L-serine dehydratase from Arthrobacter globiformis.

L-serine dehydratase from Arthrobacter globiformis has been purified over 1000-fold, to a point approaching homogeneity. An activity stain for the enzyme on poly-acrylamide gels was developed to follow this purification. The enzyme was characterised by a non-linear time course of activity in which the rate of pyruvate production slowly and progressively increased to a linear rate as the enzyme was activated. It is therefore a hysteretic enzyme (Frieden, 1970). The enzyme also had a sigmoidal substrate saturation curve and showed a requirement for cations. The concentration of L-serine affected the extent of activation whereas cations [e.g. Mg2+ or K+] affected both the rate and the extent of activation, but were not required for the catalytic activity of the enzyme. Two compounds, L-cysteine and D-serine, have been found to substitute for L-serine in the activation of L-serine dehydratase. Enzyme activated by these compounds or by L-serine showed a hyperbolic substrate saturation curve, had a linear time course of activity, and no requirement for cations. On activation, the molecular weight of the enzyme doubled. If the concentration of the activator was then reduced, the enzyme dissociated to the monomer form. These studies show that the activated enzyme was always in the dimer form and the non-activated enzyme was in the monomer form. An examination of the activation process showed it to be a second order reaction with respect to protein concentration, thereby identifying dimerisation as the slow step and the cause of the hysteretic response. A scheme, based on the equilibrium model for 'allosteric' proteins, has been presented to describe the roles played by L-serine, cations and protein in the activation of the enzyme. Investigations of the activated enzyme showed it to have the same properties as described by Bridgeland (1968) for L-serine dehydratase in toluene-tested whole cells. This suggests that the differences observed in the properties of the enzyme in vivo and in vitro are due to the maintenance of the enzyme in the activated form within the cell.