Assignment of the Q(y) absorbance bands of photosystem II chromophores by low-temperature optical spectroscopy of wild-type and mutant reaction centers

D. H. Stewart, P. J. Nixon, B. A. Diner, Gary W Brudvig

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Abstract

Photosystem II (PSII) contains a collection of pheophytins (Pheo) and chlorophylls (Chl) that have unique absorbance spectra depending on their electronic structure and the surrounding protein environment. Despite numerous efforts to identify the spectra of each cofactor, differing assignments of the chromophore absorbance bands and electrochromic effects have led to conflicting models of pigment organization and chromophore interactions in PSII. We have utilized low-temperature measurements on well-defined redox states, together with the use of site-directed mutants, to make spectral assignments of several reaction center (RC) chromophores. Cryogenic (77 K) optical spectroscopy has been used to trap the bound redox-active quinone, Q(A), in the reduced form and measure the effect of the redox state of Q(A) on PSII chromophores without interference from other redox-active cofactors. The Q(A)- minus Q(A) difference spectrum contains a number of features that represent the perturbation of Pheo and Chl absorbance bands upon Q(A) reduction. Using site-directed mutants in which the axial ligand of the D1-side monomeric core Chl, P(A), is changed (D1-H198Q) or the hydrogen-bonding environment of the D1-side Pheo is modified (D1-Q130E), we have assigned the Q(y) absorbance bands of four chromophores shifted by Q(A) reduction including both RC Pheos, the D1-side monomeric accessory Chl (B(A)), and one other Chl in PSII. The absorbance maximum of B(A) was identified at 683.5 nm from least-squares fits of the D1-H198Q minus wild type (WT) Q(A)- minus Q(A) double-difference spectrum; this assignment provides new evidence of a secondary effect of site-directed mutation on a RC chromophore. The other chromophores were assigned from simultaneous fits of the WT and D1-Q130E spectra in which the parameters of only the D1:Side Pheo were allowed to vary. The D1-side and D2-side Pheos were found to have λ(max) values at 685.6 and 669.3 nm, respectively, and another Chl influenced by Q(A)- was identified at 678.8 nm. These assignments are in good agreement with previous spectral analyses of intact PSII preparations and reveal that the number of chromophores affected by Q(A) reduction has been underestirnated previously. In addition, the assignments are generally consistent with chromophore positions that are similar in the PSH RC and the bacterial photosynthetic RC.

Original languageEnglish
Pages (from-to)14583-14594
Number of pages12
JournalBiochemistry
Volume39
Issue number47
DOIs
Publication statusPublished - Nov 28 2000

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Photosystem II Protein Complex
Chlorophyll
Chromophores
Pheophytins
Spectrum Analysis
Temperature
Oxidation-Reduction
Photosynthetic Reaction Center Complex Proteins
Hydrogen Bonding
Least-Squares Analysis
Optical spectroscopy
Accessories
Ligands
Pigments
Temperature measurement
Cryogenics
Mutation
Electronic structure
Hydrogen bonds
Proteins

ASJC Scopus subject areas

  • Biochemistry

Cite this

Assignment of the Q(y) absorbance bands of photosystem II chromophores by low-temperature optical spectroscopy of wild-type and mutant reaction centers. / Stewart, D. H.; Nixon, P. J.; Diner, B. A.; Brudvig, Gary W.

In: Biochemistry, Vol. 39, No. 47, 28.11.2000, p. 14583-14594.

Research output: Contribution to journalArticle

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