PHOTOREVERSIBLE CHANGES IN pH OF PEA PHYTOCHROME SOLUTIONS

Tyrosine 263 in Cyanobacterial Phytochrome Cph1 Optimizes Photochemistry at the prelumi‐R→lumi‐R Step

We report a low‐temperature fluorescence spectroscopy study of the PAS‐GAF‐PHY sensory module of Cph1 phytochrome, its Y263F mutant (both with known 3D structures) as well as Y263H and Y263S to connect their photochemical parameters with intramolecular interactions. None of the holoproteins showed photochemical activity at low temperature, and the activation barriers for the Pr→lumi‐R photoreaction (2.5–3.1 kJ mol−1) and fluorescence quantum yields (0.29–0.42) were similar. The effect of the mutations on Pr→Pfr photoconversion efficiency (ΦPr→Pfr) was observed primarily at the prelumi‐R S0 bifurcation point corresponding to the conical intersection of the energy surfaces at which the molecule relaxes to form lumi‐R or Pr, lowering ΦPr→Pfr from 0.13 in the wild type to 0.05–0.07 in the mutants. We suggest that the Ea activation barrier in the Pr* S1 excited state might correspond to the D‐ring (C19) carbonyl – H290 hydrogen bond or possibly to the hindrance caused by the C131/C171 methyl groups of the C and D rings. The critical role of the tyrosine hydroxyl group can be at the prelumi‐R bifurcation point to optimize the yield of the photoprocess and energy storage in the form of lumi‐R for subsequent rearrangement processes culminating in Pfr formation.

Tyrosine 263 in cyanobacterial phytochrome Cph1 optimizes photochemistry at the prelumi-R→lumi-R step

We report a low-temperature fluorescence spectroscopy study of the PAS-GAF-PHY sensory module of Cph1 phytochrome, its Y263F mutant (both with known 3D structures) as well as Y263H and Y263S to connect their photochemical parameters with intramolecular interactions. None of the holoproteins showed photochemical activity at low temperature, and the activation barriers for the Pr→lumi-R photoreaction (2.5-3.1 kJ mol(-1)) and fluorescence quantum yields (0.29-0.42) were similar. The effect of the mutations on Pr→Pfr photoconversion efficiency (ΦPr→Pfr) was observed primarily at the prelumi-R S0 bifurcation point corresponding to the conical intersection of the energy surfaces at which the molecule relaxes to form lumi-R or Pr, lowering ΦPr→Pfr from 0.13 in the wild type to 0.05-0.07 in the mutants. We suggest that the Ea activation barrier in the Pr* S1 excited state might correspond to the D-ring (C19) carbonyl - H290 hydrogen bond or possibly to the hindrance caused by the C13(1) /C17(1) methyl groups of the C and D rings. The critical role of the tyrosine hydroxyl group can be at the prelumi-R bifurcation point to optimize the yield of the photoprocess and energy storage in the form of lumi-R for subsequent rearrangement processes culminating in Pfr formation.

Proton transfer in the photoreceptors phytochrome and photoactive yellow protein

Light-induced activation of the photoreceptors phytochrome and photoactive yellow protein (PYP) is accompanied by protonation changes of the respective chromophores and key residues in the protein moiety. For both systems, proton exchange with the external medium could be observed with pH electrode measurements and with UV-visible absorption spectroscopy using appropriate pH indicator dyes. From these signals, the stoichiometry of proton release and uptake, respectively, was determined by different calibration procedures which will be presented and discussed. Kinetic information on these processes is only available from time-resolved measurements with pH indicator dyes. Vibrational spectroscopy methods such as Fourier transform infrared spectroscopy and resonance Raman scattering provided information on the protonation state of individual functional groups suggesting that internal proton transfer processes are involved as well. Deuterium kinetic isotope effects that occurred in the Pr --> Pfr phototransformation of the bacteriophytochromes Cph1 and Agp1 were consistent with proton transfer reactions as rate-limiting steps. In contrast, the apparent rate constants in the photocycle of PYP exhibited only small kinetic isotope effects that could not be interpreted conclusively. Possible mechanisms of proton transfer in the activation of phytochrome and PYP will be discussed.

Chemistry of the phytochrome photoconversions

The photoconversions of phytochrome, P r ⇌ P fr , occur both in vivo and in vitro . Structural differences between P r and P fr are discussed for chromophore and apoprotein. The chemical structure of the P r chromophore has been established. The P fr chromophore was recently demonstrated to be the 15 E isomer. The red shift of absorption to 730 nm in native P fr is discussed as interaction between chromophore and apoprotein. The nature of this interaction is still unknown. Small changes in the apoprotein surface are of particular interest because they could be part of the signal chain in photoperception.

