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首页> 外文期刊>Biochemistry >Slow ligand binding kinetics dominate ferrous hexacoordinate hemoglobin reactivities and reveal differences between plants and other species
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Slow ligand binding kinetics dominate ferrous hexacoordinate hemoglobin reactivities and reveal differences between plants and other species

机译：慢的配体结合动力学支配六价铁配位的血红蛋白反应性，并揭示了植物与其他物种之间的差异

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Hexacoordinate hemoglobins are found in many living organisms ranging from prokaryotes to plants and animals. They are named "hexacoordinate" because of reversible coordination of the heme iron by a histidine side chain located in the heme pocket. This endogenous coordination competes with exogenous ligand binding and causes multiphasic relaxation time courses following rapid mixing or flash photolysis experiments. Previous rapid mixing studies have assumed a steady-state relationship between hexacoordination and exogenous ligand binding that does not correlate with observed time courses for binding. Here, we demonstrate that this assumption is not valid for some hexacoordinate hemoglobins, and that multiphasic time courses are due to an appreciable fraction of pentacoordinate heme resulting from relatively small equilibrium constants for hexacoordination (K-H). CO binding reactions initiated by rapid mixing are measured for four plant hexacoordinate hemoglobins, human neuroglobin and cytoglobin, and Synechocystis hemoglobin. The plant proteins, while showing a surprising degree of variability, differ from the others in having much lower values of K-H. Neuroglobin and cytoglobin display dramatic biphasic time courses for CO binding that have not been observed using other techniques. Finally, an independent spectroscopic quantification of K-H is presented that complements rapid mixing for the investigation of hexacoordination. These results demonstrate that hexacoordination could play a much larger role in regulating affinity constants for ligand binding in human neuroglobin and cytoglobin than in the plant hexacoordinate hemoglobins.
机译：从原核生物到动植物，在许多活生物体中发现六配位血红蛋白。由于它们通过位于血红素袋中的组氨酸侧链对血红素铁的可逆配位而被称为“六配位”。这种内源性配位与外源性配体结合竞争，并在快速混合或快速光解实验后引起多相弛豫时间过程。先前的快速混合研究已假定六配位与外源配体结合之间存在稳态关系，该关系与观察到的结合时程无关。在这里，我们证明该假设对某些六配位血红蛋白无效，并且多相时程是由于六配位（K-H）的平衡常数相对较小而导致的五配位血红素的可观分数所致。通过快速混合引发的CO结合反应被测量为四种植物六配体血红蛋白，人神经血红蛋白和细胞血红蛋白以及集胞藻血红蛋白。植物蛋白虽然表现出令人惊讶的可变性，但与其他蛋白相比，其K-H值要低得多。神经血红蛋白和细胞血红蛋白对于CO结合表现出戏剧性的双相时程，而其他技术尚未观察到。最后，提出了一种独立的K-H光谱定量方法，该方法可补充快速混合法用于六配位研究。这些结果表明，与植物六配体血红蛋白相比，六配位在调节人神经球蛋白和细胞血红蛋白配体结合亲和常数方面可以发挥更大的作用。

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