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pH Titration studies of an SH2 domain‐phosphopeptide complex: Unusual histidine and phosphate p K a values
Author(s) -
Singer Alex U.,
FormanKay Julie D.
Publication year - 1997
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1002/pro.5560060912
Subject(s) - phosphopeptide , histidine , chemistry , sh2 domain , residue (chemistry) , protonation , crystallography , arginine , hydrogen bond , aspartic acid , titration , protein structure , stereochemistry , binding site , amino acid , biochemistry , peptide , phosphorylation , inorganic chemistry , molecule , organic chemistry , ion , receptor tyrosine kinase
Electrostatic interactions in a complex of the phospholipase C‐γ, C‐terminal SH2 domain with a high‐affinity binding phosphopeptide representing the sequence around Tyr 1021 of the β platelet‐derived growth factor receptor were studied by p K a determination of various titratable groups over the pH range of 5 to 8. A histidine residue that is highly conserved among SH2 domains (His βD4) and the phosphotyrosine (pTyrj phosphate group show p K a values significantly lower than average for these residue types in proteins. The reduced p K a of these two groups is due to the proximity of the highly positively charged pTyr binding pocket. The unusual p K , of His PD4 is also due to burial from solvent in a hydrogen‐bonding network that appears necessary for the positioning of arginine residues involved in pTyr binding. Mutation of the analogous histidine in other SH2 domains has been shown to abrogate pTyr binding. In addition to these large shifts in p K a , values, smaller effects were observed for the titratable groups of a glutamic acid and histidine near the C‐terminus of the the second helix due to its helical dipole. Finally, exchange behavior of arginine guanidinium protons with solvent as a function of pH in this SH2 domain‐phosphopeptide complex confirms previous descriptions of the roles of different arginines in the structure and function of this protein.