Volume and enthalpy changes after photoexcitation of bovine rhodopsin: laser-induced optoacoustic studies

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Rhodopsins carrying modified chromophores--the 'making of', structural modelling and their light-induced reactivity

A series of vitamin-A aldehydes (retinals) with modified alkyl group substituents (9-demethyl-, 9-ethyl-, 9-isopropyl-, 10-methyl, 10-methyl-13-demethyl-, and 13-demethyl retinal) was synthesized and their 11-cis isomers were used as chromophores to reconstitute the visual pigment rhodopsin. Structural changes were selectively introduced around the photoisomerizing C11=C12 bond. The effect of these structural changes on rhodopsin formation and bleaching was determined. Global fit of assembly kinetics yielded lifetimes and spectral features of the assembly intermediates. Rhodopsin formation proceeds stepwise with prolonged lifetimes especially for 9-demethyl retinal (longest lifetime τ3 = 7500 s, cf., 3500 s for retinal), and for 10-methyl retinal (τ3 = 7850 s). These slowed-down processes are interpreted as either a loss of fixation (9dm) or an increased steric hindrance (10me) during the conformational adjustment within the protein. Combined quantum mechanics and molecular mechanics (QM/MM) simulations provided structural insight into the retinal analogues-assembled, full-length rhodopsins. Extinction coefficients, quantum yields and kinetics of the bleaching process (μs-to-ms time range) were determined. Global fit analysis yielded lifetimes and spectral features of bleaching intermediates, revealing remarkably altered kinetics: whereas the slowest process of wild-type rhodopsin and of bleached and 11-cis retinal assembled rhodopsin takes place with lifetimes of 7 and 3.8 s, respectively, this process for 10-methyl-13-demethyl retinal was nearly 10 h (34670 s), coming to completion only after ca. 50 h. The structural changes in retinal derivatives clearly identify the precise interactions between chromophore and protein during the light-induced changes that yield the outstanding efficiency of rhodopsin.

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.

Location of the retinal chromophore in the activated state of rhodopsin*

Rhodopsin is a highly specialized G protein-coupled receptor (GPCR) that is activated by the rapid photochemical isomerization of its covalently bound 11-cis-retinal chromophore. Using two-dimensional solid-state NMR spectroscopy, we defined the position of the retinal in the active metarhodopsin II intermediate. Distance constraints were obtained between amino acids in the retinal binding site and specific (13)C-labeled sites located on the beta-ionone ring, polyene chain, and Schiff base end of the retinal. We show that the retinal C20 methyl group rotates toward the second extracellular loop (EL2), which forms a cap on the retinal binding site in the inactive receptor. Despite the trajectory of the methyl group, we observed an increase in the C20-Gly(188) (EL2) distance consistent with an increase in separation between the retinal and EL2 upon activation. NMR distance constraints showed that the beta-ionone ring moves to a position between Met(207) and Phe(208) on transmembrane helix H5. Movement of the ring toward H5 was also reflected in increased separation between the Cepsilon carbons of Lys(296) (H7) and Met(44) (H1) and between Gly(121) (H3) and the retinal C18 methyl group. Helix-helix interactions involving the H3-H5 and H4-H5 interfaces were also found to change in the formation of metarhodopsin II reflecting increased retinal-protein interactions in the region of Glu(122) (H3) and His(211) (H5). We discuss the location of the retinal in metarhodopsin II and its interaction with sequence motifs, which are highly conserved across the pharmaceutically important class A GPCR family, with respect to the mechanism of receptor activation.

The A-Fx to F(A/B) step in synechocystis 6803 photosystem I is entropy driven

We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P(700) to F(A/B) iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem I: (1) P(700) --> A(1)(-)F(X) (<10 ns) and (2) A(1)(-)F(X) --> F(A/B)(-) (20-200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (<10 ns) with large negative enthalpy (-0.8 +/- 0.1 eV) and large volume change (-23 +/- 2 A(3)), which are assigned to the P(700) --> A(1)(-)F(X) step, and (2) a component with approximately 200 ns lifetime, which has a positive enthalpy (+0.4 +/- 0.2 eV) and a small volume change (-3 +/- 2 A(3)) that are attributed to the A(1)(-)F(X) --> F(A/B)(-) step. For the fast reaction using the redox potentials of A(1)F(X) (-0.67 V) and P(700) (+0.45 V) and the energy of P(700) (1.77 eV), the free energy change for the P(700) --> A(1)(-)F(X) step is -0.63 eV, and thus the entropy change (TDeltaS, T = 25 degrees C) is -0.2 +/- 0.3 eV. For the slow reaction, A(1)(-)F(X) --> F(A/B)(-), taking the free energy of -0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283-310], the entropy change (TDeltaS) is positive, +0.54 +/- 0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A(-)F(X) to F(A/B)(-) step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.

