Molecular mechanism of chorismate mutase activity of promiscuos MbtI

Salicylate synthase from Mycobacterium tuberculosis, MbtI, initiates the biosynthesis of siderophores by converting chorismate to salicylate. Nevertheless, three more distinct activities for wild-type MbtI have been detected in vitro: isochorismate synthase, isochorismate pyruvate lyase, and chorismate mutase. In this work, hybrid Quantum Mechanics/Molecular Mechanics methods have been used to get the first simulation of the chorismate mutase activity of MbtI. The results show how the reaction proceeds by means of a [3,3] sigmatropic rearrangement with free energy barrier in very good agreement with experiments. From an analysis of the averaged structures, we show that the lower chorismate mutase activity of MbtI with respect to BsCM is reflected in the lesser diaxial character of reactants in the active site. This information was used to propose the I207F mutation. The resulting free energy of activation would represent an enhancement of the rate constant by a factor of 7 at 310 K