Abstract:
Catalase dismutes H20 2 to O2 and H20. In successive twoelectron
reactions H20 2 induces both oxidation and reduction at
the heme group. In the first step the protoheme prosthetic
group of beef liver catalase forms compound I, in which the
heme has been oxidized from Fe3+ to Fe4+=0 and a porphyrin
radical has been created. Compound II is formed by the oneelectron
reduction of comp I. It retains Fe4+=0 but lacks the
porphyrin radical and is catalytically inert. Molecular
structures are available for Escherichia coli Hydroperoxidase
II, Micrococcus Iysodeiktus, Penicillium vitale and beef liver
enzymes, which contain different hemes and heme pockets.
In the present work, the pockets and substrate access channels
of protoheme (beef liver & Micrococcus) and heme d (HPII of E.
coli and Penicillium) catalases have been analysed using
Quanta™ and CharmMTM molecular modeling packages on the
Silicon Graphics Iris Indigo 2 computer. Experimental studies
have been carried out with two catalases, HPII (and its
mutants) and beef liver. Fluoride and formate' are inhibitors of
both enzymes, and their binding is modulated by the heme and
by distal residues N201 & H128. Both HPII and beef liver
enzymes form compound I with H202 or peracetate. The reduction of beef liver enzyme compound I to II and the decay
of compound II are accelerated by fluoride. The decay of
compound II is also accelerated by formate, and this reagent
acts as a 2-electron donor towards compound I of both
enzymes.
It is concluded that heme d enzymes (Penicillium and HPII of E.
coli) are formed by autocatalytic transformation of protoheme
in a modified pocket which contains a characteristic serine
residue as well as a partially occluded heme channel. They are
less active than protoheme enzymes but also do not form the
inactive compound II species. Binding of peroxide as well as
fluoride and formate is prevented by mutation of H128 and
modulated by mutation of N201.
