Quantum Chemical Study on the Reaction Mechanism of 2,5-Furandicarboxylic Acid Decarboxylase.

2,5-Furandicarboxylic acid decarboxylase (HmfF) belongs to the UbiD enzyme family, which employs the cofactor prenylated FMN (prFMN) for catalysis. This enzyme catalyzes the reversible decarboxylation of 2,5-furandicarboxylic acid (FDCA) to produce 2-furancarboxylic acid (F2C). In the present study, quantum chemical calculations are employed to investigate the substrate binding mode and reaction mechanism of HmfF. The calculations demonstrate that HmfF follows a nucleophilic attack mechanism, rather than the 1,3-dipolar cycloaddition mechanism, which was believed to be more commonly adopted by the prFMN-dependent decarboxylases. Interestingly, the five-membered heterocyclic intermediate characteristic of 1,3-dipolar cycloaddition can also be located. However, it is calculated to be only a fleeting intermediate that does not contribute to the catalysis. In the proposed mechanism of HmfF, the reaction initiates with a single C-C bond formation between FDCA and prFMN. Then, the C-C bond between the carboxylate and the furan group of FDCA breaks to release CO2, which is followed by a proton transfer from Glu259 to the decarboxylated intermediate, and the subsequent C-C bond cleavage to generate the F2C product. Additionally, the infeasibility of HmfF in promoting the decarboxylation of F2C is evaluated computationally, and the obtained information is helpful in designing mutations to enable this reactivity.