Team:Hong Kong-CUHK/Project/Halorhodopsin

Halorhodopsin

1.     General Information

Fig. 1 The protein structure of halorhodopsin.

Halorhodopsin is an inward-directed light-driven chloride ion pump originating from Halobacterium. It utilizes light to pump chloride ions against chloride concentration difference into cells from the environment. Halobacterium is a genus of the Halobacteriaceae, which can live in extremely high salinity environment. Halorhodopsin, together with bacteriorhodopsin, helps maintain cell osmolality and proliferation while reducing the consumption of metabolic energy. It is one of the important features for Halobacterium to live under high salinity conditions¹.

Halorhodopsin consists of seven transmembrane helices surrounding a central pore. The helices together form a ion channel for passing chloride ion, making use of retinal chromophore which is covalently bound to a lysine residue (Lys242) via a protonated Schiff base². The ion channel is divided into two half-channels: an extracellular half and a cytoplasmic half. Halorhodopsin can use a photon to transfer one chloride ion from the environment to cytoplasm against electrochemical potential².

2.     Mechanism of halorhodopsin

Fig. 2 The whole process can be divided into three stages. Firstly, chloride ion binds to the protonated Schiff base in the halorhodopsin molecule. Secondly, using the energy of one photon, the ion is transferred to the cytoplasmic half-channel by the bound retinal’s conformational change, the chloride ion is then released to cytoplasm. Finally, the channel changes back to the initial state for the next round of ion transfer² .

Halorhodopsin harnesses the photon energy by converting it into molecular energy via photoisomeriztion: upon taking one proton, trans-retinal changes conformation to cis-retinal³.This action flips the N-H dipole of the protonated Schiff base from extracellular side to cytoplasmic side. This meta-stable state is called K state. Photoisomerization is followed by L1 state, in which the chloride ion is translocated from the extracellular side to the cytoplasmic side². Further conformational change opens a channel, allowing chloride ion releasing to cytoplasm².

After the chloride ion leaving the channel, halorhodopsin changes to O1 state: cis-retinal reisomerizes back to trans-retinal and the N-Hdipole of the protonated Schiff base flips from cytoplasmic side to the extracellular side. Afterwards,the halorhodopsin changes to O2 state, allowing chloride ion entering the channel toward the extracellular binding site. To enhance the binding of chloride ion to the channel, during the formation of O1 intermediate, the guanidinium group of Arginine 108 turns to the extracellular side. As guanidinium group is positively charged, when it is facing the extracellular side, uptake of chloride ion can be enhanced by electrostatic interaction². After the uptake of chloride ion, the guanidinium group returns to the cytoplasmic side and the protein changes from N state back to ground state².

References

1.        Lanyi, J.K. Halorhodopsin, a light-driven electrogenic chloride-transport system. Physiological reviews 70, 319(1990).

2.        Essen, L.O. Halorhodopsin: light-driven ion pumping madesimple? Current opinion in structural biology 12, 516–522(2002).

3.        Haupts, U. et al. General concept for ion translocation byhalobacterial retinal proteins: the isomerization/switch/transfer (IST) model. Biochemistry36, 2–7(1997).