Huber R E, Hurlburt K L
Arch Biochem Biophys. 1986 Apr;246(1):411-8. doi: 10.1016/0003-9861(86)90487-x.
The reversion reactions of beta-galactosidase (Escherichia coli) produced beta-galactosyl-galactoses and beta-galactosyl-glucoses. About 10 beta-galactosyl-galactose and 10 beta-galactosyl-glucose gas-liquid chromatographic peaks were detected and it is thus very likely that every possible isomer of beta-galactosyl-galactose and beta-galactosyl-glucose was formed by the reversion reactions (taking into account both anomers for each isomer). The presence of lactose and allolactose among the beta-galactosyl-glucoses was confirmed with standards. An important finding relating to the role of allolactose as an inducer of the lac operon was that allolactose (beta-D-galactosyl-(1----6)-D-glucose) was the only disaccharide formed initially, and at equilibrium it was present in the largest amount (50%). Obviously the enzyme is specific in its ability to form allolactose, and allolactose is the most stable beta-galactosyl-glucose, both important inducer properties. The equilibrium constant (concentration of disaccharides divided by the concentration of reactants at equilibrium) of the reaction was about 9.5 mM-1. This is the first report of an equilibrium constant for the beta-galactosidase reaction. Of mechanistic significance is the fact that only three compounds were able to replace D-galactose as a reversion reactant. Two of these (L-arabinose and D-fucose) had alterations at carbon 6. The 6 position, therefore, is not essential for reactivity. The third compound was D-galactal. Any other sugars tested (even with very minor changes relative to D-galactose) did not react. Of special consequence is the 2 position. The results strongly suggest that there has to be either an equatorial hydroxyl at the 2 position of a sugar or a special reactivity (as with D-galactal) in order for the enzyme to catalyze the beta-galactosidase reaction.
β-半乳糖苷酶（大肠杆菌）的逆反应生成了β-半乳糖基-半乳糖和β-半乳糖基-葡萄糖。检测到了约10个β-半乳糖基-半乳糖和气-液色谱峰以及10个β-半乳糖基-葡萄糖气-液色谱峰，因此很可能β-半乳糖基-半乳糖和β-半乳糖基-葡萄糖的每种可能异构体都是通过逆反应形成的（每种异构体均考虑两种异头物）。用标准品确认了β-半乳糖基-葡萄糖中乳糖和别乳糖的存在。与别乳糖作为乳糖操纵子诱导剂的作用相关的一个重要发现是，别乳糖（β-D-半乳糖基-(1→6)-D-葡萄糖）是最初形成的唯一二糖，并且在平衡时其含量最高（50%）。显然，该酶形成别乳糖的能力具有特异性，且别乳糖是最稳定的β-半乳糖基-葡萄糖，这两种特性对于诱导剂而言都很重要。该反应的平衡常数（二糖浓度除以平衡时反应物浓度）约为9.5 mM⁻¹。这是关于β-半乳糖苷酶反应平衡常数的首次报道。具有机制意义的事实是，只有三种化合物能够替代D-半乳糖作为逆反应反应物。其中两种（L-阿拉伯糖和D-岩藻糖）在碳6处有改变。因此，6位对于反应性并非必需。第三种化合物是D-半乳糖醛。测试的任何其他糖类（即使相对于D-半乳糖只有非常微小的变化）都不发生反应。特别重要的是2位。结果强烈表明，为了使该酶催化β-半乳糖苷酶反应，糖类的2位必须有一个平伏羟基或者具有特殊的反应性（如D-半乳糖醛）。
