Hassan Md Imtaiyaz, Bilgrami Sameeta, Kumar Vijay, Singh Nagendra, Yadav Savita, Kaur Punit, Singh T P
Department of Biophysics, All India Institute of Medical Sciences, New Delhi 110029, India.
J Mol Biol. 2008 Dec 19;384(3):663-72. doi: 10.1016/j.jmb.2008.09.072. Epub 2008 Oct 9.
This is the first report on the formation of a complex between zinc alpha2-glycoprotein (ZAG) and prolactin-inducible protein (PIP). The complex was purified from human seminal plasma and crystallized using 20% polyethylene glycol 9000 and 5% hexaethylene glycol. The structure of the complex has been determined using X-ray crystallographic method and refined to an R(cryst) of 0.199 (R(free)=0.239). The structure of ZAG is broadly similar to the structure of serum ZAG. The scaffolding of PIP consists of seven beta-strands that are organized in the form of two antiparallel beta-pleated sheets, resulting in the formation of a sandwiched beta-sheet. The amino acid sequence of PIP contains one potential N-glycosylation site at Asn77, and the same is found glycosylated with four sugar residues. The structure of the complex shows that the beta-structure of PIP is ideally aligned with the beta-structure of domain alpha3 of ZAG to form a long interface between two proteins. The proximal beta-strands at the long interface are arranged in an antiparallel manner. There are 12 hydrogen bonds and three salt bridges between ZAG and PIP. At the two ends of vertical interface, two salt bridges are formed between pairs of Lys41-Asp233 and Lys68-Glu229. On the perpendicular interface involving alpha1-alpha2 domains of ZAG and a loop of PIP, another salt bridge is formed. The internal space at the corner of the L-shaped structure is filled with solvent molecules including a carbonate ion. The overall buried area in the complex is approximately 914 A(2), which is considerably higher than the 660 A(2) reported for the class I major histocompatibility complex structures.
这是关于锌α2-糖蛋白（ZAG）与催乳素诱导蛋白（PIP）形成复合物的首次报道。该复合物从人精浆中纯化出来，并使用20%聚乙二醇9000和5%六甘醇进行结晶。复合物的结构已通过X射线晶体学方法确定，并精修至R(cryst)为0.199（R(free)=0.239）。ZAG的结构与血清ZAG的结构大致相似。PIP的支架由七条β链组成，这些β链以两个反平行β折叠片层的形式排列，形成一个夹心β片层。PIP的氨基酸序列在Asn77处含有一个潜在的N-糖基化位点，并且发现该位点被四个糖残基糖基化。复合物的结构表明，PIP的β结构与ZAG的α3结构域的β结构理想地对齐，在两种蛋白质之间形成一个长界面。长界面处近端的β链以反平行方式排列。ZAG和PIP之间有12个氢键和三个盐桥。在垂直界面的两端，Lys41-Asp233和Lys68-Glu229之间形成了两个盐桥。在涉及ZAG的α1-α2结构域和PIP的一个环的垂直界面上，形成了另一个盐桥。L形结构角落处的内部空间充满了包括碳酸根离子在内的溶剂分子。复合物中的总掩埋面积约为914 Ų，这大大高于I类主要组织相容性复合体结构报道的660 Ų。
