Time-resolved thermodynamic changes photoinduced in 5,12-trans-locked bacteriorhodopsin. Evidence that retinal isomerization is required for protein activation

Time-resolved photoacoustics of channelrhodopsins: early energetics and light-driven volume changes

In biological photoreceptors, the energy stored in early transient species is a key feature to drive the photocycle or a chain of reactions. Time-resolved photoacoustics (PA) can explore the energy landscape of transient species formed within few ns after photoexcitation, as well as volumetric changes (ΔV) of these intermediates with respect to the parental state. In this work, PA identified these important parameters for several channelrhodopsins, namely CaChR1 from Chlamydomonas augustae and CrChR2 from Chlamydomonas reinhardtii and various variants. PA has access to the sub-ns formation of the early photoproduct P1 and to its relaxation, provided that this latter process occurs within a few μs. We found that ΔVP1 for CaChR1 is ca. 12 mL/mol, while it is much smaller for CrChR2 (4.7 mL/mol) and for H. salinarum bacteriorhodopsin (HsBR, ΔVK = 2.8 mL/mol). PA experiments on variants strongly indicate that part of this large ΔVP1 value for CaChR1 is caused by the protonation dynamics of the Schiff base counterion complex involving E169 and D299. PA data further show that the energy level of P1 is higher in CrChR2 (ca. 96 kJ/mol) than in CaChr1 (ca. 46 kJ/mol), comparable to the energy level of the K state of HsBR (60 kJ/mol). Instrumental to gain these molecular values from the raw PA data was the estimation of the quantum yield (Φ) for P1 formation via transient spectroscopy; for both channelrhodopsins, ΦP2 was evaluated as ca. 0.4.

Graphical Abstract

Submillisecond conformational changes in proteins resolved by photothermal beam deflection

Photothermal beam deflection together with photo-acoustic calorimetry and thermal grating belongs to the family of photothermal methods that monitor the time-profile volume and enthalpy changes of light induced conformational changes in proteins on microsecond to millisecond time-scales that are not accessible using traditional stop-flow instruments. In addition, since overall changes in volume and/or enthalpy are probed, these techniques can be applied to proteins and other biomacromolecules that lack a fluorophore and or a chromophore label. To monitor dynamics and energetics of structural changes associated with Ca(2+) binding to calcium transducers, such neuronal calcium sensors, a caged calcium compound, DM-nitrophen, is employed to photo-trigger a fast (τ < 20 μsec) increase in free calcium concentration and the associated volume and enthalpy changes are probed using photothermal beam deflection technique.

Evidence from Chlamydomonas on the photoactivation of rhodopsins without isomerization of their chromophore

Attachment of retinal to opsin forms the chromophore N-retinylidene, which isomerizes during photoactivation of rhodopsins. To test whether isomerization is crucial, custom-tailored chromophores lacking the β-ionone ring and any isomerizable bonds were incorporated in vivo into the opsin of a blind mutant of the eukaryote Chlamydomonas reinhardtii. The analogs restored phototaxis with the anticipated action spectra, ruling out the need for isomerization in photoactivation. To further elucidate photoactivation, responses to chromophores formed from naphthalene aldehydes were studied. The resulting action spectral shifts suggest that charge separation within the excited chromophore leads to electric field-induced polarization of nearby amino acid residues and altered hydrogen bonding. This redistribution of charge facilitates the reported multiple bond rotations and protein rearrangements of rhodopsin activation. These results provide insight into the activation of rhodopsins and related GPCRs.

Methodology of pulsed photoacoustics and its application to probe photosystems and receptors

We review recent advances in the methodology of pulsed time-resolved photoacoustics and its application to studies of photosynthetic reaction centers and membrane receptors such as the G protein-coupled receptor rhodopsin. The experimental parameters accessible to photoacoustics include molecular volume change and photoreaction enthalpy change. Light-driven volume change secondary to protein conformational changes or electrostriction is directly related to the photoreaction and thus can be a useful measurement of activity and function. The enthalpy changes of the photochemical reactions observed can be measured directly by photoacoustics. With the measurement of enthalpy change, the reaction entropy can also be calculated when free energy is known. Dissecting the free energy of a photoreaction into enthalpic and entropic components may provide critical information about photoactivation mechanisms of photosystems and photoreceptors. The potential limitations and future applications of time-resolved photoacoustics are also discussed.

