Protein and Solvent Reorganization Drives Radical Pair Stability in Avian Cryptochrome 4a

Within the photoreceptor cells of avian retinas, cryptochrome-4a
(Cry4a) has been proposed as a magnetic compass sensor in migratory
songbirds. Recent photochemistry studies have demonstrated that Cry4a
from the night-migratory European robin (Erithacus
rubecula) exhibits greater magnetic sensitivity compared
to the nonmigratory bird species. In ErCry4a, blue-light
excitation of the flavin triggers stepwise electron transfer along
a tryptophan tetrad to the flavin, forming well-separated radical
pairs that have been proposed to play a role in magnetoreception.
Herein, we employ first-principles electronic structure methods and
hybrid quantum mechanical/molecular mechanical (QM/MM) simulations
to investigate the stabilization, interconversion, and recombination
of these well-separated radical pairs. Reorganization of the protein
environment and the aqueous solvent substantially stabilizes the long-range
charge-transfer states associated with the anionic flavin radical
and the cationic radical localized on the third or fourth tryptophan,
rendering these radical-pair states energetically comparable to those
in the charge-neutral state. Free energy analysis combined with electronic
coupling calculations provides supporting evidence for the previously
proposed idea of a “composite” radical pair, involving
both the third and fourth tryptophans, as the functional magnetoreceptor.
To provide guidance in probing the potential role of the composite
radical pair in magnetoreception, we identify key amino acid residues
whose mutation may significantly alter the relative stabilization
and chemical dynamics of these radical pairs and consequently affect
magnetic field sensitivity of ErCry4a.

在鸟类视网膜感光细胞中，隐花色素4a（cryptochrome-4a, Cry4a）被认为是迁徙鸣禽的磁罗盘传感器。近期光化学研究表明，来自夜间迁徙欧亚鸲（Erithacus rubecula）的Cry4a，其磁敏感性优于非迁徙鸟类。在欧亚鸲Cry4a（ErCry4a）中，黄素的蓝光激发会触发沿色氨酸四聚体向黄素的分步电子转移，形成间距较远的自由基对，这类自由基对被认为在磁感受中发挥作用。本研究采用第一性原理电子结构方法与混合量子力学/分子力学（QM/MM）模拟，对这类间距较远的自由基对的稳定化、相互转化与重组过程展开研究。蛋白质微环境与水溶液溶剂的重构，可显著稳定由阴离子黄素自由基与定位于第三或第四色氨酸的阳离子自由基构成的长程电荷转移态，使这类自由基对态的能量水平与电荷中性态相当。结合自由能分析与电子耦合计算的结果，为此前提出的“复合自由基对”假说提供了支撑证据——该假说认为，由第三和第四色氨酸共同参与构成的复合自由基对，是发挥磁感受功能的核心物种。为探究复合自由基对在磁感受中的潜在作用，本研究还筛选出关键氨基酸残基，对其进行突变可显著改变这类自由基对的相对稳定性与化学动力学过程，进而影响ErCry4a的磁场灵敏度。