Ahmad F, McPhie P
Biochim Biophys Acta. 1978 Dec 20;537(2):247-54. doi: 10.1016/0005-2795(78)90508-1.
The ionization of tyrosine residues in diazotized pepsin under various solvent conditions was studied. All tyrosyl residues of the protein titrated normally with a pK of 10.02 in 6 M guanidine hydrochloride solution. On the other hand, two stages in the phenolic group titration curve were observed for the inactivated protein in the absence of guanidine hydrochloride; only about 10 tyrosine residues ionized reversibly up to pH 11, above which titration was irreversible. The irreversible titration zone corresponds to the pH range 11--13 in which unfolding, leading to the random coil state, was shown to occur by circular dichroism and viscosity measurements. The number of tyrosine residues exposed in the native and alkali-denatured (pH 7.5) states of diazotized protein were also studied by solvent perturbation techniques; 10 and 12 groups are exposed in the native and denatured states, respectively.
研究了重氮化胃蛋白酶在各种溶剂条件下酪氨酸残基的电离情况。在6M盐酸胍溶液中，该蛋白质的所有酪氨酰残基正常滴定，其pK值为10.02。另一方面，在没有盐酸胍的情况下，观察到失活蛋白质的酚基团滴定曲线有两个阶段；在pH值达到11之前，只有约10个酪氨酸残基可逆电离，高于此pH值滴定则不可逆。不可逆滴定区对应于pH值范围11 - 13，在此范围内，通过圆二色性和粘度测量表明会发生导致无规卷曲状态的解折叠。还通过溶剂扰动技术研究了重氮化蛋白质在天然和碱变性（pH 7.5）状态下暴露的酪氨酸残基数量；在天然和变性状态下分别有10个和12个基团暴露。
