Tyrosyl Radical-Mediated Sequential Oxidative Decarboxylation of Coproporphyrinogen III through PCET: Theoretical Insights into the Mechanism of Coproheme Decarboxylase ChdC.

The peroxide-dependent coproheme decarboxylase ChdC from Geobacillus stearothermophilus catalyzes two key steps in the synthesis of heme b, i.e., two sequential oxidative decarboxylations of coproporphyrinogen III (coproheme III) at propionate groups P2 and P4. In the binding site of coproheme III, P2 and P4 are anchored by different residues (Tyr144, Arg217, and Ser222 for P2 and Tyr113, Lys148, and Trp156 for P4); however, strong experimental evidence supports that the generated Tyr144 radical acts as an unique intermediary for hydrogen atom transfer (HAT) from both reactive propionates. So far, the reaction details are still unclear. Herein, we carried out quantum mechanics/molecular mechanics calculations to explore the decarboxylation mechanism of coproheme III. In our calculations, the coproheme Cpd I, Fe(IV) = O coupled to a porphyrin radical cation (por•+) with four propionate groups, was used as a reactant model. Our calculations reveal that Tyr144 is directly involved in the decarboxylation of propionate group P2. First, the proton-coupled electron transfer (PCET) occurs from Tyr144 to P2, generating a Tyr144 radical, which then abstracts a hydrogen atom from the Cβ of P2. The β-H extraction was calculated to be the rate-limiting step of decarboxylation. It is the porphyrin radical cation (por•+) that makes the PCET from Tyr144 to P2 to be quite easy to initiate the decarboxylation. Finally, the electron transfers from the Cβ• through the porphyrin to the iron center, leading to the decarboxylation of P2. Importantly, the decarboxylation of P4 mediated by Lys148 was calculated to be very difficult, which suggests that after the P2 decarboxylation, the generated harderoheme III intermediate should rebind or rotate in the active site so that the propionate P4 occupies the binding site of P2, and Tyr144 again mediates the decarboxylation of P4. Thus, our calculations support the fact that Tyr144 is responsible for the decarboxylation of both P2 and P4.