A brief guide to bacteriorhodopsin

A brief guide to bacteriorhodopsin (bR)

Bacteriorhodopsin (bR) is the simplest photoactive energy conversion biological device that is known to the world. It is found as a two-dimensional crystal in the cell membrane of the salt marsh archeon Halobacterium salinarum. When the bacteria is under osmotic shock it produces intensely purple coloured patches, the so-called purple membranes (PM) (Hampp 2000b). The existent of bR in the bacterial cell is critical for the survival of the organism in an oxygen-deficient environment. This is because bR is used by the bacterium to convert sunlight directly into chemical energy, thereby switching from a respiratory process to photosynthesis. When a bR molecule absorbs a photon (or a light quantum), it acts as a light driven proton pump by setting up an electric potential gradient across the membrane. A positively charge particle (H⁺) is then transported from the inner side of the cell membrane to the outer medium. This subsequently induces a sequence of transitions (or spectral changes), the so-called photochemical cycle (Downie and Smithey 1996, Hampp 2000b, Kalaidzidis et al 2001).

Figure 1. Simplified representation of the photochemical cycle of genetically modified bacteriorhodopsin films (bR-D96N). The broken and solid lines indicate the photo-induced (hn) and thermal-induced transitions, respectively.

The photochemical cycle of bR-D96N can be classified as the following states or transitions: D→bR (ground state) → J→K→L→M→N→bR (ground state), as shown in figure 1. When bR is left in the dark, the bR protein stays in the D state (dark-adapted state). After an illumination, the protein moves from the D-state to the bR state (purple bR state or B state). The initial transition from the bR state to the J state is driven by a photon or a light quantum whereas the other transitions are driven thermally. The intermediate M state (yellow M state), which is a meta-stable state, has the longest lifetime within the bR photochemical cycle, and optical information can be stored within the bR sample. This is because the bR state (ground state) has an absorption peak at 570nm (recording and retrieving optical information) which is different from the meta-stable M state that has an absorption peak at 410nm (erasing optical information) (Juchem and Hampp, 2001).

In addition, the intermediate meta-stable M state in bR can be thermally or genetically enhanced to increase its lifetime transition from milliseconds to a few tens of seconds (Okamoto et al 1999).