Bioorganic chemistry of rhodopsins
Bacteriorhodopsin and rhodopsin are integral membrane proteins containing retinal as a light sensitive group. Light energy is used by halophilic microorganisms for ATP synthesis and other vital functions. In visual cells light absorption activates several enzymes in the rod outer segment. Comparative study of the structural organization of these proteins was necessary to elucidate their functioning as light energy transducers. The complete amino acid sequences of bacteriorhodopsin and rhodopsin were established and their polypeptide chain arrangement in the corresponding membranes was determined. Monoclonal antibodies were used to define the topography of bacteriorhodopsin in the purple membrane. Much more groups of the polypeptide chain were demonstrated to be exposed into the aqueous phase in rhodopsin than in bacteriorhodopsin. The polypeptide chain of rhodopsin traverses the photoreceptor membrane seven times, the N— and C— terminal parts being located at the opposite sides of the membrane. A ge— neral feature for both rhodopsin and bacteriorhodopsin is location of the retinal chromophore binding lysine residue at the C—terminal intramembrane fragment. Data will be presented on the orientation of the retinal and aldimine bond in the membrane. Knowledge of the molecular mechanism of photoreception is a key problem in the study of the visual process as a whole. Rhodopsin, a basic protein component of disk membranes from outer segments of rods from the vertebrate retina, plays a substantial role in photoreception initiating the visual perception. Therefore structure—functional investigations of rhodopsin are essential for understanding physicochemical processes of vision. Of much significance is the mechanism of light energy transformation from the rhodopsin molecule which absorbs light quanta to a cytoplasmic membrane and further to the synaptic ending of the cell, viz, mechanism of photoreception. Nowadays two possible mechanisms of the process are discussed, each suggests a mediator that regulates the degree of polarization of the plasma membrane. Cyclic GMP is one of the main mediators whose concentration lowering in cytoplasm, resulted from light—dependent activation of cyclo—GMP—phosphodiesterase, leads to blocking sodium channels. Calcium ions are also considered as probable mediators (ref. 1). A special GTP— binding protein or transducin, a transmitter of the visual signal from rhodopsin to phospho— diesterase and its amplifyer, participates in coupling between photoactivable rhodopsin and phosphodiesterase. Transducin, a peripheral membrane protein, consists of three subunits: n (39 kDa), (37 kDa), and y (8 kDa) (ref. 2). The major pathways of the model proposed by Fung and Stryer (ref. 2, 3) are summarized as follows. Photolyzed rhodopsin catalyzes the exchange of GTP for GDP bound to the n—subunit of transducin. Photolyzed rhodopsin has a high affinity for transducin—GDP but a low affinity for transducin—GTP. The shift in the binding affinity enables the photolyzed rhodopsin to recycle rapidly and to activate many trans— ducin molecules. The binding of GTP leads to the dissociation of the n—y rhodopsin complex. Since dissociation of the n—subunit—GTP complex is much faster than hydrolysis of GTP, the level of the n—subunit—GTP complex increases rapidly. The n—subunit—GTP complex is an activator of phosphodiesterase. It interacts with phosphodiesterase by removing an inhibitory effect of the y—polypeptide on the n—and —polypeptides of the phosphodiesterase. The enzyme activity is again inhibited when the GTP on the n—subunit is hydrolyzed to GDP. The y—subunit serves to bring the n—subunit back onto rhodopsin then the whole cycle is repeated again. Until recently rhodopsin was the only known light—transducing protein using retinal as a chromophore. The unexpected discovery, seemingly bearing no direct relation to visual excitation, gave a new impetus to these investigations (ref. 4). Halophilic microorganisms of the Halobacterium family utilize the solar radiation energy due to the presence of bacteriorhodopsin, a light—driven primary proton translocase. Bacterio—