Simulations of electron transfer in the NADPH-bound catalase from Proteus mirabilis PR.

Catalase-bound NADPH both prevents and reverses the accumulation of compound II, an inactive form of catalase that is generated from the normal active intermediate form (compound I) when catalase is exposed to a steady flow of hydrogen peroxide. The mechanism for the regeneration reaction is unknown although NADPH could act either as a one-electron or a two-electron donor. Recently, a reaction scheme has been proposed in which the formation of compound II from compound I generates a neighboring radical species within the protein. NADPH would then donate two electrons, one to compound II for reduction of the iron and the other to the protein free radical. In this paper, we report calculations to find the dominant electron tunneling pathways between NADPH and the heme iron in the catalase from the peroxide-resistant mutant of Proteus mirabilis. Two major tunneling pathways are found which fuse together on Ser-196. It is suggested that the sequence Gly-Ser of the loop that divides the beta 5-strand is the key element for shielding a radical amino acid.