:1 complexes (denoted E*I*) with their target proteinases that release free enzyme and cleaved inhibitor only very slowly. The mechanism of E*I* formation is incompletely understood and continues to be a source of controversy. Kinetic evidence exists that formation of E*I* proceeds via a Michaelis complex (E . I) and so involves at least two steps. In this paper, we determine the rate of E*I* formation from alpha-chymotrypsin and alpha(1)-antichymotrypsin using two approaches: first, by stopped-flow spectrofluorometric monitoring of the fluorescent change resulting from reaction of alpha-chymotrypsin with a fluorescent derivative of alpha(1)-antichymotrypsin (derivatized at position P7 of the reactive center loop); and second, by a rapid mixing/quench approach and SDS-polyacrylamide gel electrophoresis analysis. In some cases, serpins are both substrates and inhibitors of the same enzyme. Our results indicate the presence of an intermediate between E . I and E*I* and suggest that the partitioning step between inhibitor and substrate pathways precedes P1-P1' cleavage.