Allosteric Activation of the Hydrolysis of Specific Substrates by Chymotrypsin

Abstract

A variety of azobenzene compounds having bis‐quaternary nitrogens have been shown to accelerate the hydrolyis by chymotrypsin of certain specific substrates by an allosteric mechanism. One of the most potent, 2,2′‐bis[α‐(benzyldimethylammonium)methyl]azobenzene dibromide (2,2′‐Q‐Bzl) accelerated the hydrolysis of glutaryl‐l‐phenylalanine p‐nitroanilide 40‐fold at saturating concentration. Acceleration was by increasing kcat without altering Km. The hydrolysis of acetyl‐l‐tyrosine p‐nitroanilide and acetyl‐l‐tyrosine anilide was also accelerated by Q‐Bzl (25‐fold and 1.8‐fold respectively) while the hydrolysis of hemoglobin, azocoll and a number of esters was not affected. The inactivation of chymotrypsin by diphenylcarbamyl chloride and diphenylcarbamyl fluoride was accelerated by 2,2′‐Q‐Bzl. Reactivation in the presence of NH2OH was also accelerated, but in the absence of added nucleophile (i.e. of NH2OH) no increase in rate was detectable. An allosteric effector was covalently attached to chymotrypsinogen A by reaction with 2,2′‐bis[α‐(o‐bromomethylbenzyldimethylammonium)methyl]azobenezene dibromide. The product, when converted to active enzyme, was about 4 times more active than chymotrypsin as a result of an increase in kcat of hydrolysis; Km was unaffected. The mechanism of the allosteric acceleration process is not known but, because for all of the substrates affected acylation of the enzyme is rate‐limiting, it is tentatively suggested that the effectors facilitate proton transfer to the leaving group by an inductive effect on the ‘charge relay system’. Spectral studies indicate that the allosteric site is a portion of the enzyme with a polarity near that of water, possibly on the outside surface of the enzyme molecule. Copyright © 1976, Wiley Blackwell. All rights reserved