SIMULATIONS OF ELECTRON-TRANSFER IN THE NADPH-BOUND CATALASE FROM PROTEUS-MIRABILIS PR

Catalase-bound NADPH both prevents and reverses the accumulation of co mpound II, an inactive form of catalase that is generated from the nor mal 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 propose d in which the formation of compound II from compound I generates a ne ighboring radical species within the protein. NADPH would then donate two electrons, one to compound II for reduction of the iron and the ot her to the protein free radical. In this paper, we report calculations to find the dominant electron tunneling pathways between NADPH and th e heme iron in the catalase from the peroxide-resistant mutant of Prot eus mirabilis. Two major tunneling pathways are found which fuse toget her 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 rad ical amino acid.