The mechanism of glycine oxidation reaction by PQQ in aqueous solution was investigated. A new crystal structure of PQQ under alkaline conditions was referred to calculate on a quantum chemical basis. Two mechanisms are investigated by this calculation: a 'stepwise' mechanism, namely, a nucleophilic attack on C5 or C4 by the nitrogen atom of glycine, and proton and electron transfer to PQQQ. The second mechanism is a 'concerted' mechanism, namely, simultaneous reaction, which does not include the nucleophilic attack and include proton and electron transfer to PQQ. (C) 2014 Elsevier B.V. All rights reserved.