Palmer T N, Ryman B E, Whelan W J
Eur J Biochem. 1976 Oct 1;69(1):105-15. doi: 10.1111/j.1432-1033.1976.tb10863.x.
Amylomaltase, the inducible 4-alpha-glucanotransferase of Escherichia coli strain ML, has been purified to homogeneity. Its specific activity with a commercial maltose substrate was 500 mkat/kg protein (30 mumol glucose formed min-1 mg protein-1). The purified enzyme, dependent on buffer concentration, exists in interconvertible low-molecular-weight (apparent molecular weight 71000) and high-molecular-weight (apparent molecular weight 370000) forms. The specificity of amylomaltase has been redefined. Hitherto, the enzyme was thought to be a glucosyltransferase, catalysing the transfer of single glucosyl units, and maltose has been regarded as its most important substrate. Amylomaltase is now shown to exhibit both glucosyl-transfer and 4-alpha-glucanosyl-transfer specificity. 4-alpha-Glucanosyl chains containing up to at least nine glucosyl units can be transferred. However, it is concluded that the transfer reaction by which amylomaltase action was originally expressed, does not take place, i.e., Maltose + maltose in equilibrium Maltotriose + glucose and that maltose has a restricted role as a substrate. This may be due to the inability of maltose to function as a donor substrate, serving only as an acceptor substrate. It is confirmed that when a maltodextrin serves as a donor, that portion of the molecule transfered by the enzyme is that containing the nonreducing-end-group. Enzyme action on chromatographically pure maltose is characterized by a lag phase in the time course of glucose release. The lag pahse is overcome by addition of 'priming' (catalytic) concentrations of maltotriose or higher maltodextrins. An autocatalytic reaction mechanism involving the generation of primer molecules is proposed to explain the action of the enzyme on maltose. The redefined action pattern of amylomaltase is consistent with the redefined role of the enzyme in the utilization of exogenous and endogenous 1,4-alpha-glucans by E. coli.
淀粉麦芽糖酶是大肠杆菌ML菌株中的一种可诱导型4-α-葡聚糖转移酶，已被纯化至同质。其对市售麦芽糖底物的比活性为500 mkat/kg蛋白质（每分钟形成30 μmol葡萄糖/mg蛋白质）。纯化后的酶根据缓冲液浓度的不同，以可相互转化的低分子量（表观分子量71000）和高分子量（表观分子量370000）形式存在。淀粉麦芽糖酶的特异性已被重新定义。迄今为止，该酶被认为是一种葡糖基转移酶，催化单个葡糖基单元的转移，麦芽糖一直被视为其最重要的底物。现在发现淀粉麦芽糖酶同时具有葡糖基转移和4-α-葡聚糖基转移特异性。可以转移含有至少九个葡糖基单元的4-α-葡聚糖链。然而，可以得出结论，最初表达淀粉麦芽糖酶作用的转移反应不会发生，即麦芽糖+麦芽糖⇌麦芽三糖+葡萄糖，并且麦芽糖作为底物的作用有限。这可能是由于麦芽糖不能作为供体底物发挥作用，仅作为受体底物。已证实，当麦芽糊精作为供体时，酶转移的分子部分是含有非还原端基的部分。酶对色谱纯麦芽糖的作用在葡萄糖释放的时间进程中具有滞后阶段。通过添加“引发”（催化）浓度的麦芽三糖或更高的麦芽糊精可以克服滞后阶段。提出了一种涉及引物分子生成的自催化反应机制来解释该酶对麦芽糖的作用。重新定义的淀粉麦芽糖酶作用模式与该酶在大肠杆菌利用外源和内源1,4-α-葡聚糖中的重新定义作用一致。
