Alternative binding modes in abortive NADH-alcohol complexes of horse liver alcohol dehydrogenase

Enzymes typically have high specificity for their substrates, but the structures of substrates and products differ, and multiple modes of binding are observed. In this study, high resolution X-ray crystallography of complexes with NADH and alcohols show alternative modes of binding in the active site. Enzyme crystallized with the good substrates NAD⁺ and 4-methylbenzyl alcohol was found to be an abortive complex of NADH with 4-methylbenzyl alcohol rotated to a "non-productive" mode as compared to the structures that resemble reactive Michaelis complexes with NAD⁺ and 2,2,2-trifluoroethanol or 2,3,4,5,6-pentafluorobenzyl alcohol. The NADH is formed by reduction of the NAD⁺ with the alcohol during the crystallization. The same structure was also formed by directly crystallizing the enzyme with NADH and 4-methylbenzyl alcohol. Crystals prepared with NAD⁺ and 4-bromobenzyl alcohol also form the abortive complex with NADH. Surprisingly, crystals prepared with NAD⁺ and the strong inhibitor 1H,1H-heptafluorobutanol also had NADH, and the alcohol was bound in two different conformations that illustrate binding flexibility. Oxidation of 2-methyl-2,4-pentanediol during the crystallization apparently led to reduction of the NAD⁺. Kinetic studies show that high concentrations of alcohols can bind to the enzyme-NADH complex and activate or inhibit the enzyme. Together with previous studies on complexes with NADH and formamide analogues of the carbonyl substrates, models for the Michaelis complexes with NAD⁺-alcohol and NADH-aldehyde are proposed.