sentences:  In B. subtilis, PRPP is bound to these two sites with a Kd of 33mM.When orotate is present, pyrophosphate binding affinity is increased fourfold and the reaction undergoes burst kinetics, with rapid phosphoribosyl transfer and then slow release of products.This slow release is thought to be due to the solvent-exposed loop of orotate phosphoribosyltransferase that protects the active site during the first step.