Primrose W U, Scott A I, Mackenzie N E, Malthouse J P
Biochem J. 1985 Nov 1;231(3):677-82. doi: 10.1042/bj2310677.
The 13C-n.m.r. titration shifts of the alpha-methylene group of N-alkylated imidazoles are shown to be a sensitive probe of the ionization of the imidazolium ion. The 13C-n.m.r. titration shifts of both the intact and denatured/autolysed 2-13C- and 1-13C-enriched trypsin-7-amino-3-benzyloxycarbonylamino-1-chloroheptan-2-one (Z-Lys-CH2Cl) complexes are compared. The titration shift for the denatured/autolysed complex confirms that this ionization is due to deprotonation of the N-alkylated imidazolium ring of histidine-57. In the intact trypsin-inhibitor complex the titration shift due to the 1-13C-enriched carbon is anomalous. We conclude that this titration shift cannot arise solely from the ionization of the imidazolium ion of histidine-57 and that the pKa of the imidazolium ion of histidine-57 is raised in the trypsin-inhibitor complex. The relevance of these studies to the mechanism of action of the serine proteinases is discussed.
N-烷基化咪唑α-亚甲基基团的13C-核磁共振滴定位移被证明是咪唑鎓离子电离的灵敏探针。比较了完整的和变性/自溶的富含2-13C和1-13C的胰蛋白酶-7-氨基-3-苄氧羰基氨基-1-氯庚烷-2-酮(Z-赖氨酸-CH2Cl)复合物的13C-核磁共振滴定位移。变性/自溶复合物的滴定位移证实这种电离是由于组氨酸-57的N-烷基化咪唑环去质子化所致。在完整的胰蛋白酶-抑制剂复合物中,富含1-13C的碳引起的滴定位移是异常的。我们得出结论,这种滴定位移不能仅由组氨酸-57的咪唑鎓离子电离产生,并且在胰蛋白酶-抑制剂复合物中组氨酸-57的咪唑鎓离子的pKa升高。讨论了这些研究与丝氨酸蛋白酶作用机制的相关性。