Phn. Celie et al., Hydrophobic contact between the two epidermal growth factor-like domains of blood coagulation factor IX contributes to enzymatic activity, J BIOL CHEM, 275(1), 2000, pp. 229-234
The three-dimensional structure of activated factor IX comprises multiple c
ontacts between the two epidermal growth factor (EGF)-like domains. One of
these is a salt bridge between Glu(78) and Arg(94), which is essential for
binding of factor Ma to its cofactor factor VIII and for factor VIII-depend
ent factor X activation (Christophe, O. D., Lenting, P. J., Kolkman, J. A.
Brownlee, G. G., and Mertens, K. (1998) J. Biol. Chem. 273, 222-227). We no
w addressed the putative hydrophobic contact at the interface between the E
GF-like domains, Recombinant factor IX chimeras were constructed in which h
ydrophobic regions Phe(75)-Phe(77) and Lys(106)-Val(108) were replaced by t
he corresponding sites of factor X and factor VII, Activated factor IX/fact
or X chimeras were indistinguishable from normal factor Ma with respect to
factor Ma enzymatic activity. In contrast, factor IXa(75-77)/factor VII dis
played similar to 2-fold increased factor X activation in the presence of f
actor VIII, suggesting that residues 75-77 contribute to cofactor-dependent
factor X activation. Activation of factor X by factor IX106-108/factor VII
was strongly decreased, both in the absence and presence of factor VIII. A
ctivity could be restored by simultaneous substitution of the hydrophobic s
ites in both EGF-like domains for factor VII residues. These data suggest t
hat factor Ma enzymatic activity requires hydrophobic contact between the t
wo EGF-like domains.