de Mel VS, Doscher MS, Martin PD, Edwards BF. The occupancy of two distinct conformations by active-site histidine-119 in crystals of ribonuclease is modulated by pH.
FEBS Lett 1994;
349:155-60. [PMID:
8045294 DOI:
10.1016/0014-5793(94)00664-4]
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Abstract
Structures of a semisynthetic RNase have been obtained to a resolution of 2.0 A at pH values of 5.2, 6.5, 7.5, and 8.8, respectively. The principle structural transformation occurring over this pH range is the conversion of the side chain of active site residue His-119 from one conformation (chi 1 = -43 degrees to -57 degrees) at low pH to another (chi 1 = +159 degrees to +168 degrees) at higher pH values. On the basis of this observation, a model is proposed that reconciles the disparate pK values for His-119 in the enzyme-substrate complex that have been deduced from kinetic studies and from proton NMR measurements in the presence of pseudosubstrates.
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