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. The purple color of the water below in Owens Lake, CA is due to the purple membranes of halobacteria.
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. This mechanism 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.
|Bacteriorhodopsin is made up of three identical subunits.|
|A subunit of bacteriorhodopson colored according to secondary structure.|
|Retinal - a key cofactor for bacteriorhodopsin.|
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 meaning that bacteriorhodopsin is not only a beautifully elegant enzyme for halobacteria but is also a potential source of renewable energy for us! While bacteriorhodopsin may not have the most exciting structure of any enzyme, the simple yet extremely effective mechanism by which it captures the energy of the sun makes it worthy of the title for Protein of the Year.