Sulfate potentiation of the chloride activation of angiotensin converting enzyme

The angiotensin converting enzyme (ACE)-catalyzed hydrolysis of furanacryloyl-Phe-Gly-Gly is activated by monovalent anions, notably chloride. This activation is enhanced by sulfate; at pH 7.5, the effect is maximal at 0.8 M sulfate and is mediated through a specific interaction of the divalent anion with the enzyme, not through an increase in ionic strength. Sulfate decreases the apparent binding constant for chloride which manifests as a decrease of the apparent KM value, but it does not change kcat. Thus, at pH 7.5, sulfate solely affects substrate binding in accord with the ordered bireactant mechanism of chloride activation that pertains with this substrate [Bünning, P., & Riordan, J.F. (1983) Biochemistry 22, 100-116]. Increasing the pH from 6 to 9 in the absence of sulfate increases the apparent binding constant for chloride almost 60-fold from 3.3 to 190 mM. In the presence of 0.8 M sulfate, however, the change is only about 6-fold, from 0.7 to 4.2 mM. Over the same pH range, the apparent KM for furanacryloyl-Phe-Gly-Gly obtained with saturating chloride concentrations shifts from 0.14 to 0.48 mM, while in the presence of 0.8 M sulfate about 3-fold lower apparent KM values are obtained. Sulfate does not appear to affect the pK of a group on the enzyme that controls the mechanism of chloride activation but rather decreases the apparent KM by reducing the apparent binding constant for chloride.