DFT calculations suggest a new type of self-protection and self-inhibition mechanism in the mammalian heme enzyme myeloperoxidase: nucleophilic addition of a functional water rather than one-electron reduction.

The mammalian heme enzyme myeloperoxidase (MPO) catalyzes the reaction of Cl(-) to the antimicrobial-effective molecule HOCl. During the catalytic cycle, a reactive intermediate "Compound I" (Cpd I) is generated. Cpd I has the ability to destroy the enzyme. Indeed, in the absence of any substrate, Cpd I decays with a half-life of 100 ms to an intermediate called Compound II (Cpd II), which is typically the one-electron reduced Cpd I. However, the nature of Cpd II, its spectroscopic properties, and the source of the additional electron are only poorly understood. On the basis of DFT and time-dependent (TD)-DFT quantum chemical calculations at the PBE0/6-31G* level, we propose an extended mechanism involving a new intermediate, which allows MPO to protect itself from self-oxidation or self-destruction during the catalytic cycle. Because of its similarity in electronic structure to Cpd II, we named this intermediate Cpd II'. However, the suggested mechanism and our proposed functional structure of Cpd II' are based on the hypothesis that the heme is reduced by charge separation caused by reaction with a water molecule, and not, as is normally assumed, by the transfer of an electron. In the course of this investigation, we found a second intermediate, the reduced enzyme, towards which the new mechanism is equally transferable. In analogy to Cpd II', we named it Fe(II'). The proposed new intermediates Cpd II' and Fe(II') allow the experimental findings, which have been well documented in the literature for decades but not so far understood, to be explained for the first time. These encompass a) the spontaneous decay of Cpd I, b) the unusual (chlorin-like) UV/Vis, circular dichroism (CD), and resonance Raman spectra, c) the inability of reduced MPO to bind CO, d) the fact that MPO-Cpd II reduces SCN(-) but not Cl(-), and e) the experimentally observed auto-oxidation/auto-reduction features of the enzyme. Our new mechanism is also transferable to cytochromes, and could well be viable for heme enzymes in general.