The Allosteric Activation of Mammalian α‐Amylase by Chloride

Abstract

α‐Amylase from hog pancreas is shown to possess one binding site for Cl− per molecule of enzyme with a dissociation constant of 3 × 10−4 at 25 °C. The chloride anion functions as an activating effector increasing kcat 30‐fold towards either starch or p‐nitrophenylmaltoside. No change in Km for either substrate is seen. The other monovalent anions Br−, I−, NO2−, NO3−, ClO4−, SCN−, N3− and CNO− can substitute for chloride but their effect on kcat decreases as their anionic radius increases. Chloride binding induces a subtle conformational change in the enzyme reflected by the suppression of the exchange of 26 protons and a 240‐fold increase in the amylase‐Ca2+ binding constant from 8.3 × 108 to 2 × 1011 M−1. The conformational change is minor since it is not detected by circular dichroism, protein fluorescence or by specific probes attached to the enzyme. The ability of small structural changes to induce large catalytic acceleration is discussed. Copyright © 1974, Wiley Blackwell. All rights reserved