The malate (MDH) and lactate (LDH) dehydrogenases belong to the homologous class of 2-ketoacid dehydrogenases. The specificity for their respective substrates depends on residues differing at two or three regions within each molecule. Theoretical peptide-mass fingerprinting and PROSITE analysis of nine MDH and six LDH molecules were used to describe conserved sites related to function. A unique LDH is described which probably also confers MDH activity within the 580 kbp genome of Mycoplasma genitalium (class: Mollicutes). A single hydrophilic arginine residue was found in the active site of the M. genitalium LDH enzyme, differing from an hydrophobic residue normally present in these molecules. The effect of this residue may be to alter active site substrate specificity, allowing the enzyme to perform two closely related tasks. Evidence for a single gene affording dual enzymatic function is discussed in terms of genome size reduction in the simplest of free-living organisms. Since Mollicutes are thought to lack enzymes of the tricarboxylic acid cycle that would otherwise bind and interact with MDH in bacterial species possessing this pathway, active site modification of M. genitalium LDH is the sole requirement for MDH activity of this molecule. The closely related helical Mollicute, Spiroplasma melliferum, was shown to possess two distinct gene products for MDH/LDH activity.