Cl(-) concentration dependence of photovoltage generation by halorhodopsin from Halobacterium salinarum

Three-Step Isomerization of the Retinal Chromophore during the Anion Pumping Cycle of Halorhodopsin

The anion pumping cycle of halorhodopsin from Natronomonas pharaonis ( pHR) is initiated when the all- trans/15- anti isomer of retinal is photoisomerized into the 13- cis/15- anti configuration. A recent crystallographic study suggested that a reaction state with 13- cis/15- syn retinal occurred during the anion release process, i.e., after the N state with the 13- cis/15- anti retinal and before the O state with all- trans/15- anti retinal. In this study, we investigated the retinal isomeric composition in a long-living reaction state at various bromide ion concentrations. It was found that the 13- cis isomer (^csHR'), in which the absorption spectrum was blue-shifted by ∼8 nm compared with that of the trans isomer (^taHR), accumulated significantly when a cold suspension of pHR-rich claret membranes in 4 M NaBr was illuminated with continuous light. Analysis of flash-induced absorption changes suggested that the branching of the trans photocycle into the 13- cis isomer (^csHR') occurs during the decay of an O-like state (O') with 13- cis/15- syn retinal; i.e., O' can decay to either ^csHR' or O with all- trans/15- anti retinal. The efficiency of the branching reaction was found to be dependent on the bromide ion concentration. At a very high bromide ion concentration, the anion pumping cycle is described by the scheme ^taHR -( hν) → K → L_1a ↔ L_1b ↔ N ↔ N' ↔ O' ↔ ^csHR' ↔ ^taHR. At a low bromide ion concentration, on the other hand, O' decays into ^taHR via O.

The mechanism of photo-energy storage in the Halorhodopsin chloride pump

The light-driven pump Halorhodopsin (hR) uses the energy stored in an initial meta-stable state (K), in which the bound retinal chromophore has been photoisomerized from all-trans to 13-cis, to drive the translocation of one chloride anion across the membrane. Thus far it was unclear whether retinal twisting or charge separation between the positive Schiff base of the retinal and the chloride anion is the primary mechanism of energy storage. Here, combined quantum mechanical/molecular mechanical (QM/MM) simulations show that the energy is predominantly stored by charge separation. However, a large variability in retinal twisting is observed, thus reconciling the contradictory hypotheses for storage. Surprisingly, the energy stored in the K-state amounts to only one-fifth of the photon energy. We explain why the protein cannot store more: even though this would accelerate chloride pumping, raising the K-state also reduces the relative energy barriers against unproductive decay, in particular via the premature cis to trans back-isomerization. Indeed, the protein has maximized its storage so that the back-isomerization barriers are just high enough (> or =18 kcal/mol) to keep the decay rate (1/100 ms) slower than the remaining photocycle (1/20 ms). This need to stabilize the captured photon-energy until it can be used in subsequent steps is inherent to light-driven proteins.

The nitrate transporting photochemical reaction cycle of the pharaonis halorhodopsin

Time-resolved spectroscopy, absorption kinetic and electric signal measurement techniques were used to study the nitrate transporting photocycle of the pharaonis halorhodopsin. The spectral titration reveals two nitrate-binding constants, assigned to two independent binding sites. The high-affinity binding site (K(a) = 11 mM) contributes to the appearance of the nitrate transporting photocycle, whereas the low-affinity constant (having a K(a) of approximately 7 M) slows the last decay process in the photocycle. Although the spectra of the intermediates are not the same as those found in the chloride transporting photocycle, the sequence of the intermediates and the energy diagrams are similar. The differences in spectra and energy levels can be attributed to the difference in the size of the transported chloride or nitrate. Electric signal measurements show that a charge is transferred across the membrane during the photocycle, as expected. A new observation is an apparent release and rebinding of a small fraction of the retinal, inside the retinal pocket, during the photocycle. The release occurs during the N-to-O transition, whereas the rebinding happens in several seconds, well after the other steps of the photocycle are over.