Hemoglobins (Hbs) utilize heme b as a cofactor and are found in all kingdoms of life. The current knowledge reveals an enormous variability of Hb primary sequences, resulting in topological, biochemical and physiological individuality. As Hbs appear to modulate their reactivities through specific combinations of structural features, predicting the characteristics of a given Hb is still hardly possible. The unicellular green alga Chlamydomonas reinhardtii contains 12 genes encoding diverse Hbs of the truncated lineage, several of which possess extended N- or C-termini of unknown function. Studies on some of the Chlamydomonas Hbs revealed yet unpredictable structural and biochemical variations, which, along with a different expression of their genes, suggest diverse physiological roles. Chlamydomonas thus represents a promising system to analyze the diversification of Hb structure, biochemistry and physiology. Here, we report the crystal structure, resolved to 1.75 Å, of the heme-binding domain of cyanomet THB11 (Cre16.g662750), one of the pentacoordinate algal Hbs, which offer a free Fe-coordination site in the reduced state. The overall fold of THB11 is conserved, but individual features such as a kink in helix E, a tilted heme plane and a clustering of methionine residues at a putative tunnel exit appear to be unique. Both N- and C-termini promote the formation of oligomer mixtures, and the absence of the C terminus results in reduced nitrite reduction rates. This work widens the structural and biochemical knowledge on the 2/2Hb family and suggests that the N- and C-terminal extensions of the Chlamydomonas 2/2Hbs modulate their reactivity by intermolecular interactions.

中文翻译：

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THB11 的独特结构特性，一种五配位莱茵衣藻截短的血红蛋白，具有 N 和 C 端延伸。

血红蛋白 (Hbs) 利用血红素b作为辅因子，在生命的所有王国中都有发现。目前的知识揭示了 Hb 一级序列的巨大变异性，导致拓扑、生化和生理学的个性化。由于 Hb 似乎通过结构特征的特定组合来调节它们的反应性，因此预测给定 Hb 的特征仍然是几乎不可能的。单细胞绿藻莱茵衣藻含有 12 个编码截断谱系的不同 Hb 的基因，其中几个基因具有未知功能的扩展 N 或 C 末端。对一些衣藻的研究Hbs 揭示了不可预测的结构和生化变异，连同其基因的不同表达，表明了不同的生理作用。衣藻因此，它代表了一个有前途的系统来分析 Hb 结构、生物化学和生理学的多样化。在这里，我们报告了 cyanomet THB11 (Cre16.g662750) 的血红素结合域的晶体结构，解析为 1.75 Å，这是五坐标藻类 Hb 之一，它在还原状态下提供一个自由的 Fe 配位位点。THB11 的整体折叠是保守的，但诸如螺旋 E 中的扭结、倾斜的血红素平面和假定的隧道出口处的甲硫氨酸残基聚集等个别特征似乎是独一无二的。N-和C-末端都促进低聚物混合物的形成，并且C末端的缺失导致亚硝酸盐还原率降低。这项工作拓宽了 2/2Hb 家族的结构和生化知识，并表明衣藻的 N 端和 C 端延伸 2/2Hbs 通过分子间相互作用调节它们的反应性。