CHAPTER 16
Structure, Dynamics and Function
of the Proton Pump Bacteriorhodopsin
G. BULDT I , J. HEBERLEI, R. SCHLESINGER I and H.-J. SASSI
1
Introduction
The integral membrane protein bacteriorhodopsin (bR) is an excellent example
to show how information is obtained from different biophysical methods to
understand the function of this protein on the basis of its structure and dynamics. There are only a few such proteins, which attracted the interest of a large
number of research groups.
1.1
The Photocycle and Proton Translocation
Bacteriorhodopsin is densly packed in hexagonal two dimensional lattices the socalled purple membranes (PM) in the plasma membrane of the Halobacterium
salinarum. It was discovered by Oesterhelt and Stockenius (1973) that bR contains the chromophore, retinal, covalently bound via a Schiff base to Lys 216,
which enables this protein to pump protons out of the cell (Fig. 16.1). The photocycle of bR (Fig. 16.2) is linked to the active transport of protons over the membrane by delivering part of the energy of the absorbed photon to this process. In
the ground state (bRs68}of the photo cycle the retinal is in an all-trans conformation, the Schiff's base is protonated (pK13) and positively charged. Upon absorption of a photon the retinal goes via an excited state to a 13-cis retinal conformation, which results in a dramatic pK down-shift to 3. Hence the Schiff's base proton is taken up by Asp 85 (Fig. 16.1) and another proton appears at the extracellular surface ofbR. These steps pass intermediate states of the protein which can be
monitored by colour changes of the chromophore (Fig. 16.2), which originates
from the altered electric field at the retinal position and its change in the isomerisation state. At this point of the photo cycle bR has reached the M412 intermediate which is easily detected by the blue shift of the absorption maximum to 412
nm. In order to complete the cycle and to return to the ground state, the Schiff's
base has to be reprotonated from the protonated Asp96 (Fig. 16.1). This amino
acid will then be reprotonated and the retinal is driven back to its all-trans conformation. (For recent reviews see Lanyi, 1997 and Oesterhelt, 1998.)
I Forschungszentrurn Jiilich, IBI-2: Biologische Strukturforschung, D-52425 Jillich.
Structure, Dynamics and Function
of the Proton Pump Bacteriorhodopsin
G. BULDT I , J. HEBERLEI, R. SCHLESINGER I and H.-J. SASSI
1
Introduction
The integral membrane protein bacteriorhodopsin (bR) is an excellent example
to show how information is obtained from different biophysical methods to
understand the function of this protein on the basis of its structure and dynamics. There are only a few such proteins, which attracted the interest of a large
number of research groups.
1.1
The Photocycle and Proton Translocation
Bacteriorhodopsin is densly packed in hexagonal two dimensional lattices the socalled purple membranes (PM) in the plasma membrane of the Halobacterium
salinarum. It was discovered by Oesterhelt and Stockenius (1973) that bR contains the chromophore, retinal, covalently bound via a Schiff base to Lys 216,
which enables this protein to pump protons out of the cell (Fig. 16.1). The photocycle of bR (Fig. 16.2) is linked to the active transport of protons over the membrane by delivering part of the energy of the absorbed photon to this process. In
the ground state (bRs68}of the photo cycle the retinal is in an all-trans conformation, the Schiff's base is protonated (pK13) and positively charged. Upon absorption of a photon the retinal goes via an excited state to a 13-cis retinal conformation, which results in a dramatic pK down-shift to 3. Hence the Schiff's base proton is taken up by Asp 85 (Fig. 16.1) and another proton appears at the extracellular surface ofbR. These steps pass intermediate states of the protein which can be
monitored by colour changes of the chromophore (Fig. 16.2), which originates
from the altered electric field at the retinal position and its change in the isomerisation state. At this point of the photo cycle bR has reached the M412 intermediate which is easily detected by the blue shift of the absorption maximum to 412
nm. In order to complete the cycle and to return to the ground state, the Schiff's
base has to be reprotonated from the protonated Asp96 (Fig. 16.1). This amino
acid will then be reprotonated and the retinal is driven back to its all-trans conformation. (For recent reviews see Lanyi, 1997 and Oesterhelt, 1998.)
I Forschungszentrurn Jiilich, IBI-2: Biologische Strukturforschung, D-52425 Jillich.