A correlation between proton pumping and the bacteriorhodopsin photocycle.

The removal of metal cations inhibits the deprotonation process of the protonated Schiff base during the photocycle of bacteriorhodopsin. To understand the nature of the involvement of these cations, a spectroscopic and kinetic study was carried out on bacteriorhodopsin samples in which the native Ca2' and Mg2e were replaced by Eu3+, a luminescent cation. The decay of Eu3' emission in bacteriorhodopsin can be fitted to a minimum of three decay components, which are assigned to Eu31 emission from three different sites. This is supported by the response of the decay components to the presence of 2H20 and to the changes in the Eu3+/bR molar ratio. The number of water molecules coordinated to Eu3' in each site is determined from the change in its emission lifetime when 2H20 replaces H20. Most of the emission originates from two "wet" sites of low crystal-field symmetry--e.g., surface sites. Protonated Schiff base deprotonation has no discernable effect on the emission decay of protein-bound Eu3+, suggesting an indirect involvement of metal cations in the deprotonation process. Adding Eu3+ to deionized bacteriorhodopsin increases the emission intensity of each Eu3+ site linearly, but the extent of the deprotonation (and color) changes sigmoidally. This suggests that if only the emitting Eu3+ ions cause the deprotonation and bacteriorhodopsin color change, ions in more than one site must be involved-e.g., by inducing protein conformation changes. The latter could allow deprotonation by the interaction between the protonated Schiff base and a positive field of cations either on the surface or within the protein.Bacteriorhodopsin (bR) is the only protein found in the purple membrane of Halobacterium halobium, a light-harvesting bacterium. It contains a single chromophore molecule, retinal, covalently bound via a protonated Schiff base (PSB) linkage to the E-amino group of a lysine residue in the protein (1, 2). Upon absorbing a photon, it undergoes a photochemical cycle (3) consisting of at least four intermediates on time scales varying from pico-to milliseconds: bR-7* K610 L55 -M412 06 O --, bR570.During this photocycle, protons are pumped across the cell membrane to the outside, establishing an electrochemical proton gradient used by the organism for metabolic processes such as ATP synthesis (ref. 4 and refs. therein). Protons are ejected from the cell at a rate comparable to the formation of the M412 intermediate (5,6). A good correlation has been found between the number of protons pumped and the amount of the slow-decaying form of M412 (7). This intermediate is the only one in which the Schiff base is unprotonated (8, 9). Consequently, many studies have inferred that the PSB deprotonation is closely associated with the proton pump mechanism (for review see ref. 6).Recently, a very simple electrostatic model (the cation model) was proposed (10) to account for the strong coupling between (10-14), and thus the similar mechanism of deprotonation of, the PSB and an acid with a pKa value of 8.6-10 [possibly tyro...

mentioning

confidence: 75%

Evidence for the involvement of more than one metal cation in the Schiff base deprotonation process during the photocycle of bacteriorhodopsin

Corcoran

Ismail

El‐Sayed

1987

“…4). The decay of There is also some evidence indicating distinct forms of M, cays of Mf and M' (10, but it is mostly either indirect (6,18,19) or extremely brief ons for the origin of the (20). Most other spectroscopic attempts to show that the two st (and simplest) is that forms of M are spectrally or physically distinct have not been mation of the R intersuccessful (2,7,21), probably because of lack of resolution y at this wavelength.…”

