Protoporphyrin IX iron complex (heme) is an important cofactor for oxygen transfer, oxygen storage, oxygen activation, and electron transfer when bound to the heme proteins hemoglobin, myoglobin, cytochrome P450 and cytochrome c, respectively. In addition to these prototypical heme proteins, there are emergent, critical roles of exchangeable/labile heme in signal transduction. Specifically, it has been shown that association/dissociation of heme to/from heme-responsive sensors regulates numerous functions, including transcription, DNA binding, microRNA splicing, translation, protein kinase activity, protein degradation, heme degradation, K⁺ channel function, two-component signal transduction, and many other functions. In this review, we provide a comprehensive overview of structure–function relationships of heme-responsive sensors and describe new, additional roles of exchangeable/labile heme as functional inhibitors and activators. In order to complete the description of the various roles of heme in heme-bound proteins, we also mention heme as a novel chemical reaction centre for aldoxime dehydratase, cis–trans isomerase, N–N bond formation, hydrazine formation and S–S formation, and other functions. These unprecedented functions of exchangeable/labile heme and heme proteins should be of interest to biological chemists. Insight into underlying molecular mechanisms is essential for understanding the new role of heme in important physiological and pathological processes.

中文翻译：

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血红素：血红素在信号传导，功能调节和催化中心中的新兴作用。

原卟啉IX铁配合物（血红素）当分别结合血红蛋白，血红蛋白，肌红蛋白，细胞色素P450和细胞色素c时，是氧转移，氧存储，氧活化和电子转移的重要辅助因子。除了这些原型血红素蛋白外，在信号转导中还出现了可交换/不稳定血红素的重要作用。具体而言，已显示血红素与血红素反应性传感器之间的缔合/解离调节多种功能，包括转录，DNA结合，微小RNA剪接，翻译，蛋白激酶活性，蛋白质降解，血红素降解，K ⁺通道功能，两分量信号转导以及许多其他功能。在这篇综述中，我们提供了血红素敏感型传感器的结构-功能关系的全面概述，并描述了可交换/不稳定血红素作为功能抑制剂和激活剂的新的，附加的作用。为了完整描述血红素在血红素结合蛋白中的各种作用，我们还提到血红素是醛肟脱水酶顺式-反式的新型化学反应中心异构酶，N–N键形成，肼形成和S–S形成等功能。可交换/不稳定的血红素和血红素蛋白的这些空前的功能应该引起生物化学家的兴趣。深入了解潜在的分子机制对于了解血红素在重要的生理和病理过程中的新作用至关重要。