Premkumar Lakshmanane, Greenblatt Harry M, Bageshwar Umesh K, Savchenko Tatyana, Gokhman Irena, Sussman Joel L, Zamir Ada
Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
Proc Natl Acad Sci U S A. 2005 May 24;102(21):7493-8. doi: 10.1073/pnas.0502829102. Epub 2005 May 13.
Protein molecular adaptation to drastically shifting salinities was studied in dCA II, an alpha-type carbonic anhydrase (EC 4.2.1.1) from the exceptionally salt-tolerant unicellular green alga Dunaliella salina. The salt-inducible, extracellular dCA II is highly salt-tolerant and thus differs from its mesophilic homologs. The crystal structure of dCA II, determined at 1.86-A resolution, is globally similar to other alpha-type carbonic anhydrases except for two extended alpha-helices and an added Na-binding loop. Its unusual electrostatic properties include a uniformly negative surface electrostatic potential of lower magnitude than that observed in the highly acidic halophilic proteins and an exceptionally low positive potential at a site adjoining the catalytic Zn(2+) compared with mesophilic homologs. The halotolerant dCA II also differs from typical halophilic proteins in retaining conformational stability and solubility in low to high salt concentrations. The crucial role of electrostatic features in dCA II halotolerance is strongly supported by the ability to predict the unanticipated halotolerance of the murine CA XIV isozyme, which was confirmed biochemically. A proposal for the functional significance of the halotolerance of CA XIV in the kidney is presented.
在二碳酸酐酶II（dCA II）中研究了蛋白质分子对急剧变化盐度的适应性，dCA II是一种α型碳酸酐酶（EC 4.2.1.1），来自极具耐盐性的单细胞绿藻杜氏盐藻（Dunaliella salina）。盐诱导的细胞外dCA II具有高度耐盐性，因此与其嗜温同源物不同。以1.86埃分辨率测定的dCA II晶体结构在整体上与其他α型碳酸酐酶相似，但有两个延伸的α螺旋和一个额外的钠结合环。其不寻常的静电性质包括表面静电势均匀为负，幅度低于在高酸性嗜盐蛋白中观察到的情况，并且与嗜温同源物相比，在与催化性锌离子（Zn(2+)）相邻的位点具有极低的正电势。耐盐的dCA II在低至高盐浓度下保持构象稳定性和溶解性方面也与典型的嗜盐蛋白不同。通过预测小鼠CA XIV同工酶出人意料的耐盐性（已通过生化方法证实）的能力，有力地支持了静电特征在dCA II耐盐性中的关键作用。本文提出了CA XIV在肾脏中耐盐性功能意义的建议。