Hydrogen-bond network probed by time-resolved optoacoustic spectroscopy: photoactive yellow protein and the effect of E46Q and E46A mutations

The enthalpy and structural volume changes (delta Hi and delta Vi) produced upon photoinduced formation and decay of the red-shifted intermediate (pR = I1) in the photoactive yellow protein (WT-PYP) from Halorhodospira halophila and the mutated E46Q-PYP and E46A-PYP, were determined by laser-induced optoacoustic spectroscopy (LIOAS) using the two-temperatures method, at pH 8.5. These mutations alter the hydrogen bond between the phenolate oxygen of the chromophore and the residue at position 46. Hydrogen bonding is still possible in E46Q-PYP via the delta-NH2 group of glutamine, whereas it is no longer possible with the methyl group of alanine in E46A-PYP. In all three proteins, pR decays within hundreds of ns to micros into the next intermediate, pR'. The delta H values for the formation of pR (delta H pR) and for its decay into pR'(delta H pR-->pR') are negligibly affected by the E46Q and the E46A substitution. In all three proteins the large delta H pR value drives the photocycle. Whereas delta V pR is a similar contraction of ca. 15 ml mol(-1) for E46Q-PYP and WT-PYP, attributed to strengthening the hydrogen bond network (between 4 and 5 hydrogen bonds) inside the protein chromophore cavity, an expansion is observed for E46A-PYP, indicating just an enlargement of the chromophore cavity upon chromophore isomerization. The results are discussed in the light of the recent time-resolved room temperature, crystallographic studies with WT-PYP and E46Q-PYP. Formation of pR' is somewhat slower for E46Q-PYP and much slower for E46A-PYP. The structural volume change for this transition, delta V pR-->pR', is relatively small and positive for WT-PYP, slightly larger for E46Q-PYP, and definitely larger for the hydrogen-bond lacking E46A-PYP. This indicates a larger entropic change for this transition in E46A-PYP, reflected in the large pre-exponential factor for the pR to pR' decay rate constant determined in the 5-30 degrees C temperature range. This decay also shows an activation entropy that compensates the larger activation energy in E46A-PYP, as compared to the values for WT-PYP and E46Q-PYP.

Time-resolved photothermal methods: accessing time-resolved thermodynamics of photoinduced processes in chemistry and biology

Photothermal methods are currently being employed in a variety of research areas, ranging from materials science to environmental monitoring. Despite the common term which they are collected under, the implementations of these techniques are as diverse as the fields of application. In this review, we concentrate on the recent applications of time-resolved methods in photochemistry and photobiology.

Light-induced hydrolysis and rebinding of nonisomerizable bacteriorhodopsin pigment

Bacteriorhodopsin (bR) is characterized by a retinal-protein protonated Schiff base covalent bond, which is stable for light absorption. We have revealed a light-induced protonated Schiff base hydrolysis reaction in a 13-cis locked bR pigment (bR5.13; lambda(max) = 550 nm) in which isomerization around the critical C13==C14 double bond is prevented by a rigid ring structure. The photohydrolysis reaction takes place without isomerization around any of the double bonds along the polyene chain and is indicative of protein conformational alterations probably due to light-induced polarization of the retinal chromophore. Two photointermediates are formed during the hydrolysis reaction, H450 (lambda(max) = 450 nm) and H430 (lambda(max) = 430 nm), which are characterized by a 13-cis configuration as analyzed by high-performance liquid chromatography. Upon blue light irradiation after the hydrolysis reaction, these intermediates rebind to the apomembrane to reform bR5.13. Irradiation of the H450 intermediate forms the original pigment, whereas irradiation of H430 at neutral pH results in a red shifted species (P580), which thermally decays back to bR5.13. Electron paramagnetic resonance (EPR) spectroscopy indicates that the cytoplasmic side of bR5.13 resembles the conformation of the N photointermediate of native bR. Furthermore, using osmotically active solutes, we have observed that the hydrolysis rate is dependent on water activity on the cytoplasmic side. Finally, we suggest that the hydrolysis reaction proceeds via the reversed pathway of the binding process and allows trapping a new intermediate, which is not accumulated in the binding process.

