Half-of-the-sites and all-of-the-sites reactivity in human plasma blood coagulation factor XIIIa.

The reactivities of human plasma factor XIIIa toward iodoacetic acid and toward alpha-bromo-4-hydroxy-3-nitroacetophenone have been studied under conditions where this dimeric enzyme reacts with the reagents in half-of-the-sites fashion and under conditions where it reacts with the reagents in all-of-the-sites fashion. Direct measurements of alkylation of active site -- SH groups in the apparently identical subunits of the enzyme as functions of remaining catalytic activity are in agreement with the observed reactivities. In addition to extending earlier evidence for half-of-the-sites reactions in factor XIIIa (Chung, S. I., Lewis, M. S., and Folk, J. E. (1974) J. Biol. Chem. 249, 940-950), the present findings suggest that the all-of-the-sites reactivity results from a positively cooperative interaction between enzyme subunits.