Deciphering the role of the distal pocket in Staphylococcus aureus coproheme decarboxylase.

Coproheme decarboxylase (ChdC) catalyzes the sequential oxidative decarboxylation of coproheme III propionate side chains at positions 2 and 4 to form heme b by activation of two molecules of H2O2 at its substrate's iron center. The coproheme III binding pocket lacks the distal His-Arg pair that polarizes and acts as a catalytic base toward activation of coordinated H2O2 in canonical heme-dependent peroxidases. Instead ChdC from Staphylococcus aureus has a Gln (Q185). This report presents thermodynamic, kinetic, and spectroscopic results that provide comparative insight into how wild type (WT) and Q185A and Q185R variant ChdCs activate H2O2. Reactivities with H2O2 and cyanide affinities at pH 7.5 follow the trend: WT > Q185R > Q185A. Both variants exhibited greater catalase efficiency than WT ChdC. Vibrational resonance Raman signatures of ferric coproheme-CN- and ferrous coproheme-CO complexes of WT, Q185A, and Q185R SaChdCs revealed that the Arg mutation does not significantly alter the distal environment while Q185A has a more open active site. Together these data are consistent with a modest role for Q185 in promoting the decarboxylation reaction. A model for the proton transfer required for H2O2 activation that involves the Gln185 iminol tautomer is presented. The three ChdCs reacted with chlorite to generate harderoheme III and heme b to varying extents. In reaction with chlorite, coproheme III:SaChdC was cleanly converted to harderoheme III:SaChdC, which exhibited vinyl bending and stretching modes at 423 and 1622 cm-1, respectively. Differences in SaChdC reactivity with ClO2- and H2O2 relative to those of chlorite dismutase and peroxidases, respectively, are discussed.