Structures of active conformations of UMP kinase from Dictyostelium discoideum suggest phosphoryl transfer is associative.

UMP/CMP kinase from Dictyostelium discoideum (UmpKdicty) catalyzes the specific transfer of the terminal phosphate of ATP to UMP or CMP. Crystal structures of UmpKdicty with substrates and the transition state analogs AlF3 or BeF2 that lock UmpKdicty in active conformations were solved. The positions of the catalytic Mg2+ and the highly conserved lysine of the P loop are virtually invariant in the different structures. In contrast, catalytic arginines move to stabilize charges that develop during this reaction. The location of the arginines indicates formation of negative charges during the reaction at the transferred phosphoryl group, but not at the phosphate bridging oxygen atoms. This is consistent with an associative phosphoryl transfer mechanism but not with a dissociative one.