Hoy Julie A, Robinson Howard, Trent James T, Kakar Smita, Smagghe Benoit J, Hargrove Mark S
Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, IA 50011, USA.
J Mol Biol. 2007 Aug 3;371(1):168-79. doi: 10.1016/j.jmb.2007.05.029. Epub 2007 May 18.
The evolution of oxygen transport hemoglobins occurred on at least two independent occasions. The earliest event led to myoglobin and red blood cell hemoglobin in animals. In plants, oxygen transport "leghemoglobins" evolved much more recently. In both events, pentacoordinate heme sites capable of inert oxygen transfer evolved from hexacoordinate hemoglobins that have unrelated functions. High sequence homology between hexacoordinate and pentacoordinate hemoglobins in plants has poised them for potential structural analysis leading to a molecular understanding of this important evolutionary event. However, the lack of a plant hexacoordinate hemoglobin structure in the exogenously ligand-bound form has prevented such comparison. Here we report the crystal structure of the cyanide-bound hexacoordinate hemoglobin from barley. This presents the first opportunity to examine conformational changes in plant hexacoordinate hemoglobins upon exogenous ligand binding, and reveals structural mechanisms for stabilizing the high-energy pentacoordinate heme conformation critical to the evolution of reversible oxygen binding hemoglobins.
氧运输血红蛋白的进化至少发生在两个独立的时期。最早的事件导致了动物体内肌红蛋白和红细胞血红蛋白的出现。在植物中，氧运输“豆血红蛋白”的进化则要晚得多。在这两个事件中，能够进行惰性氧转移的五配位血红素位点是从具有无关功能的六配位血红蛋白进化而来的。植物中六配位和五配位血红蛋白之间的高度序列同源性使其具备了进行潜在结构分析的条件，从而能够从分子层面理解这一重要的进化事件。然而，缺乏外源性配体结合形式的植物六配位血红蛋白结构阻碍了这种比较。在此，我们报道了来自大麦的氰化物结合型六配位血红蛋白的晶体结构。这首次提供了一个机会，来研究植物六配位血红蛋白在外源性配体结合时的构象变化，并揭示了稳定对可逆氧结合血红蛋白进化至关重要的高能五配位血红素构象的结构机制。
