The presence of a histidine-aspartic acid pair in the active site of 2-hydroxyacid dehydrogenases. X-ray refinement of cytoplasmic malate dehydrogenase.

The structure of cytoplasmic malate dehydrogenase has been partially refined by crystallographic least squares methods. Using x-ray phases based on the refined coordinates, analysis of the resultant electron density maps has led to a new model of cytoplasmic malate dehydrogenase and a tentative "x-ray sequence." The two crystallographically independent subunits comprising the dimeric enzyme are nearly identical in structure and are related to each other by roughly 2-fold rotational symmetry. The best fit of the molecular structure of cytoplasmic malate dehydrogenase to that of lactate dehydrogenase has been obtained by least squares methods. The active sites of these two enzymes contain similarly oriented His-Asp pairs linked by a hydrogen bond which may function as a proton relay system during catalysis. This pair could also provide an explanation for the relatively stronger binding by cytoplasmic malate dehydrogenase and lactate dehydrogenase of NADH versus NAD. Similar His-Asp pairs have been observed in the serine proteases, thermolysin, and phospholipase A2, and the His-Asp pair may play a similar functional role in all of these enzymes.