A protein fold with multiple functions: Chlorite dismutase, HemQ and DyP-type peroxidase

Redox Cofactor Rotates during Its Stepwise Decarboxylation: Molecular Mechanism of Conversion of Coproheme to Heme b

Coproheme decarboxylase (ChdC) catalyzes the last step in the heme biosynthesis pathway of monoderm bacteria with coproheme acting both as redox cofactor and substrate. Hydrogen peroxide mediates the stepwise decarboxylation of propionates 2 and 4 of coproheme. Here we present the crystal structures of coproheme-loaded ChdC from Listeria monocytogenes (LmChdC) and the three-propionate intermediate, for which the propionate at position 2 (p2) has been converted to a vinyl group and is rotated by 90° compared to the coproheme complex structure. Single, double, and triple mutants of LmChdC, in which H-bonding interactions to propionates 2, 4, 6, and 7 were eliminated, allowed us to obtain the assignment of the coproheme propionates by resonance Raman spectroscopy and to follow the H2O2-mediated conversion of coproheme to heme b. Substitution of H2O2 by chlorite allowed us to monitor compound I formation in the inactive Y147H variant which lacks the catalytically essential Y147. This residue was demonstrated to be oxidized during turnover by using the spin-trap 2-methyl-2-nitrosopropane. Based on these findings and the data derived from molecular dynamics simulations of cofactor structures in distinct poses, we propose a reaction mechanism for the stepwise decarboxylation of coproheme that includes a 90° rotation of the intermediate three-propionate redox cofactor.

The hydrogen bonding network of coproheme in coproheme decarboxylase from Listeria monocytogenes: Effect on structure and catalysis

Coproheme decarboxylase (ChdC) catalyzes the oxidative decarboxylation of coproheme to heme b, i.e. the last step in the recently described coproporphyrin-dependent pathway. Coproheme decarboxylation from Listeria monocytogenes is a robust enzymatic reaction of low catalytic efficiency. Coproheme acts as both substrate and redox cofactor activated by H₂O₂. It fully depends on the catalytic Y147 close to the propionyl group at position 2. In the present study we have investigated the effect of disruption of the comprehensive and conserved hydrogen bonding network between the four propionates and heme cavity residues on (i) the conformational stability of the heme cavity, (ii) the electronic configuration of the ferric redox cofactor/substrate, (iii) the binding of carbon monoxide and, (iv) the decarboxylation reaction mediated by addition of H₂O₂. Nine single, double and triple mutants of ChdC from Listeria monocytogenes were produced in E. coli. The respective coproheme- and heme b-complexed proteins were studied by UV–Vis, resonance Raman, circular dichroism spectroscopy, and mass spectrometry. Interactions of propionates 2 and 4 with residues in the hydrophobic cavity are crucial for maintenance of the heme cavity architecture, for the mobile distal glutamine to interact with carbon monoxide, and to keep the heme cavity in a closed conformation during turnover. By contrast, the impact of substitution of residues interacting with solvent exposed propionates 6 and 7 was negligible. Except for Y147A and K151A all mutant ChdCs exhibited a wild-type-like catalytic activity. The findings are discussed with respect to the structure-function relationships of ChdCs.