黑曲霉突变株葡萄糖淀粉酶的底物特异性

SUBSTRATE SPECIFICITY OF GLUCOAMYLASE FROM ASPERGILLUS NIGER MUTANT

黑曲霉(Aspergillus niger)突变株T-21葡萄糖淀粉酶(GAI)仅能水解多种淀粉及麦芽低聚糖生成唯一产物β-葡萄糖,其水解麦芽糖及麦芽三糖的速度分别为200和570mg葡萄糖·h^(-1)·mg^(-1).GAI水解α-1,4键的速度比水解α-1.6键快100多倍.除了马铃薯淀粉外,对其它淀粉及麦芽低聚糖几乎都能100%地水解,但不能水解环状糊精,其水解各麦芽低聚糖的最先产物都比原底物少一个葡萄糖单位,说明GAI为一外切型淀粉酶.GAI对麦芽糖、麦芽三糖、可溶性淀粉、糯米淀粉、糊精及糖原的Km值分别1.92mmol/L、0.38mmol/L、0.053%、0.045%、0.059%、及0.076%,V_(max)分别为590、1370、1270、1520、1120和1220mg葡萄糖·h^(-1)·mg^(-1).D-葡萄糖酸-δ-内酯及麦芽糖醇对此酶分别具有反竞争性抑制和混合性抑制.

Glucoamylase GAI from Aspergillus niger mutant T-21 hydrolysed only some kinds of starch and maltooligosaccharides and the sole product in each case was glucose. The initial rate of hydrolysis of maltose was 100 times more than that of the isomaltose. The initial hydrolysis velocity of poly-and oligo-saccharides were about 1000 mg Glc. h-1, mg-1, however, maltose and maltotriose only 200 and 570 mg Glc. h-1, mg-1 respectively. The degree of hydrolysis of some kinds of starch and maltooligosaccharides were 100% or approximately 100%, but the potato starch somewhat lower. The results of intermediate product analysis of the enzyme's hydrolysing maltooligosaccharides showed that the first product was one glucose residual less than the original substrate, and one more glucose eliminated in successional cut, the process went on untill glucose as the final product. The GAI couldn't hydrolyse cyclodextrins, which indicated GAI was an exo-amylase. The Km values of GAI for maltose, maltotriose, soluble starch, glutinous rice starch, dextrin and glycogen were 1. 92 mmol/L, 0. 38 mmol/L, 0. 053%, 0. 045%. 0.059% and 0.076% respectively; Vmax values were 590. 1370, 1270, 1520, 1120 and 1220 mg Glc. h-1, mg-1 respectively. The D-gluconic acid lactone was an anticompetitive inhibitor of GAI and maltitol was a mixed inhibitor.