Maghuin-Rogister G, Degelaen J, Roberts G C
Eur J Biochem. 1979 May 2;96(1):59-68. doi: 10.1111/j.1432-1033.1979.tb13013.x.
The titration curves of the histidine residues of porcine lutropin and its isolated alpha and beta subunits have been determined by following the pH-dependence of the imidazole C-2 proton resonances. The isolated alpha subunit contains a buried histidine, whose C-2 proton does not exchange with solvent, and which has the unusually low pK of 3.3. In the native hormone all the histidine residues have relatively normal pK values (between 5.7 and 6.2). The four histidine C-2 proton resonances have been assigned to specific residues in the amino-acid sequence, by means of deuterium and tritium exchange experiments on the alpha subunit and its des(92-96) derivative. The histidine with a pK of 3.3 is identified as His-alpha87. The effects of pH on tyrosine and methyl proton resonances show that the titration of His-87 in the isolated alpha subunit is accompanied by a significant conformational change which involves loosening of the protein structure but which is not a normal unfolding transition. The role of conformational changes in the generation of biological activity by subunit association in the glycoprotein hormones is discussed.
通过跟踪咪唑C-2质子共振的pH依赖性，测定了猪促黄体激素及其分离的α和β亚基中组氨酸残基的滴定曲线。分离出的α亚基含有一个埋藏的组氨酸，其C-2质子不与溶剂交换，且其pK值异常低，为3.3。在天然激素中，所有组氨酸残基的pK值相对正常（在5.7至6.2之间）。通过对α亚基及其去（92-96）衍生物进行氘和氚交换实验，已将四个组氨酸C-2质子共振指定为氨基酸序列中的特定残基。pK值为3.3的组氨酸被鉴定为His-α87。pH对酪氨酸和甲基质子共振的影响表明，分离的α亚基中His-87的滴定伴随着显著的构象变化，这涉及蛋白质结构的松弛，但不是正常的解折叠转变。讨论了糖蛋白激素中通过亚基缔合产生生物活性过程中构象变化的作用。
