Abstract
The ribonuclease A derivative N π-[ 13 C 1] carboxymethyl-histidine-119 ribonuclease prepared by using [ 13C 1]bromoacetate as alkylating reagent has been investigated with high resolution 13C NMR spectroscopy. In the 13C NMR spectra two carbon resonances of relatively high intensity appear which can be assigned to carboxyl groups attached to His-119 and Met-30, their intensity ratio being 10:1. The pH dependence of the carbon resonance of the carboxymethyl group bound to the N π of His-119 differs in the absence and presence of Cyd-2′- P, thus indicating that the catalytically inactive derivative does bind nucleotides. A mechanism of the alkylation reaction at pH 5.6 is proposed in which the ϵ-amino group of Lys-41 acts as the binding site for the carboxyl group of bromoacetate pushing the bromomethylene group towards the N π of His-119 or the N τ of His-12.
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