Bacteriorhodopsin as a Biosensor and its Responsiveness to Anesthetics

Abstract

Proteins with covalently bound retinal, visual rhodopsin and bacteriorhodopsin, have different spectral forms due to the retinal-protein interactions and a "fine tuning" mechanism of maximal absorbance adjustment. Bacteriorhodopsin existing in the purple membrane contains seven transmembrane helices and is extremely stable membrane protein. The bacteriorhodopsin spectrum has been found to have a high responsiveness to stimulation by anesthetics. The effects of volatile anesthetics (Enflurane, Halothane and 1-alcohols) on the structure of bacteriorhodopsin in the purple membrane in solution were investigated with spectrophotometry. Enflurane shifted the maximum absorption of bacteriorhodopsin from 568 to 480 nm, while halothane and 1-alcohols, in a concentration dependent manner, induced a further shift from 480 to 380 nm. The 480 nm-form can be reverted to the original 568 nm-form by evaporation of the anesthetics. However, the shift from 480 to 380 nm was irreversible. The reversibility is essential for anesthesia. Next the structural stability of bacteriohodopsin in an ultra thin film as a sensitive layer of an anesthetic sensor was studied using a monolayer technique which utilizes the air-water interface as a field to allow amphiphilic molecules to orient and form a condensed monolayer film. The stability of a bacteriorhodopsin film on the water surface depends significantly on the incubation time until the start of film compression. The α-helical bR molecule in the purple membrane film is partially transformed into a β-sheet with an increase in incubation time and released retinal from the inside of the protein. This destabilization of the α-helical bacteriohodopsin monolayer films is depressed by the addition of small amounts (50 mM) of Cs^+ and I-- ions. The absorbance profile at 568 nm of a self-assembled bR film transferred on a quartz plate showed a reversible conversion in regard to "off" of the vapor flow of anesthetic. The bR with covalently bound retinal chromophore can be expected to serve as an anesthetic biosensor.