Characterization of carotenoid and chlorophyll photooxidation in photosystem II.
Photosystem II (PSII) contains two accessory chlorophylls (Chl(Z), ligated to D1-His118, and Chl(D), ligated to D2-His117), carotenoid (Car), and heme (cytochrome b(559)) cofactors that function as alternate electron donors under conditions in which the primary electron-donation pathway from the O(2)-evolving complex to P680(+) is inhibited. The photooxidation of the redox-active accessory chlorophylls and Car has been characterized by near-infrared (near-IR) absorbance, shifted-excitation Raman difference spectroscopy (SERDS), and electron paramagnetic resonance (EPR) spectroscopy over a range of cryogenic temperatures from 6 to 120 K in both Synechocystis PSII core complexes and spinach PSII membranes. The following key observations were made: (1) only one Chl(+) near-IR band is observed at 814 nm in Synechocystis PSII core complexes, which is assigned to Chl(Z)(+) based on previous spectroscopic studies of the D1-H118Q and D2-H117Q mutants [Stewart, D. H., Cua, A., Chisholm, D. A., Diner, B. A., Bocian, D. F., and Brudvig, G. W. (1998) Biochemistry 37, 10040-10046]; (2) two Chl(+) near-IR bands are observed at 817 and 850 nm in spinach PSII membranes which are formed with variable relative yields depending on the illumination temperature and are assigned to Chl(Z)(+), and Chl(D)(+), respectively; (3) the Chl and Car cation radicals have significantly different stabilities at reduced temperatures with Car(+) decaying much faster; (4) in Synechocystis PSII core complexes, Car(+) decays by recombination with Q(A)(-) and not by Chl(Z)/Chl(D) oxidation, with multiphasic kinetics that are attributed to an ensemble of protein conformers that are trapped as the protein is frozen; and (5) in spinach PSII membranes, Car(+) decays mainly by recombination with Q(A)(-), but also partly by formation of the 850 nm Chl cation radical. The greater stability of Chl(Z)(+) at low temperatures enabled us to confirm that resonance Raman bands previously assigned to Chl(Z)(+) are correctly assigned. In addition, the formation and decay of these cations provide insight into the alternate electron-donation pathways to P680(+).
Tracewell
C A
CA
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
Cua
A
A
Stewart
D H
DH
Bocian
D F
DF
Brudvig
G W
GW
D002338:Carotenoids / Q000737:chemistry*; D002734:Chlorophyll / Q000737:chemistry*; D000458:Cyanobacteria / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D045342:Light-Harvesting Protein Complexes; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D019265:Spectroscopy, Near-Infrared; D013059:Spectrum Analysis, Raman; D018724:Spinacia oleracea; D013696:Temperature; D020524:Thylakoids / Q000737:chemistry
D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D002338:Carotenoids