Håkansson K, Svensson B
Department of Chemistry, Carlsberg Laboratory, Copenhagen Valby.
Carlsberg Res Commun. 1989;54(4):145-56. doi: 10.1007/BF02907184.
Treatment of glucoamylase G2 with large excesses of different group specific reagents resulted in modification of 25% of the histidyl, 15% of the tyrosyl, 20-40% of the arginyl, 30-50% of the lysyl and none of the methionyl residues. The modified groups were not critical since the various derivatives possessed from 50% to 100% residual enzymatic activity and retained the thermostability. Carboxamidomethylation occurred specifically at His254 with essentially no change of the kinetic parameters for hydrolysis of maltose and starch. Removal of the two N-linked sugar units by endoglycosidase H was similarly without effect on activity, thermostability and chemical reactivity of the histidyl residues. H(+)-titration revealed that glucoamylase G2 carries a lower net charge throughout the pH-range 3-11 than predicted from its amino acid composition.
用大量不同的基团特异性试剂处理糖化酶G2，导致25%的组氨酸残基、15%的酪氨酸残基、20 - 40%的精氨酸残基、30 - 50%的赖氨酸残基被修饰，而甲硫氨酸残基无一被修饰。修饰的基团并非关键基团，因为各种衍生物具有50%至100%的残余酶活性并保留了热稳定性。羧甲基化特异性地发生在His254处，麦芽糖和淀粉水解的动力学参数基本没有变化。用内切糖苷酶H去除两个N - 连接的糖单元同样对组氨酸残基的活性、热稳定性和化学反应性没有影响。H⁺滴定显示，在pH值3 - 11范围内，糖化酶G2的净电荷比根据其氨基酸组成预测的要低。
