Brown F F, Parsons T F, Sigman D S, Pierce J G
J Biol Chem. 1979 Jun 10;254(11):4335-8.
The pK values of the 3 histidine residues in the common alpha subunits of bovine and equine glycoprotein hormones have been determined from titration curves generated from their C-2 proton nuclear magnetic resonances at different pH values. Assignment of resonances to specific histidines is based on a comparison between the two species, which have 1 histidine residue in different positions in their sequences, and of the bovine alpha subunit after removal of its histidine 94 by treatment with carboxypeptidases. In both species, those histidines closest to the COOH terminus titrate with near normal pK values of 6.2. The histidine residue found in the bovine subunit at position 87 titrates with an approximate pK value of 5.4. Histidine 83, adjacent to an oligosaccharide moiety in both species, does not titrate over a pH range of 4.0 to 8.0 and thus appears inaccessible to solvent. Similarly, in bovine lutropin-beta, 1 of 3 histidine residues does not titrate between pH 5.0 and 7.0. In the intact hormone, 2 "nontitratable" histidine residues are found. Changes in the characteristics of the signals, however, preclude unambiguous assignment of these two resonances to the nontitrating histidines in the isolated subunits. It appears that changes in the environment of at least some histidines occur when the subunits combine to yield intact hormone.
已通过在不同pH值下对牛和马糖蛋白激素常见α亚基的C-2质子核磁共振产生的滴定曲线，确定了这两种亚基中3个组氨酸残基的pK值。根据两种物种（它们在序列中的不同位置有1个组氨酸残基）之间以及用羧肽酶处理去除其组氨酸94后的牛α亚基之间的比较，将共振分配给特定的组氨酸。在这两种物种中，那些最接近COOH末端的组氨酸以接近正常的6.2的pK值进行滴定。在牛亚基第87位发现的组氨酸残基以约5.4的pK值进行滴定。在这两种物种中，与寡糖部分相邻的组氨酸83在4.0至8.0的pH范围内不发生滴定，因此似乎无法与溶剂接触。同样，在牛促黄体激素β中，3个组氨酸残基中的1个在pH 5.0至7.0之间不发生滴定。在完整的激素中，发现了2个“不可滴定”的组氨酸残基。然而，信号特征的变化使得无法将这两个共振明确分配给分离亚基中的不可滴定组氨酸。当亚基结合产生完整激素时，至少一些组氨酸的环境似乎发生了变化。
