Wiesmann C, Hengstenberg W, Schulz G E
Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität, Freiburg im Breisgau, Germany.
J Mol Biol. 1997 Jun 27;269(5):851-60. doi: 10.1006/jmbi.1997.1084.
The initial structural model of 6-phospho-beta-galactosidase from Lactococcus lactis was refined to an R-factor of 16.4% (R[free] = 23.6%) to 2.3 A resolution (1 A = 0.1 nm), and the structures of three other crystal forms were solved by molecular replacement. The four structural models are essentially identical. The catalytic center of the enzyme is approximately at the mass center of the molecule and can only be reached through a 20 A long channel, which is observed with an "open" or "closed" entrance. The closed entrance is probably too small for the educt lactose-6-phosphate to enter, but large enough for the first product glucose to leave. Among the presented structures is a complex between an almost inactive mutant and the second product galactose-6-phosphate, which is exclusively bound at side-chains. A superposition (onto the native enzyme) of galactose-6-phosphate as bound to the mutant suggests the geometry of a postulated covalent intermediate. The binding mode of the educt was modeled, starting from the bound galactose-6-phosphate. A tightly fixed tryptophan is used as a chopping-board for splitting the disaccharide, and several other aromatic residues in the active center cavity are likely to participate in substrate transport/binding.
乳酸乳球菌6-磷酸-β-半乳糖苷酶的初始结构模型被精修至R因子为16.4%（R[自由] = 23.6%），分辨率达到2.3 Å（1 Å = 0.1 nm），并且通过分子置换解析了其他三种晶体形式的结构。这四个结构模型基本相同。该酶的催化中心大致位于分子的质心位置，只能通过一条20 Å长的通道到达，该通道有“开放”或“封闭”的入口。封闭的入口可能对于底物乳糖-6-磷酸来说太小而无法进入，但对于第一个产物葡萄糖来说足够大可以离开。在给出的结构中，有一个几乎无活性的突变体与第二个产物半乳糖-6-磷酸之间的复合物，半乳糖-6-磷酸仅结合在侧链上。与突变体结合的半乳糖-6-磷酸（叠加到天然酶上）表明了一个假定的共价中间体的几何结构。从结合的半乳糖-6-磷酸开始对底物的结合模式进行了建模。一个紧密固定的色氨酸用作切割二糖的砧板，活性中心腔内的其他几个芳香族残基可能参与底物运输/结合。
