Modified Microperoxidases Exhibit Different Reactivity Towards Phenolic Substrates

The reactivity of several microperoxidase derivatives with different
distal-site environments has been studied. The distal-site environments
of these heme peptides include a positively charged
one, an uncharged environment, two bulky and doubly or triply
positively charged ones, and one containing aromatic apolar residues.
The reactivity in the catalytic oxidation of two representative
phenols, carrying opposite charges, by hydrogen peroxide
has been investigated. This allows the determination of the binding
constants and of the electron-transfer rate from the phenol
to the catalyst in the substrate/microperoxidase complex. The
electron-transfer rates scarcely depend on the redox and charge
properties of the phenol, but depend strongly on the microperoxidase.
Information on the disposition of the substrate in the adducts
with the microperoxidases has been obtained through determination
of the paramagnetic contribution to the 1H NMR relaxation
rates of the protons of the bound substrates. The data
show that the electron-transfer rate drops when the substrate
binds too far away from the iron and that the phenols bind to
microperoxidases at similar distances to those observed with peroxidases.
While the reaction rate of microperoxidases with peroxide
is significantly smaller than that of the enzymes, the efficiency
in the one-electron oxidation of phenolic substrates is almost
comparable. Interestingly, the oxyferryl form of the triply positively
charged microperoxidases shows a reactivity larger than that
exhibited by horseradish peroxidase