New insights into the catalytic mechanism of aldose reductase: a QM/MM study

Aldose reductase is the first enzyme of the polyol pathway in which glucose is converted to fructose via sorbitol. The understanding of this key enzyme is important as it has been linked to some diabetes mellitus complications. The mechanism of the enzyme was investigated using a hybrid quantum mechanics/molecular mechanics (QM/MM) method. It was found that depending on the protonation state of His110 the mechanism can be concerted or stepwise and the proton donor can be either Tyr48 or His110. These findings are different from the previous theoretical studies based on QM/MM calculations using either AM1 or HF/4-31G, in which the reduction is, respectively, a stepwise or one-step process. The QM/MM energy barriers for the reduction of d-glyceraldehyde were evaluated at a B3LYP/6-31G* level for both HIP and HIE protonation states of His110. These were, respectively, 6.5 ± 2.2 and 16.7 ± 1.0 kcal/mol, which makes only the HIE protonation state consistent with the experimental value of 14.8 kcal/mol derived from kinetics experiments and makes Tyr48 the most probable proton donor.