Monomeric and aggregated bacteriorhodopsin: Single-turnover proton transport stoichiometry and photochemistry

Abstract

The question of the basic functional transport unit of bacteriorhodopsin (BR) has been addressed by comparing the proton pumping stoichiometry as well as the photocycle kinetics of monomeric and aggregated BR in phospholipid vesicles. When time-resolved laser spectroscopy was used in combination with the optical pH-indicator pyranine, single-turnover experiments revealed approximately 0.5-0.8 and 0.8-1.2 protons vectorially translocated per photocycling monomeric and aggregated BR molecule, respectively. Since both these values are akin and very similar to the pumping stoichiometry of crystalline BR molecules in the purple membrane, the BR monomer has been proven to be the essential transport unit. The natural arrangement of the photopigments in a crystalline array of immobilized trimers is not required for efficient vectorial proton translocation.The light-energy-converting membrane protein bacteriorhodopsin (BR), which has a covalently bound retinal as chromophore (1), has gained special attention due to its relative simplicity. However, the molecular mechanism by which, during the complex photochemical cycle, one proton per BR molecule (2-6) is vectorially translocated across the membrane is far from understood. Interpretation of measured functional data is hampered by the extraordinary organization of the photopigment in the cytoplasmic membrane of the cell. As the only protein of the so-called purple membrane (PM), it is arranged in a two-dimensional hexagonal lattice of protein trimers (7,8) which are separated from neighboring trimer clusters by only one shell of lipids (9). To interpret various experimental results with the aim of constructing a working model for BR's proton-transport mechanism, the basic functional unit of this light-driven proton pump has to be defined; the functional unit could be the monomer, the trimer, or even the lattice.BR monomers do exhibit vectorial H+-transport activity (10-13). However, the important question whether or not monomeric BR has the same efficiency as the aggregate is controversial (12)(13)(14) To accelerate the relaxation of a pH gradient and a membrane potential generated after a single flash or after light adaptation, the vesicles were incubated with 1 ,M of the ion channel gramicidin above the phase transition temperature of the lipids for 10 min and thereafter sonicated for 20 s to improve the incorporation. The sample was then divided into two 1-ml aliquots, one was incubated additionally with 50 ,M pyranine in the extravesicular medium, and both were degassed at 0.1 torr (13 Pa) for 3 min. After pH adjustment (pH 7.0) at the temperature of the flash-spectroscopic experiments, the measurements were carried out at once. The buffer capacity of the samples containing pyranine was determined by titration with KOH and HCl in the range of pH 6.7-7.3. Flash Spectroscopy. The flash spectrometer (excitation wavelength Aex = 580 ± 20 nm; 1 mJ; flash duration 5-10 ns) equipped with a logarithmic time base has been described elsewhere (6). Before the flash and ...

Bacteriorhodopsin expressed in Schizosaccharomyces pombe pumps protons through the plasma membrane.

“…3 Formation of the M intermediate is a prerequisite for proton pumping activity of bR. To measure time-resolved pH changes in the aqueous phase the pH indicator pyranine was used (16,25). We were able to clearly detect proton transfer reactions in the yeast sample (Fig.…”

Substitution of amino acids Asp-85, Asp-212, and Arg-82 in bacteriorhodopsin affects the proton release phase of the pump and the pK of the Schiff base.

“…The data suggest that Asp-85 and Asp-212 are deprotonated in the ground state and become protonated in the L -* M transition (6). About 30% of the internal charge displacement is associated with the L -k M transition ,us) (8), and proton release is detected in the aqueous solution on the extracellular side of the membrane in this time range (9). To further investigate the H+ release step, we have studied the time course of the photocycle, photovoltage, and release and subsequent reuptake of protons in single-substitution mutants of amino acids Asp-85, Asp-96, Asp-115, Asp-212, and Arg-82.…”

“…The pH changes associated with proton release and uptake were measured at pH 7.3 by using the indicator dye pyranine (8-hydroxyl-1,3,6-pyrenetrisulfonate; pK = 7.2) as described (3,9). The stoichiometry of protons released per bR cycling was calculated from the measured absorbance difference due to pyranine (AAA450), the The inverse of the second factor is the concentration of bR that has entered the photocycle.…”

Projection Structure of Channelrhodopsin-2 at 6 Å Resolution by Electron Crystallography

“…For example, bR forms 2D crystals either as a trimer 18 or as a monomer, 19 and halorhodopsin (HR) forms 2D crystals as a tetramer, 20 but the functional unit of both ion pumps is the protomer. [21][22][23][24] The strong ChR2 dimer is so far unique among microbial rhodopsins and might suggest that, in this case, the functional unit is dimeric. The projection structure of the ChR2 dimer at 6 Å resolution is shown in Fig.…”