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Assignment of the histidine 12‐threonine 45 hydrogen‐bonded proton in the nmr spectrum of ribonuclease A in H 2 O
Author(s) -
Patel Dinshaw J.,
Canuel Lita L.,
Woodward Clare,
Bovey Frank A.
Publication year - 1975
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1975.360140506
Subject(s) - chemistry , histidine , imidazole , protonation , resonance (particle physics) , hydrogen bond , rnase p , ribonuclease , chemical shift , stereochemistry , proton , proton nmr , crystallography , enzyme , molecule , rna , organic chemistry , biochemistry , ion , physics , particle physics , quantum mechanics , gene
Described herein are proton nmr experiments on chemically modified derivatives of ribonuclease A designed to elucidate the origin of an exchangeable resonance, assigned previously to a histidine ring N proton that titrates between 11 to 13 ppm with a p K a of 6.1 in H 2 O solution. Histidines 48 and 105, which are distant from the active site, are eliminated as candidates for this resonance from inhibitor binding studies on the enzyme in acetate–water solutions. This exchangeable resonance titrates with modified p K a 's and constant area over the above pH range in His‐119‐ N 1 ‐carboxymethylated‐RNase A and des‐(121–124)‐RNase A, thus eliminating the imidazole N 3 proton in the His 119‐Asp 121 hydrogen bond. In His‐12‐ N 1 ‐carboxymethylated‐RNase A, this resonance is also observable, but broadens on raising the pH above 7 and at elevated temperatures above neutrality. It exhibits a pH‐independent chemical shift characteristic of the protonated state of histidine. On the basis of these findings, this exchangeable resonance, designated a , is assigned to the imidazole N 1 proton of His 12, which is hydrogen‐bonded to the carbonyl oxygen of Thr 45 in the crystal.