Catalase dismutes H20 2 to O2 and H20. In successive twoelectronreactions H20 2 induces both oxidation and reduction atthe heme group. In the first step the protoheme prostheticgroup of beef liver catalase forms compound I, in which theheme has been oxidized from Fe3+ to Fe4+=0 and a porphyrinradical has been created. Compound II is formed by the oneelectronreduction of comp I. It retains Fe4+=0 but lacks theporphyrin radical and is catalytically inert. Molecularstructures are available for Escherichia coli HydroperoxidaseII, Micrococcus Iysodeiktus, Penicillium vitale and beef liverenzymes, which contain different hemes and heme pockets.In the present work, the pockets and substrate access channelsof protoheme (beef liver & Micrococcus) and heme d (HPII of E.coli and Penicillium) catalases have been analysed usingQuanta™ and CharmMTM molecular modeling packages on theSilicon Graphics Iris Indigo 2 computer. Experimental studieshave been carried out with two catalases, HPII (and itsmutants) and beef liver. Fluoride and formate' are inhibitors ofboth enzymes, and their binding is modulated by the heme andby distal residues N201 & H128. Both HPII and beef liverenzymes form compound I with H202 or peracetate. The reduction of beef liver enzyme compound I to II and the decayof compound II are accelerated by fluoride. The decay ofcompound II is also accelerated by formate, and this reagentacts as a 2-electron donor towards compound I of bothenzymes.It is concluded that heme d enzymes (Penicillium and HPII of E.coli) are formed by autocatalytic transformation of protohemein a modified pocket which contains a characteristic serineresidue as well as a partially occluded heme channel. They areless active than protoheme enzymes but also do not form theinactive compound II species. Binding of peroxide as well asfluoride and formate is prevented by mutation of H128 andmodulated by mutation of N201.
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