The mechanism of photo-energy storage in the halorhodopsin chloride pump.

Christoph Pfisterer, Andreea D. Gruia, Stefan Fischer*

J. Biological Chemistry, vol. 284, p.13562-13569 (2009).


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 photo-isomerized from all-trans to 13-cis, to drive the translocation of one chloride anion across the membrane. 

The energy stored in the K-state amounts to only one fifth of the photon energy. 
A combined quantum mechanical/molecular mechanical (QM/MM) analysis of the protein explains why the protein did not evolve to store more energy: Even though it would accelerate chloride pumping, a higher energy level of the K-state leads to reduced energy barriers against thermal relaxation along unproductive pathways, in particular via the premature cis to tran back-isomerisation.

Minimum Energy Paths (MEP) computed by Conjugate Peak Refinement (CPR) show that the protein has evolved to finely balance storage versus unproductive dissipation:  The back-isomerization barriers are just high enough (≥18kcal/mol) to keep the decay rate (1/100ms) slower than the remaining photocycle (1/20ms). This need to stabilize the captured photon-energy until it can be used in slower subsequent steps is inherent to light-driven proteins, as seen for example in the bacteriorhodopsin proton pump.
The four stationary states of retinal isomerization :
(protonated Schiff-base (PSB) of the retinal in yellow)

A) All-trans ground state: the positively charged PSB makes favorable interactions with the nearby negative groups (chloride and Asp238).
Transition state ‡(Ser115) of the counter-clockwise route for cistrans isomerization.
Transition state ‡(Asp238) of the clockwise route for cistrans isomerization.
D) 13-cis planar K-state: the PSB makes poorer interactions with the negative groups than in A). This is the energy storage mechanism.
4 states along the trans-cis pathway
Minimum Energy Pathway (MEP) along a 360° rotation of the PSB:

QM/MM energy profile of the pathway through the four structures shown on the left:
trans → ‡(Ser115) → 13‑cis→  ‡(Asp238) → trans

The total energy (continuous black line) is the sum of:
- the retinal self energy (in blue)
- the opsin self energy (everything excluding retinal, in red)
- and the interaction between retinal and the opsin (in green).

The arrows show the two possible decay routes of 13-cis.
Molecular movies showing these transitions are shown below.

Energy profile of cis-trans isomerization

Movie 1.  Decay of K-state via ‡(Asp238),  clockwise rotation of the PSB.
The energy barrier is 18 kcal/mol relative to the 13-cis K-state. This is the preferred route for 13-cis → trans backisomerisation.
  Download the movie  (0.4Mb)

Movie 2.  Decay of K-state via ‡(Ser115),  counter-clockwise rotation of the PSB.
The energy barrier is 23 kcal/mol relative to the 13-cis K-state.
  Download the movie  (0.6Mb)

Movie 3.  360° rotation of the PSB,  trans → ‡(Ser115) → 13‑cis→  ‡(Asp238) → trans.
Concatenation of movies 1 and 2. Same order of events as in the energy profile shown above.
  Download the movie  (0.9Mb)

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