Hemoglobin: Some (Dis)Assembly Required
Hemoglobin: Some (Dis)Assembly Required
复制标题
血红蛋白：需要一些（分解）组装
DOI：
10.1016/j.bpj.2019.12.041
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发表时间：
2020
影响因子：
3.4
通讯作者：
Lecomte, Juliette T.J.
中科院分区：
文献类型：
--
作者：
Lecomte, Juliette T.J.
血红蛋白的显着化学性质吸引了研究人员超过150年。历史记载让我们想起了拉瓦锡的工作，并从一个开创性的贡献进行到下一个，因为他们把脊椎动物血液中发现的双氧载体描绘成几个现代自然科学领域起源的星星。作为研究最广泛的蛋白质之一，人类血红蛋白是生物化学教科书的主要内容。它的基本特征，如不同构象和连接状态下的三维结构和变构效应物的作用模式，现在被广泛表征。然而，根本问题依然存在。血红蛋白是如何组装的，又是如何分解的？迄今为止，这些方面都无法用数量来描述。绘制装配和拆卸模型的挑战确实相当大。人成人血红蛋白（HbA）是ab-异源二聚体的二聚体。形成两种类型的接口，a1 b1，这是由坚固的包装接触，和a1 b2，这形成了较弱的滑动接触负责四元变化链接到双氧绑定。虽然蛋白质亚基相互作用的研究似乎不是特别值得注意，但血红蛋白系统有一个真正有问题的特征：它含有血红素基团。这种辅因子以1：1的化学计量与a链和b链紧密结合（图1），具有氧化还原活性，几乎不溶于水。因此，它使旨在解开功能性四聚体拆解的机械特征的实验工作复杂化。在这一期的《生物物理学杂志》上，塞缪尔等人。(1)提出了铁血红蛋白变性的平衡模型。几十年来对肌红蛋白（血红蛋白的单体对应物）的研究已经预示了他们任务的艰巨性（2，3）。新的血红蛋白研究建立在肌红蛋白的例子和先前对脱辅基血红蛋白解折叠的分析的基础上（4），提出了可逆Hb分解的全面观点。氧合血红蛋白容易发生铁氧化，这是一种导致“满足”状态的反应。高铁血红蛋白不仅没有功能，而且相对不稳定，它的形成标志着血红蛋白分解的开始。因此，Samuel及其同事选择详细绘制高铁血红蛋白的命运，为此，他们用重组HbA野生型和变体以及重组胎儿血红蛋白（HbF）进行了化学变性实验。氯化胍是保持血红素（亚铁血红素的Fe（III）形式）在溶液中和单体状态的首选变性剂。选择圆二色性和电子吸收来监测整个过渡的二级结构和氯化血红素。通过光谱去卷积，梳理出氯化血红素的配位，以及全局拟合，整合所有可用的数据，Samuel等人能够重建HbA分解成其组分时所访问的能量景观。的光谱结果，增强了小角度X-射线散射，支持血红素辅因子是不可或缺的稳定性和压实的四聚体。正如肌红蛋白所指出的，许多部分折叠的状态，有些类似于熔化的小球，是血红蛋白变性过程所必需的。一种特殊类型的中间体，半色素，在分析中脱颖而出。半色素协调氯化血红素与近端组氨酸和第二个蛋白质侧链，传达独特的电子吸收签名的计划。对于某些蛋白质，包括HbA，半染色质状态通过功能构象的扭曲而发生（图1，C和D），而对于其他一些蛋白质，半染色质是天然的静止状态。
The remarkable chemical properties of hemoglobin have fascinated researchers for more than 150 years. Historical accounts remind us of the work of Lavoisier and proceed from one seminal contribution to the next as they paint the dioxygen carrier found in vertebrate blood as a star at the origin of several modern fields of natural sciences. As one of the most extensively studied proteins, human hemoglobin is a staple of biochemistry textbooks. Its essential features, such as three-dimensional structure in different conformational and ligation states and the mode of action of allosteric effectors, are by now extensively characterized. Yet fundamental questions remain. How is hemoglobin assembled, and how does it fall apart? These aspects have so far resisted quantitative description. The challenge of drawing models of assembly and disassembly is indeed considerable. Human adult hemoglobin (HbA) is a dimer of ab-heterodimers. Two types of interfaces are formed, a1b1, which is held by sturdy packing contacts, and a1b2, which forms the weaker sliding contacts responsible for quaternary changes linked to dioxygen binding. Although the study of protein subunit interactions may not seem particularly noteworthy, the hemoglobin system has a truly problematic trait: it contains heme groups. This cofactor, which associates tightly in a 1: 1 stoichiometry with each of the a-and b-chains (Fig. 1), is redox active and practically insoluble in water. As such, it complicates experimental work designed to unravel the mechanistic features of functional tetramer dismantling. In this issue of Biophysical Journal, Samuel et al.(1) present an equilibrium model of ferric hemoglobin denaturation. The difficulty of their task is presaged by decades of unfolding studies of myoglobin (2, 3), the monomeric counterpart of hemoglobin. The new hemoglobin study builds on the myoglobin example and a prior analysis of apohemoglobin unfolding (4) to propose a comprehensive view of reversible Hb disassembly. Oxygenated hemoglobin is prone to iron oxidation, a reaction that leads to the ‘‘met’’state. Not only is methemoglobin nonfunctional, but it is also relatively unstable, and its formation marks the onset of hemoglobin disassembly. Thus, Samuel and co-workers chose to map out the fate of methemoglobin in detail, and to do so, they performed chemical denaturation experiments with recombinant HbA wild-type and variants and recombinant fetal hemoglobin (HbF). Guanidinium chloride is the denaturant of choice to maintain hemin (the Fe (III) version of heme) in solution and in the monomeric state. Circular dichroism and electronic absorption were selected to monitor secondary structure and hemin throughout the transitions. With spectral deconvolution, to tease out hemin coordination, and global fitting, to integrate all available data, Samuel et al. were able to reconstruct the energy landscape visited by HbA as it breaks down into its components. The spectroscopy results, enhanced by small-angle x-ray scattering, support that the heme cofactor is integral to the stability and compaction of the tetramer. As myoglobin announced, a number of partially folded states, some resembling molten globules, are necessary to account for the denaturation path of Hb. One particular type of intermediate, the hemichrome, stands out in the analysis. Hemichromes coordinate hemin with the proximal histidine and a second protein side chain, a scheme that conveys distinctive electronic absorption signatures. For some proteins, HbA included, hemichrome states occur through distortions of the functional conformation (Fig. 1, C and D), whereas for some others, hemichromes are the native, resting state …