[1] One is found in the mitochondrial matrix, participating as a key enzyme in the citric acid cycle that catalyzes the oxidation of malate.

The other is found in the cytoplasm, assisting the malate-aspartate shuttle with exchanging reducing equivalents so that malate can pass through the mitochondrial membrane to be transformed into oxaloacetate for further cellular processes.

[8] Each subunit of the malate dehydrogenase dimer has two distinct domains that vary in structure and functionality.

[9] Malate dehydrogenase has also been shown to have a mobile loop region that plays a crucial role in the enzyme's catalytic activity.

[6] The three residues in particular that comprise a catalytic triad are histidine (His-195), aspartate (Asp-168), both of which work together as a proton transfer system, and arginines (Arg-102, Arg-109, Arg-171), which secure the substrate.

[12] Studies have also identified a mobile loop in malate dehydrogenase that participates in the catalytic activity of the enzyme.

This flipping of the loop to the up position to cover the active site also promotes enhanced interaction of the catalytically important amino residues on the enzyme with the substrate.

Additionally, the movement of the loop has been shown to correlate with the rate determining step of the enzyme.

[16] Additionally, pH levels control specificity of substrate binding by malate dehydrogenase due to proton transfer in the catalytic mechanism.

[17] A histidine moiety with a pK value of 7.5 has been suggested to play a role in the pH-dependency of the enzyme.

As a result, at lower pH values malate dehydrogenase binds preferentially to D-malate, hydroxymalonate, and keto-oxaloacetate.

This may be due to deviations observed in the kinetic behavior of malate dehydrogenase at high oxaloacetate and L-malate concentrations.

Additionally, the formation of this complex enables glutamate to react with aminotransferase without interfering activity of malate dehydrogenase.

The formation of this ternary complex also facilitates the release of oxaloacetate from malate dehydrogenase to aminotransferase.