Bovine beta-lactoglobulin at 1.8 angstrom resolution - Still an enigmatic lipocalin
Bovine beta-lactoglobulin at 1.8 angstrom resolution - Still an enigmatic lipocalin
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DOI：
10.1016/s0969-2126(97)00205-0
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发表时间：
1997-04-15
期刊：
影响因子：
5.7
通讯作者：
Sawyer, L
中科院分区：
文献类型：
--
作者：
Brownlow, S;Cabral, JHM;Sawyer, L
背景：β -乳球蛋白（β - lg）是反刍动物和许多其他哺乳动物牛奶中的主要乳清蛋白。它的功能尚不清楚，但它经历了至少两个与ph有关的构象变化，这可能是重要的。牛β - lg在几种不同的晶格中结晶，并发表了正交晶格Y和三角晶格Z的中分辨率结构。三斜晶格X和晶格Z晶体在pH值两侧生长，其中一个由pH引起的构象变化发生。对结构的充分了解有助于解释遗传变异的构象变化和不同的变性行为。结果：用多次同构置换法在3.0埃分辨率下重新确定了β - lg晶格Z的结构，并对其进行了部分细化（R因子= 24.8%）。以这种晶格Z结构的二聚体为搜索模型，用分子置换法求解了pH 6.5（晶格X）下生长的三斜晶型。细化晶格X在1.8埃分辨率的R因子为18.1%。我们确定的结构在几个方面与先前发表的结构不同。结论：在β - lg已发表的结构中，序列的错误穿线影响了9条β链中的4条。然而多肽链的基本脂钙蛋白折叠是不变的。在两个晶格中，β - lg二聚体中单体的相对取向不同。当pH值升高时，有一个大约5度的旋转，这打破了一些亚基间的氢键。然而，目前尚不清楚，为什么结构的稳定性如此依赖于64号残基周围的外部环或含有游离硫醇的β链，它们都表现出遗传变异。
Background: beta-Lactoglobulin (beta-Lg) is the major whey protein in the milk of ruminants and many other mammals. Its function is not known, but it undergoes at least two pH-dependent conformational changes which may be important. Bovine beta-Lg crystallizes in several different lattices, and medium-resolution structures of orthorhombic lattice Y and trigonal lattice Z have been published. Triclinic lattice X and lattice Z crystals grow at pH values either side of the pH at which one of the pH-induced conformational changes occurs. A full understanding of the structure is needed to help explain both the conformational changes and the different denaturation behaviour of the genetic variants.Results: We have redetermined the structure of beta-Lg lattice Z at 3.0 Angstrom resolution by multiple isomorphous replacement and have partially refined it (R factor = 24.8%). Using the dimer from this lattice Z structure as a search model, the triclinic crystal form grown at pH 6.5 (lattice X) has been solved by molecular replacement. Refinement of lattice X at 1.8 Angstrom resolution gave an R factor of 18.1%. The structure we have determined differs from previously published structures in several ways,Conclusions: Incorrect threading of the sequence in the published structures of beta-Lg affects four of the nine beta strands. The basic lipocalin fold of the polypeptide chain is unchanged, however. The relative orientation of the monomers in the beta-Lg dimer differs in the two lattices. On raising the pH, there is a rotation of approximately 5 degrees, which breaks a number of intersubunit hydrogen bonds. It is not yet clear, however, why the stability of the structure should depend so heavily upon the external loop around residue 64 or the beta strand with the free thiol, each of which shows genetic variation.