DIMERIC-LIKE KINETIC COOPERATIVITY OF THE BACTERIORHODOPSIN MOLECULESIN PURPLE MEMBRANES

The kinetics of the absorption changes accompanying the photocycle of bacteriorhodopsin (BR) strongly depend on the intensity of the excitin g short laser pulse. The decrease in the flash intensity dependence of the M kinetics after different extents of bleaching of the purple mem branes by hydroxylamine proves the existence of a cooperative interact ion between the photocycling BR molecules. The yield of the slow compo nent of the M decay (M(s)) is a quadratic function of the extent of th e fraction cycling. The slope of the relative weight of M(s), versus t he fraction cycling is 0.5. This slope indicates a dimeric-like cooper ative interaction, although the structural units of the purple membran es are the trimers of the BR molecules. For the most probable cooperat ive mechanism an asymmetric trimeric interaction is suggested, which a ccounts for the apparently dimeric features. A photocycling molecule m ay influence only one of its two neighbors in the trimer. From this as ymmetric feature a deformative interaction is expected to be the coope rative mechanism, which would be an allosteric regulating mechanism in the purple membrane.