Fig. 8
From: Optimized photo-stimulation of halorhodopsin for long-term neuronal inhibition
Proposed photo-cycle of NpHR. An extracellular chloride ion is bound to the Schiff base lysine of NpHR at resting state, with Km = 16 mM [11]. Photon absorption (with maximum at 580 nm) triggers the isomerization of retinal and starts the photo-cycle, containing intermediates K (omitted here), L, N, and O. The chloride ion is released into the cytosol during the transition from N to O, and uptake of a chloride ion from the extracellular side takes place in the recovery from O to the initial state. HR without a bound chloride ion is prone to deprotonation of the Schiff base in the L state (indicated by dashed line), leading to formation of M. This intermediate is long-lived and absorbs similarly as HR410 (or M412 in BR) from Halobacterium salinarum. The uptake of the proton for reprotonation of the M intermediate is very slow in dark (open arrow) but fast after absorption of a blue photon (blue arrow). Our data support deprotonation of the chloride-free L state (indicated by broken line)