The Beauty of Bacteriorhodopsin

Bacteriorhodopsin is a photoactive enzyme found in the cell membrane of halobacteria which call some of the harshest ecological niches on earth home. Bacteriorhodopsin is arranged in these bacteria as a two-dimensional crystal integrated into the cell membrane. These patches are called purple membranes and may occupy up to 80% of the membrane surface. Bacteriorhodopsin is the key enzyme in the photosynthetic system of halobacteria. It is an elegantly simple enzyme that allows for the direct conversion of sunlight to chemical energy with the use of only one enzyme. Compare this with chlorophyll-based photosynthetic systems, and the beauty of bacertiorhodopsin can be easily seen. When light strikes this enzyme, a proton is pumped from the inside to the outside of the cell and this proton gradient then drives the synthesis of ATP by ATPase. Ever since its discovery, the potential for the use of bacteriorhodospin in a whole host of technical applications has been recognized. These include uses in photoelectric, photochromic, and charge transport applications including the conversion of sunlight to electricity. The purple membranes that bacertiorhodopsin are arranged in show analogous functions for protons that silicon solar cells show for electrons. The bacertiorhodopsin molecules individually transport protons across the membrane and the purple membranes are not permeable to the back-flow of protons into the cell. These traits of bacertiorhodopsin along with the facts that it is highly photosensitive compared to synthetic materials and can be easily modified, make it a good candidate for translation to technical applications.

Hampp, N. Chem. Rev. 2000, 100, 1755-1776.

Crystal structures of bacertiorhodopsin have been identified for all the intermediates of the bacertiorhodopsin transport cycle which have allowed the mechanism of proton transport across the cell membrane to be determined. The mechanism is based on nine conformational changes of the key cofactor, retinal. Retinal is initially attached to a Lys residue of the enzyme via a Schiff base linkage, and with the input of light, retinal is converted from its trans isomer to its cis isomer. The thermal reisomerization of the cis-form back to the trans-form drives the process of pumping protons across the membrane. The relaxation of the strained conformation of retinal is made possible by local conformational changes of the enzyme which translate into global conformational changes and allow for the passage of a proton through the pump. Interestingly, these global conformational changes observed in bacertiorhodopsin are very similar to those observed in G-protein coupled receptors and methods used in determining the mechanism of bacertiorhodopsin proton pumping may prove useful in doing the same thing for G-protein coupled receptors.

Lanyi, J. K.; Schobert, B. Biochemistry 2004, 43, 3-8.

The mechanism described above includes five proton-transfer steps. First a proton is transferred from the retinal Schiff base to the nearby Asp85 residue. This residue is electrostatically coupled to an extracellular group called the proton release group which releases a proton to the outside of the cell once Asp85 is protonated. The Schiff base is then reprotonated from the cytoplasmic residue Asp96 which will in turn be reprotonated from the cytoplasm. The final step is then the transfer of a proton from the prontonated Asp85 residue to the proton release group which gave up a proton to the extracellular environment. This is a long-distance proton transfer covering a distance of about 12 angstroms. This work investigates this final transfer using computational methods and concludes that a transient intermediate is possible that involves a protonated Asp212 residue stabilized by water molecules to facilitate this long-range proton transfer.

Phatak, P.;  Frahmcke, J. S.; Wanko, M.; Hoffmann, M.; Strodel, P.; Smith, J. C.;  Suhai, S.;  Bondar, A.; Elstner, M. J. Am. Chem. Soc., 2009, 131 (20), 7064–7078.

I chose this enzyme from the Molecule of the Month on the Protein Data Bank

http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb27_1.html