Ali Fedaa, Shafaa Medhat W, Amin Muhamed
Medical Biophysics Division, Department of Physics, Faculty of Science, Helwan University, Cairo 11795, Egypt.
Genome Science and Technology, The University of Tennessee, Knoxville, TN 37996, USA.
Biology (Basel). 2022 Feb 24;11(3):362. doi: 10.3390/biology11030362.
Photosystem I is a light-driven electron transfer device. Available X-ray crystal structure from Thermosynechococcus elongatus showed that electron transfer pathways consist of two nearly symmetric branches of cofactors converging at the first iron-sulfur cluster F, which is followed by two terminal iron-sulfur clusters F and F. Experiments have shown that F has lower oxidation potential than F and F, which facilitates the electron transfer reaction. Here, we use density functional theory and Multi-Conformer Continuum Electrostatics to explain the differences in the midpoint Em potentials of the F, F and F clusters. Our calculations show that F has the lowest oxidation potential compared to F and F due to strong pairwise electrostatic interactions with surrounding residues. These interactions are shown to be dominated by the bridging sulfurs and cysteine ligands, which may be attributed to the shorter average bond distances between the oxidized Fe ion and ligating sulfurs for F compared to F and F. Moreover, the electrostatic repulsion between the 4Fe-4S clusters and the positive potential of the backbone atoms is lowest for F compared to both F and F These results agree with the experimental measurements from the redox titrations of low-temperature EPR signals and of room temperature recombination kinetics.
光系统I是一种光驱动的电子转移装置。来自嗜热栖热放线菌的现有X射线晶体结构表明，电子转移途径由两个几乎对称的辅因子分支组成，这两个分支在第一个铁硫簇F处汇聚，随后是两个末端铁硫簇F和F。实验表明，F的氧化电位低于F和F，这有利于电子转移反应。在此，我们使用密度泛函理论和多构象连续介质静电学来解释F、F和F簇中点Em电位的差异。我们的计算表明，与F和F相比，F由于与周围残基的强成对静电相互作用而具有最低的氧化电位。这些相互作用显示出以桥连硫和半胱氨酸配体为主，这可能归因于与F和F相比，F的氧化态铁离子与连接硫之间的平均键距更短。此外，与F和F相比，F的4Fe-4S簇与主链原子正电位之间的静电排斥最低。这些结果与低温EPR信号的氧化还原滴定和室温重组动力学的实验测量结果一致。