Time-Resolved Thermodynamics: Heat Capacity Change of Transient Species during Photoreaction of PYP

Heat capacity changes of short-lived transient species in different time ranges were measured for the first time by using the thermal component of the transient grating and transient lens signals at various temperatures. This method was applied to the transient intermediates of Photoactive Yellow Protein (PYP). The temperature dependence of the enthalpy change shows that the heat capacity of the short-lived intermediate pR2 (also called I1 or PYP(L)) species is the same as that of the ground state (pG) species within our experimental accuracy, whereas that of the long-lived intermediate pB (I2 or PYP(M)) is much larger (2.7 +/- 0.4 kJ/mol K) than that of pG. The larger heat capacity is interpreted in terms of the conformational change of the pB species such as melted conformation and/or exposure of the nonpolar residues to the aqueous phase. This technique can be used for photochemical reactions in general to investigate the conformational change and the hydrophobic interaction in a time domain.

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.

Energetics and volume changes of the intermediates in the photolysis of octopus rhodopsin at a physiological temperature

Enthalpy changes (Delta H) of the photointermediates that appear in the photolysis of octopus rhodopsin were measured at physiological temperatures by the laser-induced transient grating method. The enthalpy from the initial state, rhodopsin, to bathorhodopsin, lumirhodopsin, mesorhodopsin, transient acid metarhodopsin, and acid metarhodopsin were 146 +/- 15 kJ/mol, 122 +/- 17 kJ/mol, 38 +/- 8 kJ/mol, 12 +/- 5 kJ/mol, and 12 +/- 5 kJ/mol, respectively. These values, except for lumirhodopsin, are similar to those obtained for the cryogenically trapped intermediate species by direct calorimetric measurements. However, the Delta H of lumirhodopsin at physiological temperatures is quite different from that at low temperature. The reaction volume changes of these processes were determined by the pulsed laser-induced photoacoustic method along with the above Delta H values. Initially, in the transformation between rhodopsin and bathorhodopsin, a large volume expansion of +32 +/- 3 ml/mol was obtained. The volume changes of the subsequent reaction steps were rather small. These results are compared with the structural changes of the chromophore, peptide backbone, and water molecules within the membrane helixes reported previously.

Time-resolved absorption and photothermal measurements with sensory rhodopsin I from Halobacterium salinarum

An expansion accompanying the formation of the first intermediate in the photocycle of transducer-free sensory rhodopsin I (SRI) was determined by means of time-resolved laser-induced optoacoustic spectroscopy. For the native protein (SRI-WT), the absolute value of the expansion is approximately 5.5 mL and for the mutant SRI-D76N, approximately 1.5 mL per mol of phototransformed species (in 0.5 M NaCl), calculated by using the formation quantum yield for the first intermediate (S610) of Phi610 = 0.4 +/- 0.05 for SRI-WT and 0.5 +/- 0.05 for SRI-D76N, measured by laser-induced optoacoustic spectroscopy and by laser flash photolysis. The similarity in Phi610 and in the determined value of the energy level of S610, E610 = (142 +/- 12) kJ/mol for SRI-WT and SRI-D76N indicates that Asp76 is not directly involved in the first step of the phototransformation. The increase with pH of the magnitude of the structural volume change for the formation of S610 in SRI-WT and in SRI-D76N upon excitation with 580 nm indicates also that amino acids other than Asp76, and other than those related to the Schiff base, are involved in the process. The difference in structural volume changes as well as differences in the activation parameters for the S610 decay should be attributed to differences in the rigidity of the cavity surrounding the chromophore. Except for the decay of the first intermediate, which is faster than in the SRI-transducer complex, the rate constants of the photocycle for transducer-free SRI in detergent suspension are strongly retarded with respect to wild-type membranes (this comparison should be done with great care because the preparation of both samples is very different).