Coordination of amino acids by the Fe3+ porphyrin microperoxidase-8: possible role for the invariant Phe in the peroxidase enzymes

Abstract

Equilibrium constants have been determined for the substitution of coordinated H₂O in the Fe³⁺ MP-8 by various amino acids in 20% aqueous MeOH at 25 °C; the presence of and aromatic side chain significantly increases log K from, e.g. Gly 3.5 to Phe 4.8 and Trp 5.6 and causes a shift in the wavelength of the Soret band from 403 to 406 nm, which is ascribed to donor–acceptor interaction between the porphyrin and aromatic/heterocyclic rings and suggests a role for the invariant distal Phe in the peroxidases.