Butcher David, Bernad Sophie, Derrien Valerie, Sebban Pierre, Miksovska Jaroslava
Department of Chemistry and Biochemistry, Florida International University , Miami Florida 33199, United States.
Laboratoire de Chimie Physique, Université Paris-Sud 11 , 91405 Orsay, France.
J Phys Chem B. 2017 Jan 19;121(2):351-364. doi: 10.1021/acs.jpcb.6b06933. Epub 2017 Jan 10.
Type 1 nonsymbiotic hemoglobins are found in a wide variety of land plants and exhibit very high affinities for exogenous gaseous ligands. These proteins are presumed to have a role in protecting plant cells from oxidative stress under etiolated/hypoxic conditions through NO dioxygenase activity. In this study we have employed photoacoustic calorimetry, time-resolved absorption spectroscopy, and classical molecular dynamics simulations in order to elucidate thermodynamics, kinetics, and ligand migration pathways upon CO photodissociation from WT and a H73L mutant of type 1 nonsymbiotic hemoglobin from Oryza sativa (rice). We observe a temperature dependence of the resolved thermodynamic parameters for CO photodissociation from CO-rHb1 which we attribute to temperature dependent formation of a network of electrostatic interactions in the vicinity of the heme propionate groups. We also observe slower ligand escape from the protein matrix under mildly acidic conditions in both the WT and H73L mutant (τ = 134 ± 19 and 90 ± 15 ns). Visualization of transient hydrophobic channels within our classical molecular dynamics trajectories allows us to attribute this phenomenon to a change in the ligand migration pathway which occurs upon protonation of the distal His73, His117, and His152. Protonation of these residues may be relevant to the functioning of the protein in vivo given that etiolation/hypoxia can cause a decrease in intracellular pH in plant cells.
1型非共生血红蛋白存在于多种陆地植物中，对外源气体配体表现出非常高的亲和力。这些蛋白质被认为在黄化/缺氧条件下通过一氧化氮双加氧酶活性保护植物细胞免受氧化应激。在本研究中，我们采用光声量热法、时间分辨吸收光谱法和经典分子动力学模拟，以阐明一氧化碳从野生型和水稻1型非共生血红蛋白的H73L突变体光解离后的热力学、动力学和配体迁移途径。我们观察到一氧化碳从一氧化碳-rHb1光解离的解析热力学参数与温度有关，我们将其归因于血红素丙酸基团附近静电相互作用网络的温度依赖性形成。我们还观察到在轻度酸性条件下，野生型和H73L突变体中配体从蛋白质基质中逸出的速度较慢（τ = 134 ± 19和90 ± 15纳秒）。在我们的经典分子动力学轨迹中对瞬态疏水通道的可视化使我们能够将这种现象归因于远端His73、His117和His152质子化时配体迁移途径的变化。考虑到黄化/缺氧会导致植物细胞内pH值下降，这些残基的质子化可能与蛋白质在体内的功能有关。
