The dioxygen activation catalyzed by 4-hydorxylphenyl pyruvate dioxygenase(HPPD)were reinvestigated by using hybrid quantum mechanics/molecular mechanics(QM/MM)approaches at the B3LYP/6-311++G(d,p):AMBER level.These studies showed that this reaction consisted of two steps including the dioxygen addition/decarboxylation and hetero O-O bond cleavage,where the first step was found to be rate-determining.The former step initially runs on a septet potential energy surface(PES),then switches to a quintet PES after crossing a septet/quintet minimum energy crossing point(MECP)5-7M2,whereas the rest step runs on the quintet PES.The reliability of our theoretical predictions is supported by the excellent agreement of the calculated free-energy barrier value of 16.9 kcal/mol with available experimental value of 16-17 kcal/mol.The present study challenges the widely accepted view which holds that the O2activation catalyzed byα-keto glutamate(α-KG)dioxygenase mainly runs on the quintet PES and provides new insight into the catalytic mechanism ofα-KG dioxygenase and/or other related Fe(Ⅱ)-dependent oxygenase.
