Cryo-trapping intermediate states in the CarH photoreceptor

CarH is a vitamin B12-dependent photoreceptor protein that binds to operator DNA in a tetrameric complex to inhibit gene expression. The tetramer is stable in dark conditions, while exposure to light induces tetramer dissolution and dissociation from DNA, thus allowing gene expression. The suppressed gene controls the biosynthesis of carotenoids that protects bacteria from excess of sunlight (Jost et al., 2015). CarH’s dissociation mechanism has been studied by time-resolved transient absorption spectroscopy. Two photoreaction pathways have been proposed, providing insight into a number of intermediates formed during the dissociation process (Kutta et al., 2015 ; Miller et al., 2020). In both mechanisms the cobalt (Co) atom contained in the corrin of the B12 moiety plays the most important role by sensing light. Co’s redox photodynamics involves the transient formation of intermediate states that result in CarH tetramer dissociation.

CarH是一种依赖维生素B12的光受体蛋白，可通过四聚体复合物结合操纵基因DNA，进而抑制基因表达。该四聚体在黑暗环境中保持稳定，而光照会诱导四聚体解体并脱离DNA，使基因表达得以恢复。该受抑制的基因调控类胡萝卜素的生物合成，这类物质可保护细菌免受过量日光的伤害（Jost等，2015）。目前已通过时间分辨瞬态吸收光谱法对CarH的解离机制展开研究。学界已提出两种光反应途径，为解析解离过程中形成的多种中间态提供了理论依据（Kutta等，2015；Miller等，2020）。在两种机制中，B12辅因子咕啉环所含的钴（Co）原子通过感知光线发挥核心作用。钴的氧化还原光动力学过程会暂态形成多种中间态，最终引发CarH四聚体的解离。