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The primary donor of far-red photosystem II: Chl D1 or P D2? BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2020; 1861:148248. [PMID: 32565079 DOI: 10.1016/j.bbabio.2020.148248] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/31/2020] [Revised: 06/13/2020] [Accepted: 06/15/2020] [Indexed: 11/20/2022]
Abstract
Far-red light (FRL) Photosystem II (PSII) isolated from Chroococcidiopsis thermalis is studied using parallel analyses of low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies in conjunction with fluorescence measurements. This extends earlier studies (Nurnberg et al 2018 Science 360 (2018) 1210-1213). We confirm that the chlorophyll absorbing at 726 nm is the primary electron donor. At 1.8 K efficient photochemistry occurs when exciting at 726 nm and shorter wavelengths; but not at wavelengths longer than 726 nm. The 726 nm absorption peak exhibits a 21 ± 4 cm-1 electrochromic shift due to formation of the semiquinone anion, QA-. Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: PD1, PD2 ChlD1, ChlD2, PheoD1 and PheoD2. Two of these chlorins, ChlD1 and PD2, are located at a distance and orientation relative to QA- so as to account for the observed electrochromic shift. Previously, ChlD1 was taken as the most likely candidate for the primary donor based on spectroscopy, sequence analysis and mechanistic arguments. Here, a more detailed comparison of the spectroscopic data with exciton modelling of the electrochromic pattern indicates that PD2 is at least as likely as ChlD1 to be responsible for the 726 nm absorption. The correspondence in sign and magnitude of the CD observed at 726 nm with that predicted from modelling favors PD2 as the primary donor. The pros and cons of PD2 vs ChlD1 as the location of the FRL-primary donor are discussed.
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The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2016; 1857:1580-1593. [PMID: 27342201 DOI: 10.1016/j.bbabio.2016.06.007] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/15/2016] [Revised: 06/16/2016] [Accepted: 06/18/2016] [Indexed: 11/22/2022]
Abstract
The identification of low-energy chlorophyll pigments in photosystem II (PSII) is critical to our understanding of the kinetics and mechanism of this important enzyme. We report parallel circular dichroism (CD) and circularly polarized luminescence (CPL) measurements at liquid helium temperatures of the proximal antenna protein CP47. This assembly hosts the lowest-energy chlorophylls in PSII, responsible for the well-known "F695" fluorescence band of thylakoids and PSII core complexes. Our new spectra enable a clear identification of the lowest-energy exciton state of CP47. This state exhibits a small but measurable excitonic delocalization, as predicated by its CD and CPL. Using structure-based simulations incorporating the new spectra, we propose a revised set of site energies for the 16 chlorophylls of CP47. The significant difference from previous analyses is that the lowest-energy pigment is assigned as Chl 612 (alternately numbered Chl 11). The new assignment is readily reconciled with the large number of experimental observations in the literature, while the most common previous assignment for the lowest energy pigment, Chl 627(29), is shown to be inconsistent with CD and CPL results. Chl 612(11) is near the peripheral light-harvesting system in higher plants, in a lumen-exposed region of the thylakoid membrane. The low-energy pigment is also near a recently proposed binding site of the PsbS protein. This result consequently has significant implications for our understanding of the kinetics and regulation of energy transfer in PSII.
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Cox N, Hughes JL, Steffen R, Smith PJ, Rutherford AW, Pace RJ, Krausz E. Identification of the QY Excitation of the Primary Electron Acceptor of Photosystem II: CD Determination of Its Coupling Environment. J Phys Chem B 2009; 113:12364-74. [DOI: 10.1021/jp808796x] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Nicholas Cox
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
| | - Joseph L. Hughes
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
| | - Ronald Steffen
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
| | - Paul J. Smith
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
| | - A. William Rutherford
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
| | - Ron J. Pace
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
| | - Elmars Krausz
- Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and iBiTec-S, CNRS URA 2096, Bât 532, CEA Saclay, 91191 Gif-sur-Yvette, France
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Vassiliev S, Bruce D. Toward understanding molecular mechanisms of light harvesting and charge separation in photosystem II. PHOTOSYNTHESIS RESEARCH 2008; 97:75-89. [PMID: 18443918 DOI: 10.1007/s11120-008-9303-4] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/27/2007] [Accepted: 03/31/2008] [Indexed: 05/26/2023]
Abstract
Conversion of light energy in photosynthesis is extremely fast and efficient, and understanding the nature of this complex photophysical process is challenging. This review describes current progress in understanding molecular mechanisms of light harvesting and charge separation in photosystem II (PSII). Breakthroughs in X-ray crystallography have allowed the development and testing of more detailed kinetic models than have previously been possible. However, due to the complexity of the light conversion processes, satisfactory descriptions remain elusive. Recent advances point out the importance of variations in the photochemical properties of PSII in situ in different thylakoid membrane regions as well as the advantages of combining sophisticated time-resolved spectroscopic experiments with atomic level computational modeling which includes the effects of molecular dynamics.
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Affiliation(s)
- Serguei Vassiliev
- Department of Biology, Brock University, St. Catharines, ON, Canada L2S 3A1.
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Hughes JL, Smith P, Pace R, Krausz E. Charge separation in photosystem II core complexes induced by 690-730 nm excitation at 1.7 K. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2006; 1757:841-51. [PMID: 16859635 DOI: 10.1016/j.bbabio.2006.05.035] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/30/2006] [Revised: 05/01/2006] [Accepted: 05/22/2006] [Indexed: 11/24/2022]
Abstract
The illumination of oxygen-evolving PSII core complexes at very low temperatures in spectral regions not expected to excite P680 leads to charge separation in a majority of centers. The fraction of centers photoconverted as a function of the number of absorbed photons per PSII core is determined by quantification of electrochromic shifts on Pheo(D1). These shifts arise from the formation of metastable plastoquinone anion (Q(A)(-)) configurations. Spectra of concentrated samples identify absorption in the 700-730 nm range. This is well beyond absorption attributable to CP47. Spectra in the 690-730 nm region can be described by the 'trap' CP47 absorption at 689 nm, with dipole strength of approximately 1 chlorophyll a (chl a), partially overlapping a broader feature near 705 nm with a dipole strength of approximately 0.15 chl a. This absorption strength in the 700-730 nm region falls by 40% in the photoconverted configuration. Quantum efficiencies of photoconversion following illumination in the 690-700 nm region are similar to those obtained with green illumination but fall significantly in the 700-730 nm range. Two possible assignments of the long-wavelength absorption are considered. Firstly, as a low intensity component of strongly exciton-coupled reaction center chlorin excitations and secondly as a nominally 'dark' charge-transfer excitation of the 'special pair' P(D1)-P(D2). The opportunities offered by these observations towards the understanding of the nature of P680 and PSII fluorescence are discussed.
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Affiliation(s)
- Joseph L Hughes
- Research School of Chemistry, Australian National University, Canberra
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