The Unique Properties of Bacteriorhodopsins as Energy Converters

Abstract

A quarter of a century has passed since the discovery of PM (purple membrane) and the first bacterial rhodopsin (BR) was isolated in a wild cell culture of Halobacterium halobium. Since that time, microbiologists and geneticists have synthesized dozens of artificial halobacteria cell cultures, have reconstructed hundreds of BR analogs (genetic and mutant invariants), and have even discovered new bacterial rhodopsinoid molecules. Such analogs differ in their ground state absorption spectra, the lifetimes of intermediates, the ability to transport protons, etc. Only a few BR analogs have no photochemical activity (no photocycle), and are not capable of creating a potential drop. Nearly all retinal-protein complexes, to a greater or lesser extent, are capable of energy conversion. However, despite a knowledge of BR protein structure (primary, secondary, and tertiary), the retinal binding location, and despite the multitude of theories put forth, and the behemoth experimental databank, the mechanism of BR functioning is still largely a mystery!