Alterations in Carboxylate Ligation at the Active Site of Photosystem II

Abstract

Photosystem II (PSII) is the photosynthetic enzyme catalyzing the oxidation of water and reduction of plastoquinone (Q). This reaction occurs at a catalytic site containing four manganese atoms and cycling among five oxidation states, the S n states, where n refers to the number of oxidizing equivalents stored. Biochemical and spectroscopic techniques have been used previously to conclude that aspartate 170 in the D1 subunit influences the structure and function of the PSII active site (Boerner, R. J., Nguyen, A. P., Barry, B. A., and Debus, R. J. (1992) Biochemistry 31, 6660 -6672). Substitution of glutamate for aspartate 170 resulted in an assembled manganese cluster, which was capable of enzymatic turnover, but at lower steady-state oxygen evolution rates. Here, we obtained the difference (lightminus-dark) Fourier transform IR spectrum associated with the S 2 Q ؊ -minus-S 1 Q transition by illumination of oxygen-evolving wild-type and DE170D1 PSII preparations at 200 K. These spectra are known to be dominated by contributions from carboxylic acid and carboxylate residues that are close to or ligating the manganese cluster. Substitution of glutamate for aspartate 170 results in alterations in the S 2 Q ؊ -minus-S 1 Q spectrum; the alterations are consistent with a change in carboxylate coordination to manganese or calcium. In particular, the spectra are consistent with a shift from bridging/ bidentate carboxylates in wild-type PSII to unidentate carboxylate ligation in DE170D1 PSII.Photosystem II (PSII) 1 is the photosynthetic reaction center that is responsible for the light-driven oxidation of water and the reduction of quinone in plants, green algae, and cyanobacteria. The catalytic site contains a tetranuclear manganese cluster, and the catalytic cycle of water oxidation has been proposed to consist of five oxidation states (S n ) called the JoliotKok S states. The subscript n refers to the number of oxidizing equivalents stored at the active site. Absorption of four photons and four corresponding charge separations are necessary to complete one catalytic cycle. Each cycle results in the oxidation of two water molecules to form one molecule of molecular oxygen and four protons. Calcium is required for oxygen evolution and may be bound in proximity to the manganese cluster (reviewed in Refs. 1 and 2).Many studies of PSII have focused on the S 2 state (for review, see Ref. 3). Dark adaptation at room temperature sets the catalytic center to the S 1 state because S 0 converts to S 1 by reduction of tyrosine radical, D ⅐ (4, 5). Two different EPR signals, a g ϭ 2 multiline signal (6) and a g ϭ 4.1 signal (7), are observed from the S 2 state. Illumination of plant or cyanobacterial PSII at 200 K results in the formation of the g ϭ 2 multiline EPR signal (for examples, see Refs. 6 and 8) and an EPR signal from Fe 2ϩ Q A Ϫ (9). The S 1 -to-S 2 transition is associated with oxidation of manganese, probably from Mn 3ϩ to Mn 4ϩ (reviewed in Ref. 10). Recently, we have obtained the difference FT-IR spectrum associate...

Deprotonation of the 33-kDa, Extrinsic, Manganese-stabilizing Subunit Accompanies Photooxidation of Manganese in Photosystem II

“…2A, solid line). In agreement with our published work (35), alteration in illumination time from 10 min to 1 min had no effect on the spectrum obtained. The inset in Fig.…”

“…Illumination at this temperature has been used to acquire the S 2 Q A Ϫ -minus-S 1 Q A vibrational spectrum (21,31,32,35). The spectroscopic results that we have presented here are consistent with a deprotonation reaction, accompanying the S 1 to S 2 transition.…”

“…2A is compared with Fig. 2C, a variation in the ratio of the 1478 cm Ϫ1 line to broad spectral features between 1500 and 1200 cm Ϫ1 , previously assigned to S 2 -minus-S 1 (21,31,32,35), is observed. When PSII complexes in glycerol are illuminated at 200 K, the S 2 state is formed in the majority of centers, but Chl ϩ is oxidized in 33 Ϯ 9% of PSII centers (21).…”

Specific Isotopic Labeling and Photooxidation-linked Structural Changes in the Manganese-stabilizing Subunit of Photosystem II

“…1, A-C, represent S 2 Q A Ϫ -minus-S 1 Q A difference spectra acquired from PSII samples containing natural abundance, [ 13 C]Glu/Gln-, or [ 13 C]Asp/Asn-labeled MSP, respectively. The 12 C MSP spectrum is similar to data obtained previously at this temperature (25,29,47,51). Because all the spectra were corrected for path length and concentration, they were directly subtracted on a one to one basis, to generate the isotope-edited spectrum (Figs.…”

“…The origin of these spectral differences has been discussed previously (35,36). Assignments of the 200 K spectra, based on global isotopic labeling and site-directed mutagenesis, have been described (25,29,47,51). In particular, at 200 K, our previous isotope editing work has shown that a subset of the carboxylates, contributing to the spectra, is on MSP (23).…”