Department of Chemistry, University of Louisville, Louisville, KY 40292, United States of America.
Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia in Katowice, Szkolna 9, PL-40 006 Katowice, Poland.
J Photochem Photobiol B. 2020 Aug;209:111919. doi: 10.1016/j.jphotobiol.2020.111919. Epub 2020 May 28.
The discovery of naturally occurring B-depedent photoreceptors has allowed for applications of cobalamins (Cbls) in optogenetics and synthetic biology to emerge. However, theoretical investigations of the complex mechanisms of these systems have been lacking. Adenosylcobalamin (AdoCbl)-dependent photoreceptor, CarH, is one example and it relies on daylight to perform its catalytic function. Typically, in enzymes employing AdoCbl as their cofactor, the Co-C bond activation and cleavage is triggered by substrate binding. The cleavage of the Co-C bond is homolytic resulting in radical pair formation. However, in CarH, this bond is instead activated by light. To explore this peculiarity, the ground and first excited state potential energy surfaces (PESs) were constructed using the quantum mechanics/molecular mechanics (QM/MM) framework and compared with other AdoCbl-dependent enzymes. QM/MM results indicate that CarH is photolytically active as a result of the AdoCbl dual role, acting as a radical generator and as a substrate. Photo-cleavage of the Co-C bond and subsequent H-atom abstraction is possible because of the specific orientation of the H-C bond with respect to the Co(II) center. Comparison with other AdoCbl-dependent enzymes indicate that the protein environment in the CarH active center alters the photochemistry of AdoCbl by controlling the stereochemistry of the ribose moiety.
天然存在的 B 依赖性光感受器的发现使得钴胺素 (Cbl) 在光遗传学和合成生物学中的应用成为可能。然而，这些系统的复杂机制的理论研究一直缺乏。依赖腺嘌呤核苷酸钴胺素 (AdoCbl) 的光感受器 CarH 就是一个例子，它依赖日光来发挥其催化功能。通常，在使用 AdoCbl 作为辅助因子的酶中，Co-C 键的活化和断裂是由底物结合引发的。Co-C 键的断裂是均裂的，导致自由基对的形成。然而，在 CarH 中，这个键是被光激活的。为了探索这种特殊性，使用量子力学/分子力学 (QM/MM) 框架构建了基态和第一激发态势能面 (PES)，并与其他依赖 AdoCbl 的酶进行了比较。QM/MM 结果表明，由于 AdoCbl 的双重作用，CarH 具有光裂解活性，既作为自由基生成剂，又作为底物。由于 H-C 键相对于 Co(II)中心的特定取向，Co-C 键的光裂解和随后的 H 原子夺取是可能的。与其他依赖 AdoCbl 的酶的比较表明，CarH 活性中心的蛋白质环境通过控制核糖部分的立体化学来改变 AdoCbl 的光化学性质。
