ALDOSE REDUCTASE AS A TARGET FOR DRUG DESIGN - MOLECULAR MODELING CALCULATIONS ON THE BINDING OF ACYCLIC SUGAR SUBSTRATES TO THE ENZYME

In an attempt to obtain a picture of the binding conformation of aldeh yde substrates to human aldose reductase (hAR), modeling calculations have been performed on the binding of three substrate, D-xylose, L-xyl ose, and D-lyxose, to wild-type human aldose reductase and two of its site-directed mutants, It was found that the average geometry of D-xyl ose in the active site of wild-type aldose reductase is characterized by strong hydrogen bonds involving the reactive carbonyl oxygen of the substrate and both Tyr48 and His110. The calculations also suggest th e importance of Trp111 in the binding of 2'-hydroxyl-containing aldehy de substrates. A good correlation between calculated interaction entha lpies and experimental log(K-m) or log(k(cat)/K-m) values was obtained when His110 was modeled with its N epsilon 2 atom protonated and N de lta 1 unprotonated. No correlation was found for the other two configu rations of His110. On the basis of comparisons of the calculated subst rate binding conformations for the three possible His110 configuration s, and on the correlations between measured log(K-m) or log(k(cat)/K-m ) and calculated parameters, it is proposed that His110 is neutral and protonated at N epsilon 2 when an aldehyde substrate is bound to the hAR/NADPH complex. A chain of three hydrogen-bonded water molecules ha s been identified in all available crystal structures and is located i n an enzyme channel which links the N delta 1 atom of His110 to the so lvent-accessible surface of the enzyme. A possible role of this channe l in the mechanism of catalysis of aldose reductase is suggested.