MALATE LACTATE DEHYDROGENASE IN MOLLICUTES - EVIDENCE FOR A MULTIENZYME PROTEIN/

The malate (MDH) and lactate (LDH) dehydrogenases belong to the homolo gous class of 2-ketoacid dehydrogenases. The specificity for their res pective substrates depends on residues differing at two or three regio ns within each molecule. Theoretical peptide-mass fingerprinting and P ROSITE analysis of nine MDH and six LDH molecules were used to describ e conserved sites related to function. A unique LDH is described which probably also confers MDH activity within the 580 kbp genome of Mycop lasma genitalium (class: Mollicutes). A single hydrophilic arginine re sidue was found in the active site of the M. genitalium LDH enzyme, di ffering from an hydrophobic residue normally present in these molecule s. The effect of this residue may be to alter active site substrate sp ecificity, allowing the enzyme to perform two closely related tasks. E vidence for a single gene affording dual enzymatic function is discuss ed in terms of genome size reduction in the simplest of free-living or ganisms. Since Mollicutes are thought to lack enzymes of the tricarbox ylic acid cycle that would otherwise bind and interact with MDH in bac terial species possessing this pathway, active site modification of M. genitalium LDH is the sole requirement for MDH activity of this molec ule. The closely related helical Mollicute, Spiroplasma melliferum, wa s shown to possess two distinct gene products for MDH/LDH activity. (C ) 1997 Elsevier Science B.V.