Bacterial Threonine Aldolase and Serine Hydroxymethyltransferase Enzymes

for the enzyme in vivo unless an alkaline micro-environment was postulated or conditions were present in vivo which could potentiate inhibition by UMP at near-neutral pH (Yon, 19726). Substances such as deoxycholate which can bind to the hydrophobic site on the enzyme may provide such potentiation. Fig. 2 shows the effects of UMP and deoxycholate at pH values between 7 and 11. In the absence of detergent the enzyme is most active and most sensitive to UMP at pH10.6 (0, m, Fig. 2); however, between pH7 and 8.5 the enzyme is insensitive to UMP ( 5 0 ~ ~ ) at the substrate concentrations used in this study. A similar result has been obtained previously under slightly different conditions of substrate concentration (Yon, 19726). When the experiment is repeated in the presence of 0.5m~-deoxycholate ( A , A, Fig. 2) the pH-activity profile in the absence of UMP is unchanged, but inhibition by ~ O ~ M U M P is increased at all pH values. Of especial interest is the inhibition between pH7 and pH8.5; the relative increase in inhibition is much greater in this range than at more alkaline values. These results suggest that end-product inhibition by UMP at physiological pH values may be dependent in vivo on the local concentrations of substances able to bind to the hydrophobic site, thereby producing effects similar to those produced by deoxycholate in these experiments. Such a control mechanism would, presumably, involve a three-way interaction between the hydrophobic site, the UMP-binding site and the active site. It will be of interest to determine the specificity of the hydrophobic site, as judged by the potentiating effect on inhibition by UMP of various biological lipid molecules and of naturaland artificial-membrane systems.