Siroheme Decarboxylase in Complex with Iron‐Uroporphyrin III

Abstract

Siroheme decarboxylase catalyzes the conversion of siroheme to 12,18‐didecarboxysiroheme during heme d 1 biosynthesis in denitrifying bacteria and during the alternative heme biosynthesis in archaea and sulfate‐reducing bacteria. The crystal structure of siroheme decarboxylase from Hydrogenobacter thermophilus in complex with the substrate analog iron‐uroporphyrin III is available as well as the structure of siroheme decarboxylase from Desulfovibrio desulfuricans in complex with the reaction product didecarboxysiroheme. The two structures reveal two possible binding modes for the iron‐containing tetrapyrroles. In both cases, the central iron ion is coordinated by a histidine residue as the axial ligand, however, these histidines are located at different positions of the enzyme. Moreover, although identical amino acid residues coordinate the substrate analog and reaction product, the respective equivalent residues interact with different parts of the respective tetrapyrrole macrocycle. The two observed tetrapyrrole binding modes might hint toward two different binding sites for the initial substrate, siroheme, and the reaction intermediate monodecarboxysiroheme.