Factor IX Activation by Factor XIa Proceeds without Release of a Free Intermediate

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Abstract
Factor IX activation by factor XIa is thought to proceed through the singly-cleaved free intermediate, factor IXα. However, we observed no intermediate development during factor IX activation by factor XIa when using a low substrate to enzyme ratio (44:1 mol/mol). This result can be explained by one of two mechanisms: (1) factor XIa-catalyzed activation proceeds via a singly-cleaved free intermediate with a much higher efficiency of cleavage than factor IX zymogen, or (2) the reaction occurs without free intermediate generation, whereby factor XIa makes both proteolytic cleavages in a single substrate molecule before releasing the final product (processive mechanism). We compared the factor XIa cleavage rates of free factor IXα and factor IXaα with that of factor IX zymogen. In contrast to the requirements of mechanism (1), the cleavage rate constants of factor IX zymogen, factor IXα, and factor IXaα were similar: 0.38 ± 0.02 s-1, 0.34 ± 0.05 s-1, and 0.27 ± 0.01 s-1, respectively. It seems likely that factor XIa-generated intermediates observed under some reaction conditions are produced through the occasional failure of a processive mechanism. Indeed, in reactions using a high substrate to enzyme ratio (1900:1 mol/mol), we observed some factor IXα development; however, the pattern of intermediate and product development over time was inconsistent with a mechanism involving an obligate intermediate. Rather, it corresponded to behavior expected from a processive mechanism undergoing a consistent low failure. We conclude that factor XIa-catalyzed activation of factor IX proceeds via a processive mechanism without release of a free intermediate.