We present the results of combined molecular dynamics and full-quantum calculations aimed at elucidating the role of residue Thr122 of the enzyme methylamine dehydrogenase. Calculations were performed on the native structure and the T122A mutant. We found that the presence of Thr122 has a deleterious effect on the proton transfer step that is proposed to determine the rate of the reaction. Besides, at the PM3 level, the substitution of Thr122 by Ala does not significantly modify the preference of the proton by atom OD2 of Asp76. Transmission coefficients obtained form MP2/6-31G(d, p)//PBE/DZP minimum energy paths show that proton tunneling is significant. (c) 2008 Elsevier B.V. All rights reserved.