Spectroscopic evidence of the effect of hydrogen peroxide excess on the coproheme decarboxylase from actinobacterial Corynebacterium diphtheriae

The actinobacterial coproheme decarboxylase from Corynebacterium diphtheriae catalyzes the final reaction to generate heme b via the "coproporphyrin-dependent" heme biosynthesis pathway in the presence of hydrogen peroxide. The enzyme has a high reactivity toward H2O2 used for the catalytic reaction and in the presence of an excess of H2O2 new species are generated. Resonance Raman data, together with electronic absorption spectroscopy and mass spectrometry, indicate that an excess of hydrogen peroxide for both the substrate (coproheme) and product (heme b) complexes of this enzyme causes a porphyrin hydroxylation of ring C or D, which is compatible with the formation of an iron chlorin-type heme d species. A similar effect has been previously observed for other heme-containing proteins, but this is the first time that a similar mechanism is reported for a coproheme enzyme. The hydroxylation determines a symmetry lowering of the porphyrin macrocycle, which causes the activation of A2g modes upon Soret excitation with a significant change in their polarization ratios, the enhancement and splitting into two components of many Eu bands, and an intensity decrease of the non-totally symmetric modes B1g, which become polarized. This latter effect is clearly observed for the isolated ν10 mode upon either Soret or Q-band excitations. The distal His118 is shown to be an absolute requirement for the conversion to heme d. This residue also plays an important role in the oxidative decarboxylation, because it acts as a base for deprotonation and subsequent heterolytic cleavage of hydrogen peroxide.