Structure and function of Scaptodrosophila lebanonensis alcohol dehydrogenase

Abstract

Originating from a fruit fly species, Drosophilid alcohol dehydrogenases enzymes (DADHs) belong to the Short-chain dehydrogenases/reductases (SDR) family. Covering a wide range of species, SDR family members show very similar structure and chemical reaction mechanism. In human there are, so fare, at least 24 SDR enzymes that are connected to diseases. Therefore, understanding the function and chemical reaction mechanism of SDR is medically very important. Scaptodrosophilid lebanonensis alcohol dehydrogenase (SlADH) is one of the DADHs of the SDR family. Therefore, it can be used as a model enzyme to study other human disease-related SDR enzymes. SlADH oxidizes alcohol by using the cofactor NAD+. The oxidization reaction consists of proton release and hydride transfer steps. Our study showed that the replacement of the amino acid Threonine with Valine at position 114 in SlADH results in a break of an eight membered chain of water moleculs mediating transport of a proton inside SlADH. Although several enzyme kinetic parameters differ, this mutant follows the same reaction mechanism as the wild type SlADH. For example, the mutation results in a slower hydride transfer from alcohol to NAD+, and slower and weaker binding of NAD+ to the mutant enzyme. Hence, an intact water chain in SlADH is essential for optimal enzyme activity. Other factors may also contribute to these changes, such as the change of dynamical behavior of this enzyme due to the mutation and the broken water chain. A computer based quantum mechanics and molecular mechanics (QM/MM) study of the oxidation mechanism showed that during the alcohol oxidation, the hydride transfer most likely starts first and initiate the proton transfer, and when the proton transfer is finished, the hydride transfer continues until it is finished. Although the QM/MM study gives complementary explanations to the reaction order of these two steps, due to its limitation, more studies are necessary in order to gain more insights into the reaction mechanism of SlADH.