Valcour A A, Woodworth R C
Biochemistry. 1987 Jun 2;26(11):3120-5. doi: 10.1021/bi00385a026.
The binding of Ga(III) to the proteolytically derived N-terminal half-molecule of human transferrin (HTF/2N) was studied by proton nuclear magnetic resonance spectroscopy. The pH-dependent titration curves of the histidinyl C(2) proton chemical shifts were altered upon formation of the GaIIIHTF/2N(C2O4) ternary complex. Two high-pK'a histidines failed to titrate when the metal and synergistic anion formed a complex with the protein. These results implicated two histidinyl residues as direct ligands to the metal. The rates of hydrogen-deuterium exchange for the C(2) protons of certain histidinyl residues were substantially decreased by metal ion binding. The two ligand histidines were protected from exchange, and a third, low-pK'a, histidinyl residue was protected. We propose that this third histidinyl residue is involved in anion binding and may serve as the base in the putative proton-relay scheme proposed for complex formation.
通过质子核磁共振光谱研究了Ga(III)与人转铁蛋白蛋白水解衍生的N端半分子（HTF/2N）的结合。形成GaIIIHTF/2N(C2O4)三元复合物后，组氨酸C(2)质子化学位移的pH依赖性滴定曲线发生了变化。当金属和协同阴离子与蛋白质形成复合物时，两个高pK'a的组氨酸未能滴定。这些结果表明两个组氨酸残基是金属的直接配体。金属离子结合使某些组氨酸残基C(2)质子的氢-氘交换速率大幅降低。两个配体组氨酸受到交换保护，第三个低pK'a的组氨酸残基也受到保护。我们提出，这个第三个组氨酸残基参与阴离子结合，可能在复合物形成的假定质子传递方案中作为碱基。
