Experimental charge measurement at leaving oxygen in the bovine ribonuclease. A catalyzed cyclization of uridine 3'-phosphate aryl esters

Publisher Website
Google Scholar
Abstract
The title esters are demonstrated to be specific substrates of bovine pancreatic ribonuclease A (EC 3.1.27.5). The Bronsted dependence of kcat/Km at pH 7.50 for the enzyme-catalyzed cyclization versus the pKa of the leaving phenol exhibits two regression lines of almost identical slope for respectively 2-chlorophenols and 2,6-unsubstituted phenols: log kcat/Km = -0.20KaArOH + 5.47 (n = 5, r = 0.957); log kcat/Km = -0.17pKaArOH = 5.79 (n = 4, r = 0.965). Comparison of the Bronsted .beta.1g''s with that for the standard reaction where imidazole catalyzes the cyclization (.beta.1g = -0.59) indicates considerably less development of negative charge on the leaving oxygen in the enzyme case, providing experimental evidence for the hypothesis that electrophilic assistance is involved in catalysis. The existence of two essentially parallel Bronsted correlations is not reflected in the standard reaction of substrate with imidazole. Modeling studies indicate that the phenyl ring of the substrate can take up a range of positions away from the active site; the presence of ortho chloro substituents considerably restricts the motion of the phenyl leaving group.

This publication has 2 references indexed in Scilit: