Malate dehydrogenase, mitochondrial also known as malate dehydrogenase 2 is an enzyme that in humans is encoded by the MDH2 gene. Malate dehydrogenase catalyzes the reversible oxidation of malate to oxaloacetate, utilizing the NAD/NADH cofactor system in the citric acid cycle. The protein encoded by this gene is localized to the mitochondria and may play pivotal roles in the malate-aspartate shuttle that operates in the metabolic coordination between cytosol and mitochondria. Structure The protein encoded by MDH2 exists as a dimer, which indicate the important connection between protein stability and enzymatic activity. Each subunit contains two structurally and functionally distinct domains. The first is the NAD-binding domain, which exists in the amino-terminal half of each molecule, and contains a parallel-sheet structure, otherwise known as a Rosman fold motif. The core dinucleotide binding structure is composed of four beta-sheets and one alpha-helix. The other domain is a carboxy-terminal domain that contains the substrate binding site and amino acids that are necessary for catalysis. The active site of these enzymes is in a cleft between two domains. Crystallography reveals the dimer interface, which consists mainly of interacting alpha-helices that form a compact interaction. The active sites in these dimeric proteins are well separated from each other. Function Because malate dehydrogenase is closely tied to the citric acid cycle, regulation is highly dependent on TCA products. Citrate also affects MDH activity by very complex manner. It inhibits the reduction of oxaloacetate under all conditions. Citrate also inhibits malate oxidation, but only at low malate or NAD concentrations. When both malate and NAD concentrations are high (10 mmol/L and 5 mmol/L, respectively), citrate can actually augment MDH2 activity. All three effectors (malate, oxaloacetate and citrate) bind to the same putative allosteric site. Recent studies of mitochondrial malate dehydrogenase are focused into the nature of the inactivation processes. The oligomeric structure of MDH2 has a variety of biological implications. Some researches have suggested that the dimeric structure is critical for enzymatic activity. It was first proposed that the reciprocating compulsory ordered mechanism where each subunit alternates as the active and the helper subunit, but both are needed for activity. This mechanism predicts an inactive monomer, and was corroborated by studies that showed a dramatic reduction of enzymatic activity. Studies with mitochondrial MDH2 have shown that this enzyme is allosterically regulated as a complex as well. Binding experiments indicate that mitochondrial aspartate aminotransferase can associate with