Jl. Primus et al., The effect of iron to manganese substitution on microperoxidase 8 catalysed peroxidase and cytochrome P450 type of catalysis, J BIOL I CH, 4(3), 1999, pp. 274-283
This study describes the catalytic properties of manganese microperoxidase
8 [Mn(III)MP8] compared to iron microperoxidase 8 [Fe(III)MP8]. The mini-en
zymes were tested for pH-dependent activity and operational stability in pe
roxidase-type conversions, using 2-methoxyphenol and 3,3'-dimethoxybenzidin
e, and in a cytochrome P450-like oxygen transfer reaction converting:anilin
e to pam-aminophenol. For the peroxidase type of conversions the Fe to Mn r
eplacement resulted in a less than 10-fold decrease in the activity at opti
mal pH, whereas the aniline para-hydroxylation is reduced at least 30-fold.
In addition it was observed that the peroxidase type of conversions are al
l fully blocked by ascorbate and that aniline parahydroxylation by Fe(III)M
P8 is increased by ascorbate whereas aniline para-hydroxylation by Mn(III)M
P8 is inhibited by ascorbate. Altogether these results indicate that differ
ent types of reactive metal oxygen intermediates are involved in the variou
s conversions. Compound I/II, scavenged by ascorbate, may be the reactive s
pecies responsible for the peroxidase reactions, the polymerization of anil
ine and (part of) the oxygen transfer to aniline in the absence of ascorbat
e. The para-hydroxylation of aniline by Fe(III)MP8, in the presence of asco
rbate, must be mediated by another reactive iron-ore species which could be
the electrophilic metal(III) hydroperoxide anion of microperoxidase 8 [M(I
II)OOH MP8]. The lower oxidative potential of Mn, compared to Fe, may affec
t the reactivity of both compound I/II and the metal(III) hydroperoxide ani
on intermediate, explaining the differential effect of the Fe to Mn substit
ution on the pH-dependent behavior, the rate of catalysis and the operation
al stability of MP8.