The ester exchange reaction on phosphorus of N-phosphorylhistidine with primary alcohol has been studied by MNDO method. Due to the participation of imidazole group, a hexa-coordinate phosphorus intermediate was formed, in which the iso-propoxyl group opposite to the imidazole was more labile than the others and much easier to leave because the activation energy was lowered by 64kJ.mol^-^1. When this iso-propoxyl left, to a new form penta-coordinate phosphorus intermediate which then was to be attacked by an alcohol from the direction opposite to the imidazole group. The mechanism of hexa- coordinate phosphorus intermediate explains the catalytic effect of imidazole.

Key words: HISTIDINE, PHOSPHORYLATION, PHOSPHORUS COMPLEX, IMIDAZOLE, CATALYTIC REACTION, TRANSESTERIFICATION