Section: Discussionmentioning

confidence: 99%

Independent photocycles of the spectrally distinct forms of bacteriorhodopsin

Dancsházy

Govindjee

Ebrey

1988

Time-resolved, flash-induced difference absorbance spectra (300-700 am) at pH 10.5 and 5C for the bacteriorhodopsin photocycle fast and slow decaying forms of the M intermediate (M' and MS, respectively) MATERIALS AND METHODS Purple membranes were isolated from Halobacterium halobium, S9 strain, and suspended in 30 mM piperazine/30 mM glycylglycine, pH 10.5. All samples contained 40%o (vol/vol) glycerol except those used for the flash-induced absorbance changes in Fig. 4. All measurements were carried out on light-adapted BR.Two spectrophotometers were used to obtain the lightinduced difference spectra. For the faster measurements, the data were taken and analyzed as described (10,11). For the slower measurements a Hewlett-Packard model 8452A diode array spectrophotometer was used to take complete lightinduced difference spectra from 300 to 700 nm with 2-nm spectral resolution. (The absorption spectrum of the lightadapted sample was used as the baseline.) The actinic source was a commercial photoflash with Coming long-pass filter (CS 3-67, wavelength >550 nm). The half-duration time was 150 tUs. Since the measuring beam of the diode array spectrophotometer was relatively strong white light, special care was taken to check and avoid a possible actinic effect of the measuring beam. Sample OD570 was between 1.3 and 1.5. RESULTSSeparation of the Difference Spectra of Intermediates M', ME, and R. From the time-dependent series of difference spectra, we can determine the difference spectra of the late intermediates involved in the BR recovery at high pH (Mf, MS, and R). We have chosen conditions for which the differences in the lifetimes and yields of these three components are maximal. We assume that the intermediates before M can be neglected because of their much shorter lifetimes and that no significant amount of the late intermediate 0 is formed because of the low temperatures and high pH values (refs. 5, 6, and 10 and unpublished data). Fig. 1 6358The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

“…A flowing sample and a tightly focused continuous wave laser beam control the interaction time ofthe sample with the laser. A fast sample flow using a peristaltic pump through a capillary (or a syringe needle) with a 10-gm laser focus resulted in an interaction time of 10 Asec and by varying the flow rate and defocusing the laser, longer times were easily obtained. The details of the time resolved Raman experimental set-up were given previously (18).…”

Section: Methodsmentioning

confidence: 99%

On the molecular mechanisms of the Schiff base deprotonation during the bacteriorhodopsin photocycle

Chronister

Corcoran

et al. 1986

Using optical flash photolysis and timeresolved Raman methods, we examined intermediates formed during the photocycle of bacteriorhodopsin (bR), as well as the bR color change, as a function of pH (in the 7.0-1.5 region) and as a function of the number of bound Ca2+ ions. It is found that at a pH just below 3 or with less than two bound Ca21 per bR, the deprotonation (the L550 -_ M412) step ceases, yet the K610 and L550 analogues are still formed as in native bR. The lack of deprotonation in the photocycle of both acid blue and deionized blue bR and the similarity of their Raman spectra as well as of their K610 and L550 analogues strongly suggest that both blue samples have nearly the same retinal active site. It is suggested that in both blue species, bound cations are removed via a proton-cation exchange equilibrium, either on the cation exchange column for the deionized sample or in solution for the acid blue sample. The proton-cation exchange equilibrium is found to quantitatively account for the pH dependence of the purple-to-blue color change. The different mechanisms responsible for the large reduction (=11 units) of the pKa value of the protonated Schiff base (PSB) during the photocycle are discussed. The absence of the L550 1M412 deprotonation process in both blue species is discussed in terms of the previously proposed cation model for the deprotonation of the PSB during the photocycle of native bR. The extent of the deprotonation and the blue-to-purple color change are found to follow the same dependence on either the pH or the amount of cations added to deionized blue bR. This observed correlation is briefly discussed.Bacteriorhodopsin (bR) is the only protein found in the purple membrane of Halabacterium halobium, a light-harvesting bacterium. It contains retinal as a chromophore, which is covalently bound via a protonated Schiff base linkage to the E-amino group of a lysine residue in the protein (1, 2). Upon absorbing a photon, it undergoes a photochemical cycle (3) The photocycle causes protons to be pumped across the cell membrane to the outside, establishing a pH gradient used by the organism for metabolic processes such as ATP synthesis (4-7). The protons are ejected from the cell at a rate comparable to that for the formation of the M412 intermediate (8,9). A good correlation has been found between the number ofprotons pumped and the amount of slow decaying (10) form of M412. This intermediate is the only one in which the Schiff base is unprotonated (11-16). Consequently, many studies have inferred that the protonated Schiff base (PSB) deprotonation is closely associated with the proton pump mechanism (17). Preceding M412 formation (or PSB deprotonation) is the formation of the early intermediates K610 and L550. The retinal in bR570 is in the all-trans form, while in K610 it has a distorted 13-cis conformation (18)(19)(20). In the L550 form, the isomerization is complete (18-20).The pKa value of the PSB is 13.3 in bR570 (21, 22), yet it deprotonates during the cycle, suggesting a lar...