Mechanistic insights into the catalytic reaction of plant allene oxide synthase (pAOS) via QM and QM/MM calculations

Abstract

QM cluster and QM/MM protein models have been employed to understand aspects of the reaction mechanism of plant allene oxide synthase (pAOS). In this study we have investigated two reaction mechanisms for pAOS. The standard pAOS mechanism was contrasted with an alternative involving an additional active site molecule which has been shown to facilitate proton coupled electron transfer (PCET) in related systems. Firstly, we found that the results from QM/MM protein model are comparable with those from the QM cluster model, presumably due to the large active site used. Furthermore, the results from the QM cluster model show that the FeIII and FeIV pathways for the standard mechanism have similar energetic and structural properties, indicating that the reaction mechanism may well proceed via both pathways. However, while the PCET process is facilitated by an additional active site bound water in other related families, in pAOS it is not, suggesting this type of process is not general to all closely related family members.