Will P450cam Hydroxylate or Desaturate Alkanes? QM and QM/MM Studies

The hydroxylation versus desaturation for the enzyme P450(cam) is addressed by comparing the reactivity of the active species Por(center dot+)(IV)=O (Cpd I) toward cyclohexane (CH), camphor (CAM), and cyclohexene (CHE). The quantum mechanics (QM)-only calculations, which reveal protein-free trends, show mixed and nonselective hydroxylation/desaturation activities, branching from the PorFe(IV)OH/R center dot intermediates. By contrast, the quantum mechanics/molecular mechanics (QM/MM) results with CAM and CHE show exclusive alcohol formation. Two distinct modes by which the protein controls the hydroxylation/desaturation selectivity were identified: (a) with the native substrate CAM, the tight binding site of the P450(cam) protein prevents the second hydrogen abstraction and leads to exclusive C-5-H hydroxylation, and (b) with the freely tumbling CHE, the protein stabilizes the polarizable electromers, Por(center dot+)Te(III)OH/R center dot, which possess intrinsic hydroxylase preference. The latter mechanism is common for substrates that are not tightly bound. It is a unique mechanism to P450 Cpd I, which possesses the Por(center dot+)-Fe(III)OH electromers that dominate the in-protein reactivity. This is contrasted with nonheme enzymes, which lack such electromers.