QM/MM theoretical study of the pentacoordinate Mn(III) and resting states of manganese-reconstituted cytochrome P450(cam).

Quantum mechanical/molecular mechanical (QM/MM) theoretical calculations were performed for the pentacoordinate Mn(III) and water-bound resting states of the Mn-reconstituted mutant of cytochrome P450(cam) (Mn-P450(cam)) in order to obtain insights into their characters, especially, their spin state ordering. The QM/MM study was carried out by use of the B3LYP and BLYP density functional theory (DFT) methods coupled to the CHARMM force field. Although the relative energies of possible spin states for the Mn-P450(cam) species varied depending on the functional, this dependence was less significant compared with previous calculations on the corresponding intermediates of wild-type P450(cam). The results suggested that both Mn-P450(cam) intermediates have quintet ground states. Additional time-dependent DFT (TDDFT) calculations were carried out for the quintet states of these species using the B3LYP and BP86 functionals with the electrostatic environmental effect included. The TDDFT results enabled us to assign the origins of the peaks observed in optical absorption spectra (Makris et al. in J. Inorg. Biochem. 100:507-518, 2006).