Abstract
A competitive labelling method, using tritiated 1-fluoro-2,4-dinitrobenzene (FDNB) as the labelling reagent, is described for determining the ionization constants and reactivities of individual histidine residues in proteins. The imidazolyl side chain of histidine-57 in α-chymotrypsin is shown to have a pK a (app) of 6.8 and its reactivity towards FDNB is an order to magnitude greater than the imidazolyl side chain of α-N-acetyl-L-histidine. This data supports the proposal that a charge relay mechanism is operative in chymotrypsin catalysis. Above pH 8 the reactivities show that the system responsible for this mechanism is disrupted in the free enzyme.
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