A lactate dehydrogenases-like structure with malate dehydrogenase enzymatic activity
The LDH-like MDH proteins have a lactate dehyhydrogenase-like (LDH-like) structure and malate dehydrogenase (MDH) enzymatic activity. This subgroup is composed of some archaeal LDH-like MDHs that prefer NADP(H) rather than NAD(H) as a cofactor. One member, MJ0490 from Methanococcus jannaschii, has been observed to form dimers and tetramers during crystalization, although it is believed to exist primarilly as a tetramer in solution. In addition to its MDH activity, MJ0490 also possesses fructose-1,6-bisphosphate-activated LDH activity. Members of this subgroup have a higher sequence similarity to LDHs than to other MDHs. LDH catalyzes the last step of glycolysis in which pyruvate is converted to L-lactate. MDH is one of the key enzymes in the citric acid cycle, facilitating both the conversion of malate to oxaloacetate and replenishing levels of oxalacetate by reductive carboxylation of pyruvate. The LDH-like MDHs are part of the NAD(P)-binding Rossmann fold superfamily, which includes a wide variety of protein families including the NAD(P)- binding domains of alcohol dehydrogenases, tyrosine-dependent oxidoreductases, glyceraldehyde-3-phosphate dehydrogenases, formate/glycerate dehydrogenases, siroheme synthases, 6-phosphogluconate dehydrogenase, aminoacid dehydrogenases, repressor rex, and NAD-binding potassium channel domains, among others.