Open AccessThe Allosteric Activation of Mammalian α‐Amylase by Chloride
Author(s) -
Levitzki Alexander,
Steer Michael L.
Publication year - 1974
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1974.tb03257.x
Subject(s) - chemistry , conformational change , guanidinium chloride , allosteric regulation , dissociation constant , chloride , enzyme , amylase , circular dichroism , crystallography , stereochemistry , biophysics , biochemistry , organic chemistry , biology , receptor
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 k cat 30‐fold towards either starch or p ‐nitrophenylmaltoside. No change in K m for either substrate is seen. The other monovalent anions Br − , I − , NO 2 − , NO 3 − , ClO 4 − , SCN − , N 3 − and CNO − can substitute for chloride but their effect on k cat 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‐Ca 2+ binding constant from 8.3 × 10 8 to 2 × 10 11 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.