Equilibrium constants of several reactions involved in the fermentation of glutamate

The equilibrium constants of the reactions catalysed by (S)-citramalate lyase and (R)-2-hydroxyglutarate dehydrogenase were determined using the purified enzymes from Clostridium tetanomorphum and Acidaminococcus fermentans, respectively. The former constant had to be determined at high ionic strength (I). Therefore it was corrected to I = 0.1 M by applying single-ion activity coefficients estimated from literature data. The result (Kapp = 4.31 +/- 0.07 M-1; direction of citramalate formation) agreed very well with the constant of the (2R,3S)-2,3-dimethylmalate lyase equilibrium when all optical isomers were taken into account. From these and other data values for the free energies of formation (delta Gzerof) of (2S,3S)-3-methylaspartate, mesaconate and (S)-citramalate were calculated. The constant of the (R)-2-hydroxyglutarate dehydrogenase equilibrium [Kapp = (1.47 +/- 0.12)10(-12) M, direction of 2-oxoglutarate formation, I = 0.1 M] was shown to lie between those for malate and lactate dehydrogenases as expected.