Physical Chemistry B, 109,
14786-14788 Full paper (PDF)
Productive proton pumping by bacteriorhodopsin requires that, after
the all-trans to 13-cis photoisomerization of the retinal chromophore,
the photocycle proceeds with proton
transfer and not with thermal back isomerization. The
of how the protein controls these events in the active site was
using Quantum Mechanical/Molecular Mechanical and reaction path
with CPR. The results indicate that
retinal twisting significantly contributes to lowering the barrier for
the thermal cis-trans back isomerization, the rate-limiting barrier for
this isomerization is still ~5 kcal/mol larger than for the first
transfer step. In this way, the retinal twisting is finely tuned so as
to store energy to drive the subsequent photocycle while preventing
|Fig. 1. The absorption of one photon by the
retinal chromophore leads to rotation of the C13-C14 bond into the
conformation. This triggers a photocycle, the net effect of which is
transfer of one proton from the cytoplasmic to the extracellular side
||Fig. 2. Photoabsorption and electronic relaxation
the protein in the K-state (13-cis retinal). From there,
cis-trans back isomerization competes with the productive first
first proton transfer step.
The schematic energy profile is based on QM/MM-optimized energies
The movies below show the retinal back-isomerization, first from L to K, then from K to BR.
Retinal back-isomerization from L to K.
Rate-limiting barrrier 9kcal/mol. Download the movie , 1Mb
Retinal back-isomerization from K to BR.
It is a very small motion, yet it has a high rate-limiting barrrier of 11kcal/mol. Download the movie , 0.5Mb
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