Crystal structure of lactose synthase reveals a large conformational change in its catalytic component, the β1,4-galactosyltransferase-1
Crystal structure of lactose synthase reveals a large conformational change in its catalytic component, the β1,4-galactosyltransferase-1
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DOI：
10.1006/jmbi.2001.4757
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发表时间：
2001-06-29
影响因子：
5.6
通讯作者：
Qasba, PK
中科院分区：
文献类型：
--
作者：
Ramakrishnan, B;Qasba, PK
乳糖合成酶（LS）是催化组分β 1，4-半乳糖基转移酶（β 4Gal-T1）和调节组分α-乳白蛋白（LA）（乳腺特异性蛋白）的1：1复合物。LA促进葡萄糖（Glc）与β 4Gal-T1的结合，从而将其糖受体特异性从N-乙酰葡糖胺（GlcNAc）改变为葡萄糖，这使得LS能够合成乳糖，乳糖是牛奶的主要碳水化合物组分。以2埃分辨率解析与各种基底结合的LS的晶体结构。这些结构表明，在底物结合β 4Gal-T1时，在包含残基345至365的区域中发生大的构象变化。这使His 347以这样的方式重新定位，即它可以参与金属离子的配位，并产生糖和LA结合位点。在糖受体结合位点，发现由残基Arg 359、Phe 360和Ile 363形成的疏水性N-乙酰基结合口袋。在葡萄糖结合结构中，不存在这种疏水口袋。对于Glc与LS的结合，Arg 359侧链发生重定向，这阻断了疏水口袋并使与Glc分子的相互作用最大化。因此，LA的作用是通过与β 4Gal-T1上的受体结合位点中的O-1羟基形成氢键来保持Glc，而β 4Gal-T1中的N-乙酰基结合口袋进行调节以使与Glc分子的相互作用最大化。这项研究提供了详细的结构基础的部分有序的动力学机制提出的乳糖合酶。
The lactose synthase (LS) enzyme is a 1:1 complex of a catalytic component, beta1,4-galactosyltransferse (beta 4Gal-T1) and a regulatory component, a-lactalbumin (LA), a mammary gland-specific protein. LA promotes the binding of glucose (Glc) to beta 4Gal-T1, thereby altering its sugar acceptor specificity from N-acetylglucosamine (GlcNAc) to glucose, which enables LS to synthesize lactose, the major carbohydrate component of milk. The crystal structures of LS bound with various substrates were solved at 2 Angstrom resolution. These structures reveal that upon substrate binding to beta 4Gal-T1, a large conformational change occurs in the region comprising residues 345 to 365. This repositions His347 in such a way that it can participate in the coordination of a metal ion, and creates a sugar and LA-binding site. At the sugar-acceptor binding site, a hydrophobic N-acetyl group-binding pocket is found, formed by residues Arg359, Phe360 and Ile363. In the Glc-bound structure, this hydrophobic pocket is absent. For the binding of Glc to LS, a reorientation of the Arg359 sidechain occurs, which blocks the hydrophobic pocket and maximizes the interactions with the Glc molecule. Thus, the role of LA is to hold Glc by hydrogen bonding with the O-1 hydroxyl group in the acceptor-binding site on beta 4Gal-T1, while the N-acetyl group-binding pocket in beta 4Gal-T1 adjusts to maximize the interactions with the Glc molecule. This study provides details of a structural basis for the partially ordered kinetic mechanism proposed for lactose synthase.