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.