Amino Acid Sequence Homology among the 2-hydroxy Acid Dehydrogenases: Mitochondrial and Cytoplasmic Malate Dehydrogenases Form a Homologous System with Lactate Dehydrogenase

The amino acid sequence of porcine heart mitochondrial malate dehydrogenase (mMDH; L-malate:NAD+oxidoreductase, EC 1.1.1.37) has been compared with the sequences of six different lactate dehydrogenases (LDH; L-lactate:NAD+oxidoreductase, EC 1.1.1.27) and with the ``x-ray'' sequence of cytoplasmic malate dehydrogenase (sMDH). The main points are that (i) all thre enzymes are homologous; (ii) invariant residues in the catalytic center of these enzymes include a histidine and an internally located aspartate that function as a proton relay system; (iii) numerous residues important to coenzyme binding are conserved, including several glycines and charged residues; and (iv) amino acid side chains present in the subunit interface common to the MDHs and LDHs appear to be better conserved than those in the protein interior. It is concluded that LDH, sMDH, and mMDH are derived from a common ancestral gene and probably have similar catalytic mechanisms.