Interconversions among four M-intermediates in the bacteriorhodopsin photocycle

Amphipol-assisted folding of bacteriorhodopsin in the presence or absence of lipids: functional consequences

Amphipols are short amphipathic polymers designed to stabilize membrane proteins in aqueous solutions in the absence of detergent. Bacteriorhodopsin (BR), a light-driven proton pump, has been denatured, either by direct solubilization of the purple membrane in sodium dodecylsulfate (SDS) solution or by a procedure that involves delipidation with organic solvent followed by transfer to SDS, and renatured in amphipol A8-35. The effect of different renaturation procedures and of the presence or absence of lipids and the cofactor retinal have been investigated. The resulting samples have been characterized by absorbance spectroscopy, size-exclusion chromatography, thermostability measurements, and determination of photocycle kinetics. Transfer to A8-35 can be achieved by SDS precipitation, dilution, or dialysis, the first route resulting in the highest yield of refolding. Functional BR can be refolded whether in the presence or absence of lipids, higher yields being achieved in their presence. Retinal is not required for the protein to refold, but it stabilizes the refolded form and, thereby, improves folding yields. Lipids are not required for BR to perform its complete photocycle, but their presence speeds up the return to the ground state. Taken together, these data indicate that a membrane or membrane-mimetic environment is not required for correct decoding of the chemical information contained in the sequence of BR; functional folding is possible even in the highly foreign environment of lipid-free amphipols. BR interactions with lipids, however, contribute to an effective photocycle.

The ability of actinic light to modify the bacteriorhodopsin photocycle revisited: heterogeneity vs photocooperativity

In 1995, evidence both for photocooperativity and for heterogeneity as possible explanations for the ability of actinic light to modify the kinetics and pathways of the bacteriorhodopsin (BR) photocycle was reviewed ( Shrager, R. I., Hendler, R. W., and Bose, S. (1995) Eur. J. Biochem. 229, 589-595 ). Because both concepts could be successfully modeled to experimental data and there was suggestive published evidence for both, it was concluded that both photocooperativity and heterogeneity may be involved in the adaptation of the BR photocycle to different levels of actinic light. Since that time, more information has become available and it seemed appropriate to revisit the original question. In addition to the traditional models based on all intermediates in strict linear sequences, we have considered both homogeneous and heterogeneous models with branches. It is concluded that an explanation based on heterogeneity is more likely to be the true basis for the variation of the properties of the photocycle caused by changes in actinic light intensity. On the basis of new information presented here, it seems that a heterogeneous branched model is more likely than one with separate linear sequences.

Electrogenic proton-pumping capabilities of the m-fast and m-slow photocycles of bacteriorhodopsin

The parallel model for the bacteriorhodopsin (BR) photocycle at neutral pH and a temperature near 20 degrees C contains an M-fast cycle with steps BR-->K-->L-->Mf-->N-->O-->BR and an M-slow cycle which contains steps BR-->K-->L-->Ms-->BR. With increasing actinic laser strength, the M-fast cycle at first rises faster than the M-slow cycle, but reaches saturation sooner and at a lower level than the M-slow cycle. The O-intermediate shows the same saturation behavior as Mf. In this paper, we show that the peak current of proton flux and the apparent voltages developed by this flux show the same saturation behavior as Ms, which is very different from that of both M f and O. It is further shown that most of the proton-charge displacement is connected with the step Ms-->BR. The optical and electrical data in these studies were collected simultaneously by a newly designed and built spectrometer which is described separately.

Simultaneous measurements of fast optical and proton current kinetics in the bacteriorhodopsin photocycle using an enhanced spectrophotometer

A one-of-a-kind high speed optical multichannel spectrometer was designed and built at NIH and described in this journal in 1997 [J.W. Cole, R.W. Hendler, P.D. Smith, H.A. Fredrickson, T.J. Pohida, W.S. Friauf. A high speed optical multichannel analyzer. J Biochem Biophys Methods 1997;35:16-174.]. The most unique aspect of this instrument was the ability to follow an entire time course from a single activation using a single sample. The instrument has been used to study rapid kinetic processes in the photon-driven bacteriorhodopsin photocycle and electron transport from cytochrome c to cytochrome aa3 and from cytochrome aa3 to oxygen. The present paper describes a second generation instrument with a number of important enhancements which significantly improve its capabilities for multichannel kinetic studies. An example application is presented in which the kinetics of photon-induced proton flow across the biological membrane is measured simultaneously with the individual steps of the photocycle determined optically. Matching the time constants for the two processes indicates which molecular transformations are associated with major proton movements.

Heterogeneity based on bending of purple membrane containing bacteriorhodopsin

The first and second derivatives of dielectric spectra have evidenced the existence of two interacting states of purple membrane (PM) that respond differently to the intensity of illuminating light providing, this way, underlying consequences to the heterogeneous behavior of bacteriorhodopsin (bR). It is of particular interest to note that the rotational diffusion coefficient of PM has exhibited non-linearity versus light intensity. The explored non-linearity in electrical properties beers, thereby, on changes in PM size. The non-linear variations in PM bending might initiate, in consequence, variations in the dipole moment (permanent and induced) and dc-conductivity of PM patches. Proposal based on PM bending has been introduced to correlate the light intensity effect to the PM lipid environment. Modulation of the global structure of PM and, in turn, its electrical properties by an external perturbation (e.g., light) could be of interest in biotechnological applications based on optoelectronic properties of bR.

Bacteriorhodopsin

Fourier transform infrared and Raman spectroscopy, solid-state NMR, and X-ray crystallography have contributed detailed information about the structural changes in the proton transport cycle of the light-driven pump, bacteriorhodopsin. The results over the past few years add up to a step-by-step description of the configurational changes of the photoisomerized retinal, how these changes result in internal proton transfers and the release of a proton to the extracellular surface and uptake on the other side, as well as the conservation and transformation of excess free energy during the cycle.