Molecular Study of a New Active Site in Soybean Malate Dehydrogenase 1

NAD/NADH dependent malate dehydrogenase (MDH, E.C.1.1.1.37) is an enzyme in the citric acid cycle that reversibly catalyzes the conversion of malate to oxaloacetate using NAD⁺. MDH usually works as dimer. Its structure is well studied. The N-terminal region is critical for MDH’s catalytic function. Histidine (His-195) and Aspartate (Asp-168) are responsible for proton transfer, and three arginines (Arg-102, Arg-109, Arg-171) secure the substrate. In present study, we are reporting another active site in MDH protein, discovered in the dysfunctional MDH1 in soybean cultivar Mandell. Mandell is a spontaneous mutation from wild-type cv. Manchu. Mitochondrial MDH1 in Mandell encoded by mdh1 gene lost its ability to catalyze the conversion of malate to oxaloacetate. Sequence comparison shows that mdh1 in Mandell differs the wild type Mdh1 gene only by a single nucleotide mutation, which changes a C-terminal residue Arg-302 to Gly-302. Thus, in this study, we are trying to elucidate the role of Arg-302 in soybean MDH1 during MDH catalysis through genetics and biochemistry methods.