To quantitatively understand the reactions taking place in the cycle, we describe the following reactions in a kinetic model (See supporting notes Scheme S1-S20 and Table ): the MAP hydrolysis to form inorganic phosphate (Fig. ), the reaction of MAP with histidine yielding 1-pHis or 3-pHis, the reaction of MAP with 1-pHis or 3-pHis to form 1,3-bisphospho-histidine (1,3-bispHis). Each of these activation reactions releases one molecule of ammonia.