Mechanism of glycine oxidation catalyzed by pyrroloquinoline quinone in aqueous solution

•Two mechanisms, stepwise and concerted, were investigated.•Stepwise mechanism is a nucleophilic attack on C5 or C4 by glycine, and then proton and electron transfer to PQQ.•Concerted mechanism is that proton and electron transfer occur simultaneously, not including a nucleophilic attack.•In stepwise reaction, a nucleophilic attack on C5 and then a proton transfer of H1′ to O5A is likely to occur.•In concerted reaction, a proton transfer of H1′ to O5A or O4A is difficult to occur.