Time-resolved thermodynamic analysis of the oat phytochrome A phototransformation. A photothermal beam deflection study

The time-resolved enthalpy and the structural volume changes after excitation of native oat phytochrome A were studied in the micro- to milliseconds range by photothermal beam deflection (PBD), a technique that follows the time-resolved refractive index changes upon decay of the excited species. The first set of intermediates, I700(1) and I700(2), stores ca 83% of the energy of the first excited state, in agreement with previous optoacoustic data, whereas the second set stores only ca 18%. The temperature dependence of the amplitudes ratio for the optical absorbances of the (I700(1) + I700(2)) intermediates set is explained on the basis of the thermochromic equilibrium between Pr,657 and Pr,672, which also is in line with the present PBD data. These data were best fitted with a parallel mechanism (with equal yield in each branch) for the production of the first set of intermediates, I700(1) and I700(2), as well as the second set of intermediates, Ibl1 and Ibl2. Thus, the final steps toward Pfr should be largely driven by positive entropic changes brought about by protein movements, in line with previous resonance Raman data. For the production of the first set of intermediates (I700(1) and I700(2)) an expansion of 18 +/- 13 mL mol-1 was determined, and a further expansion > or = 7 mL mol-1 was estimated for the decay from I700(1) to the set of Ibl intermediates, indicating that the far red-absorbing form of phytochrome (Pfr) has a larger volume than the red-absorbing form of phytochrome. This is in agreement with previous chromatographic and circular dichroism data according to which Pfr shows a larger volume and the chromophore shows a higher accessibility, respectively, in the Pfr state.

Thermodynamics of the early steps in the photocycle of Natronobacterium pharaonis halorhodopsin. Influence of medium and of anion substitution

The enthalpy (delta H) and structural volume changes (delta V) associated with the formation and decay of the early intermediate K600 in the photocycle of Natronobacterium pharaonis halorhodopsin (pHR), an inward-directed anion pump, were obtained by laser-induced optoacoustic spectroscopy. A large expansion is associated with K600 formation, its value depending on the medium and on the anion (Cl-, NO3-, Br-, I-). A smaller expansion is associated with K600 decay to L520. A contraction is found for the same step in the case of the azide-loaded pHR which is an efficient outward-directed proton pump. Thus, the conformational changes in L520 determine the direction and sign of charge translocation. The linear correlation between delta H and delta V for chloride-loaded pHR observed upon mild medium variations is attributed to enthalpy-entropy compensation effects and allows the calculation of the free-energy changes, delta GK = (97 +/- 16) kJ/mol and delta GKL = -(2 +/- 2) kJ/mol. Different from other systems, delta S correlates negatively with delta V in the first steps of the pHR photocycle. Thus, the space around the anion becomes larger and more rigid during each of these two steps. The photocycle quantum yield was 0.52 for chloride-pHR as measured by laser flash photolysis.

Nonisomerizable non-retinal chromophores initiate light-induced conformational alterations in bacterioopsin

The photoactivation of retinal proteins is usually interpreted in terms of C=C photoisomerization of the retinal moiety, which triggers appropriate conformational changes in the protein. In this work several dye molecules, characterized by a completely rigid structure in which no double-bond isomerization is possible, were incorporated into the binding site of bacteriorhodopsin (bR). Using a light-induced chemical reaction of a labeled EPR probe, it was observed that specific conformational alterations in the protein are induced following light absorption by the dye molecules occupying the binding site. The exact nature of these changes and their relationship to those occurring in the bR photocycle are still unclear. Nevertheless, their occurrence proves that C=C or C=NH(+) isomerization is not a prerequisite for protein conformational changes in a retinal protein. More generally, we show that conformational changes, leading to changes in reactivity, may be induced in proteins by optical excitation of simple nonisomerizable dyes located in the macromolecular matrix.