Mechanism of allosteric transition of bacterial L-lactate dehydrogenase

The allosteric behaviour of L-lactate dehydrogenase (L-lactate:NAD+oxidoreductase, EC 1.1.1.27, LDH) from Bifidobacterium longum aM101-2 was studied by means of the subunit hybridization technique as well as X-ray crystallography. Homotropic allosteric response of the LDH activity was found against the concentration of its substrate, pyruvate. Heterotropic allosteric activation of the enzyme was induced by the addition of fructose 1,6-bisphosphate (FBP). Two mutant enzymes, which had either altered sensitivity to FBP or altered substrate specificity from pyruvate to oxaloacetate, were made by site-directed mutagenesis. Hybrid LDHs, between the wild and the mutant LDHs, were made by in vivo subunit hybridization using double transformation of Escherichia coli with two plasmids carrying either the gene of wild or mutant LDH. Introduction of only one desensitized mutant subunit to the LDH changed the characteristic of the wild enzyme to that of the mutant LDH. Kinetic studies on hybrid enzymes consisting of subunits having different substrate specificity indicated that there was a strong cooperative interaction among subunits. These results strongly support the idea that the allosteric change of the LDH fits the concerted-symmetry model proposed by Monod, Wyman, and Changeux. We analysed the crystallographic structure of the LDH having low affinity to substrate at 1.9 A resolution. By the comparison of the structure with that of other LDHs, we concluded that the conformational transition of the LDH was mainly caused by concerted rotations of subunits.