Molecular properties of phytochrome

Chromopeptides with molecular masses of ca . 114, 62, 56, 40, 39 and 33 kDa were prepared from pea phytochrome by limited proteolysis. Absorption and circular dichroism spectra were determined and proton uptake and release investigated. The data indicate how long the chromopeptide chain must be for photoreversible changes between P r and P fr or between P 659 and P bl . Double flash-photolytic and low-temperature spectroscopic studies on the photo-transformation pathways from P r to P fr and from P fr to P r of native and degraded chromopeptides were carried out under different conditions, demonstrating that the pool size of kinetically detectable intermediates in a sample changed reversibly depending upon monomer size, and microenvironmental factors such as pH and temperature. Six monoclonal antibodies against rye phytochrome and six against pea phytochrome were raised and investigated in terms of the sites of phytochrome determinants, species specificity, and influence on spectral and other molecular properties.

Resonance Raman spectra of the intermediates in phototransformation of large phytochrome: deprotonation of the chromophore in the bleached intermediate

Resonance Raman (RR) scattering from type A large phytochrome of pea was measured at cryogenic as well as ambient temperatures to determine an intermediate in which deprotonation of the chromophore takes place. The RR bands of the red-absorbing (Pr) and far-red-absorbing forms (Pfr) of large pea phytochromes at ambient temperatures are almost the same in their frequencies as those of the intact form reported previously (Mizutani et al., 1991). The RR spectrum of large phytochrome excited at 364 nm at -120 degrees C, where Pr and a photointermediate, I700 (= lumi-R), are trapped, showed a strong band at 1625 cm-1 with a shoulder at 1648 cm-1 in the C C=C stretching region. The shoulder disappeared, and a new band appeared at 1597 cm-1 upon raising the temperature to -80 degrees C, where transformation from I700 to meta-Ra proceeds. The RR spectra remained unchanged until -10 degrees C, indicating that the RR spectra of meta-Rb and meta-Rc are close to that of meta-Ra, and we call them comprehensively the bleached intermediate, Ibl. At ambient temperatures where photo-steady-states among a few species are attained, strong RR bands were observed at 1625 and 1599 cm-1 upon excitation at 364 nm under simultaneous far-red illumination, and the 1599-cm-1 band was appreciably intensified under simultaneous red- instead of far-red illumination. By comparison of these spectra with those at low temperatures, the 1625- and 1599-cm-1 bands were reasonably assigned to Pr and Ibl, respectively. A chemically prepared model of the bleached form, Pbl, also gave a prominent band at 1599 cm-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Events in the phytochrome molecule after irradiation

The photoconversion of Pr to Pfr has been investigated by a large number of investigators. We have previously demonstrated that Z, E isomerization of the tetrapyrrole chromophore is involved in the photoconversion. It is the best candidate for the primary photoreaction. Conformation and configuration of the Pr chromophore will be compared with that of chromophores in phycocyanin. The crystal structure of phycocyanin had been elucidated by x-ray analysis. Proton transfer and/or Z, E isomerization of the tetrapyrrole are probably involved in different steps of the photoconversion in phytochrome and in photoreversible phycobiliproteins. Fluorescence decay kinetics of irradiated Pr and intermediate formation show heterogeneity. Possible reasons for this heterogeneity will be discussed.

Resonance Raman study on intact pea phytochrome and its model compounds: evidence for proton migration during the phototransformation

Resonance Raman (RR) scattering from intact pea phytochrome was observed in resonance with the blue band at ambient temperature. The relative populations of the red-light-absorbing form (Pr) and far-red-light-absorbing form (Pfr) under laser illumination were estimated from the absorption spectra. The most prominent RR band of Pr obtained by 364-nm excitation under 740-nm pumping exhibited a frequency shift between H2O and D2O solutions, but that of Pfr obtained by 407-nm excitation under 633-nm pumping did not, indicating a distinct difference in a protonation state of their chromophores. Since the protonation level of a whole molecule of intact phytochrome remains unchanged between Pr and Pfr, this observation indicates migration of a proton from the chromophore of Pr to the protein moiety of Pfr. As model compounds, octaethylbiliverdin (OEBV-h3), its deuterated and 15N derivatives, and their protonated forms were also studied with both RR and 1H and 15N NMR spectroscopies. The RR spectrum of the protonated form, for which the protonation site was determined to be C-ring pyrrole nitrogen by NMR, displayed a deuteration shift corresponding to that of Pr, suggesting a similar protonated structure for the pyrrolic rings of Pr. The RR spectral difference between OEBV-h3 and OEBV-d3 and that between H2O and D2O solutions of Pfr suggested that the N-H protons of the A-, B-, and D-rings of intact phytochrome are replaced with deuterons in D2O. A role of the 7-kDa segment of phytochrome is discussed on the basis of RR spectral differences between the intact and large phytochromes.

The molecular topography of phytochrome: chromophore and apoprotein

Phytochrome serves as the photochromic receptor for a number of morphogenic and developmental responses to red light in higher plants. The photoreversible phototransformation of 124 kDa oat phytochrome involves several structural changes in the chromophore and the apoprotein, including a configurational/conformational isomerization and secondary/tertiary structural changes respectively. For example, there appears to be a specific interaction between the chromophore and the amino terminus segment in the Pfr form of phytochrome, which results in a photoreversible peptide folding of the amino terminus peptide chain. Other structural changes also accompany the phototransformation, as has been probed by peptide mapping, phosphorylation, and monoclonal antibodies.