The occupancy of two distinct conformations by active-site histidine-119 in crystals of ribonuclease is modulated by pH.
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.
de Mel
V S
VS
Department of Biochemistry, Wayne State University School of Medicine, Detroit, MI 48201.
Doscher
M S
MS
Martin
P D
PD
Edwards
B F
BF
D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D006639:Histidine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry*; D011487:Protein Conformation; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*
D010446:Peptide Fragments; D006639:Histidine; D012259:Ribonuclease, Pancreatic