Protonation-Enhanced Energy Transfer in Xanthorhodopsin Kin4B8

Microbial rhodopsins are key light-harvesting proteins for energy conversion in aquatic environments. While typically reliant on retinal, xanthorhodopsins (XRs) also employ carotenoids as antenna pigments. Here, we investigate Kin4B8, a novel freshwater XR that exhibits a counterintuitive increase in lutein-to-retinal excitation-energy transfer (EET) efficiency under acidic conditions─from 40% at neutral pH to 55%─despite a redshift in its absorption spectrum. By integrating spectroscopic measurements with quantum chemical calculations, we examine the influence of pH-dependent protonation states of two key residues, His60 and Asp94, on EET. We find that protonation of Asp94, in particular, markedly enhances lutein-retinal electronic coupling while moderately reducing spectral overlap. The 1.24-fold increase in squared coupling compensates for the decreased overlap, resulting in a net 1.07-fold increase in the EET rate. These findings suggest that rhodopsin-carotenoid complexes employ protonation-dependent tuning of carotenoid-retinal energy transfer as a mechanism for pH-responsive adaptation to diverse light environments.