Department of Chemistry, Seattle University, 901 12th Ave., Seattle, WA, 98122, USA.
J Biol Inorg Chem. 2020 May;25(3):489-500. doi: 10.1007/s00775-020-01777-0. Epub 2020 Mar 27.
In this study, we investigate the thermodynamic mechanisms by which electron transfer proteins adapt to environmental temperature by directly comparing the redox properties and folding stability of a psychrophilic cytochrome c and a mesophilic homolog. Our model system consists of two cytochrome c proteins from diatoms: one adapted specifically to polar environments, the other adapted generally to surface ocean environments. Direct electrochemistry shows that the midpoint potential for the mesophilic homolog is slightly higher at all temperatures measured. Cytochrome c from the psychrophilic diatom unfolds with a melting temperature 10.4 °C lower than the homologous mesophilic cytochrome c. Changes in free energy upon unfolding are identical, within error, for the psychrophilic and mesophilic protein; however, the chemical unfolding transition of the psychrophilic cytochrome c is more cooperative than for the mesophilic cytochrome c. Substituting alanine residues found in the mesophile with serine found in corresponding positions of the psychrophile demonstrates that burial of the polar serine both decreases the thermal stability and decreases the midpoint potential. The mutagenesis data, combined with differences in the m-value of chemical denaturation, suggest that differences in solvent accessibility of the hydrophobic core underlie the adaptation of cytochrome c to differing environmental temperature.
在这项研究中，我们通过直接比较一种嗜冷细胞色素 c 和一种中温同系物的氧化还原性质和折叠稳定性，研究了电子转移蛋白如何通过热力学机制适应环境温度。我们的模型系统由两种来自硅藻的细胞色素 c 蛋白组成：一种专门适应极地环境，另一种通常适应海洋表面环境。直接电化学显示，在测量的所有温度下，中温同系物的中点电位略高。与同源中温细胞色素 c 相比，来自嗜冷硅藻的细胞色素 c 的解折叠温度低 10.4°C。在误差范围内，嗜冷和中温蛋白的折叠自由能变化相同；然而，嗜冷细胞色素 c 的化学展开转变比中温细胞色素 c 更具协作性。用嗜冷蛋白中相应位置的丝氨酸取代中温蛋白中的丙氨酸残基表明，极性丝氨酸的埋藏既降低了热稳定性，又降低了中点电位。突变数据，加上化学变性 m 值的差异，表明细胞色素 c 对不同环境温度的适应源于疏水性核心溶剂可及性的差异。
