Department of Biochemistry, Iowa State University, Ames, IA 50011, USA.
Biopolymers. 2009 Dec;91(12):1083-96. doi: 10.1002/bip.21256.
Plants express three phylogenetic classes of hemoglobins (Hb) based on sequence analyses. Class 1 and 2 Hbs are full-length globins with the classical eight helix Mb-like fold, whereas Class 3 plant Hbs resemble the truncated globins found in bacteria. With the exception of the specialized leghemoglobins, the physiological functions of these plant hemoglobins remain unknown. We have reviewed and, in some cases, measured new oxygen binding properties of a large number of Class 1 and 2 plant nonsymbiotic Hbs and leghemoglobins. We found that sequence classification correlates with distinct extents of hexacoordination with the distal histidine and markedly different overall oxygen affinities and association and dissociation rate constants. These results suggest strong selective pressure for the evolution of distinct physiological functions. The leghemoglobins evolved from the Class 2 globins and show no hexacoordination, very high rates of O(2) binding ( approximately 250 muM(-1) s(-1)), moderately high rates of O(2) dissociation ( approximately 5-15 s(-1)), and high oxygen affinity (K(d) or P(50) approximately 50 nM). These properties both facilitate O(2) diffusion to respiring N(2) fixing bacteria and reduce O(2) tension in the root nodules of legumes. The Class 1 plant Hbs show weak hexacoordination (K(HisE7) approximately 2), moderate rates of O(2) binding ( approximately 25 muM(-1) s(-1)), very small rates of O(2) dissociation ( approximately 0.16 s(-1)), and remarkably high O(2) affinities (P(50) approximately 2 nM), suggesting a function involving O(2) and nitric oxide (NO) scavenging. The Class 2 Hbs exhibit strong hexacoordination (K(HisE7) approximately 100), low rates of O(2) binding ( approximately 1 muM(-1) s(-1)), moderately low O(2) dissociation rate constants ( approximately 1 s(-1)), and moderate, Mb-like O(2) affinities (P(50) approximately 340 nM), perhaps suggesting a sensing role for sustained low, micromolar levels of oxygen.
植物根据序列分析表达三种系统发生类别的血红蛋白（Hb）。第 1 类和第 2 类 Hbs 是全长球蛋白，具有经典的八螺旋 Mb 样折叠，而第 3 类植物 Hbs 类似于在细菌中发现的截断球蛋白。除了专门的豆血红蛋白外，这些植物血红蛋白的生理功能仍然未知。我们已经审查了大量第 1 类和第 2 类植物非共生 Hbs 和豆血红蛋白的新氧结合特性，并在某些情况下进行了测量。我们发现，序列分类与远端组氨酸的六配位程度明显不同，以及整体氧亲和力和结合和解离速率常数有很大差异相关。这些结果表明，强烈的选择性压力促进了不同生理功能的进化。豆血红蛋白是从第 2 类球蛋白进化而来的，没有六配位，氧结合速度非常高（约 250 μM(-1) s(-1)），氧解离速度适中（约 5-15 s(-1)），氧亲和力高（K(d)或 P(50)约 50 nM）。这些特性都有利于 O(2)向呼吸固氮细菌的扩散，并降低豆类根瘤中的 O(2)张力。第 1 类植物 Hbs 表现出较弱的六配位（K(HisE7)约 2），中等的氧结合速度（约 25 μM(-1) s(-1)），氧解离速度非常小（约 0.16 s(-1)），以及惊人的高氧亲和力（P(50)约 2 nM），这表明它具有涉及 O(2)和一氧化氮（NO）清除的功能。第 2 类 Hbs 表现出强烈的六配位（K(HisE7)约 100），氧结合速度低（约 1 μM(-1) s(-1)），氧解离速率常数适中（约 1 s(-1)），以及中等的、类似 Mb 的氧亲和力（P(50)约 340 nM），这可能表明它具有对持续低浓度、微摩尔氧水平的感应作用。
