Structure of the calcium-binding echidna milk lysozyme at 1.9 angstrom resolution
Structure of the calcium-binding echidna milk lysozyme at 1.9 angstrom resolution
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DOI：
10.1107/s0907444996015831
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发表时间：
1997-07-01
影响因子：
2.2
通讯作者：
Kumar, V
中科院分区：
文献类型：
--
作者：
Guss, JM;Messer, M;Kumar, V
从单孔类动物针鼹（Tachyglossus aculeatus multiaculeatus）的乳汁中分离出一种溶菌酶（空间群P2（1），晶胞尺寸a = 37.1，B = 42.0，c = 38.1埃，β = 91度和Z = 2），并且对于分辨率范围7.0-1.9埃的所有测量数据，结构被细化到R值0.167。以前从与α-乳白蛋白的序列同源性推断，针鼹鼠乳溶菌酶（EML）每个分子将结合一个钙离子。这已经在本研究中得到证实，其中差分傅立叶合成中的最大峰与钙离子相关。EML的钙结合位点与狒狒和人α-乳白蛋白中观察到的钙结合位点以及经工程改造含有钙结合位点的人溶菌酶中观察到的钙结合位点非常相似。蛋白质的总体折叠与鸡型溶菌酶的折叠相似。EML，像鸽子溶菌酶一样，只有125个残基终止于半胱氨酸，但在EML中，这在残基9处与半胱氨酸形成二硫化物，而在已知序列的所有其他溶菌酶中，等同的半胱氨酸残基出现在位置6处。这些变化引起了一些小的结构重组。钙的结合似乎对多肽骨架构象几乎没有影响，并且仅引起配位钙离子的侧链构象的微小变化。同源性建模研究[Acharya，Stuart，菲利普斯，McKenzie & Teahan（1994）. J. Protein Chem.13（6），569-584]正确地预测了EML的总体结构及其钙结合位点的性质，但通常未能模拟在EML结构中观察到的一些更细微的差异，如同源性模型更接近于衍生该模型的起始结构而不是晶体结构的事实所证明的。
A lysozyme isolated from the milk of a monotreme, the echidna, Tachyglossus aculeatus multiaculeatus, has been crystallized (space group P2(1), with unit-cell dimensions a = 37.1, b = 42.0, c = 38.1 Angstrom, beta = 91 degrees and Z = 2) and the structure refined to an R value of 0.167 for all measured data in the resolution range 7.0-1.9 Angstrom. It had previously been inferred from sequence homology with alpha-lactalbumins that echidna milk lysozyme (EML) would bind one calcium ion per molecule. This has been confirmed in the present study in which the largest peak in a difference Fourier synthesis is associated with a calcium ion. The calcium binding site of EML is very similar to that observed in baboon and human alpha-lactalbumins, and in a human lysozyme engineered to contain a calcium-binding site. The overall fold of the protein is similar to that of chick-type lysozymes. EML, like pigeon lysozyme, has only 125 residues terminating at a cysteine but in EML this forms a disulfide with a cysteine at residue 9 whereas the equivalent cysteine residue in all other lysozymes of known sequence occurs at position 6. These changes cause some minor structural rearrangements. The binding of calcium appears to have had little effect on the polypeptide backbone conformation and caused only small changes in the conformation of side chains coordinating the calcium ion. A homology modelling study [Acharya, Stuart, Phillips, McKenzie & Teahan (1994). J. Protein Chem. 13(6), 569-584] correctly predicted the overall structure of EML and the nature of its calcium binding site but generally failed to model some more subtle differences observed in the EML structure as evidenced by the fact that the homology model more closely resembles the starting structure from which the model was derived than it does the crystal structure.