Photon Factory, National Laboratory for High Energy Physics, Ibaraki-ken, Japan.
The three-dimensional structure of allosteric L-lactate dehydrogenase from Bifidobacterium longum, the first example of a T-state structure of L-lactate dehydrogenase, has been determined to 2.0 A. A comparative study of this structure with the previously reported R-state structure from Bacillus stearothermophilus has revealed the allosteric activation mechanism of the bacterial L-lactate dehydrogenase. The fructose 1,6-bisphosphate-induced conformational change at the effector site and the substrate affinity change at the activity site are clearly shown at a molecular level. Coupling of these changes can be simply explained by a set of concerted rotations between subunits in the tetramer of the enzyme. This T to R transition is the first example for a tetrameric allosteric protein where the rotations occur around each of three axes of symmetry.