Regions 301-303 and 333-339 in the catalytic domain of blood coagulation factor IX are factor VIII-interactive sites involved in stimulation of enzyme activity.

The contribution of the Factor IX catalytic domain to Factor VIIIa binding has been evaluated by functional analysis of Factor IX variants with substitutions in alpha-helix region 333-339 and region 301-303. These regions were found to play a prominent role in Factor VIIIa-dependent stimulation of Factor X activation, but do not contribute to the high-affinity interaction with Factor VIIIa light chain. We propose that complex assembly between Factor IXa and Factor VIIIa involves multiple interactive sites that are located on different domains of these proteins.