Two Redox-Active β-Carotene Molecules in Photosystem II

Abstract

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b(559) (Cyt b(559)), chlorophyll (Chl), and beta-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electron-transfer pathways of PS II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redox-active carotenoids are formed upon low-temperature illumination. The Car(+) near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car(+) peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car(+) (Car(A)(+) has an absorbance maximum of 982 nm, and Car(B)(+) has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, we observe a shift of the Car(+) peak to shorter wavelength when oxidized tyrosine D (Y(D)*) is present in the sample that is explained by an electrostatic interaction between Y(D)* and a nearby beta-carotene that disfavors oxidation of Car(B). The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.

Construction and Characterization of Genetically Modified Synechocystis sp. PCC 6803 Photosystem II Core Complexes Containing Carotenoids with Shorter π-Conjugation than β-Carotene

“…observed in spinach and WT Synechococcus and Synechocystis PS II samples have been observed in the range of 980 -1000 nm (11). The terminal double bonds of ␤-carotene are located within the ␤-ionylidene rings, which are forced out of the plane of conjugation with the rest of the polyene chain because of steric repulsion between the methyl groups on the ring and the polyene chain.…”

“…Temperature Dependence of Carotenoid Oxidation-Tracewell and Brudvig (11) showed that the Car ϩ band in WT Synechocystis can be fit by two Gaussians, one centered at 982 nm (Car A ϩ ) and another at 1027 nm (Car B ϩ ). Although both Car A ϩ and Car B ϩ are ␤-carotenes, the difference in the absorbance maxima was attributed to differences in the local environments of the two carotenoids.…”

“…Pigment analyses of PS II reaction center preparations have identified two ␤-carotene molecules bound to the D1 and D2 polypeptides (18,19,22). Both of these ␤-carotenes have been reported to be redox-active (6,11).…”

“…Earlier studies suggested that the carotenoid molecule is the primary donor to P 680 ϩ at low temperature, followed by subsequent oxidation of Chl Z and Cyt b 559 (3,5). Recently, evidence has been obtained for the involvement of two carotenoid molecules in this pathway, most likely the all-trans-␤-carotenes in the reaction center (6,11). Although four crystal structures of PS II core complexes have been determined, the location of the carotenoids in the D1 and D2 polypeptides remains uncertain because of the limited resolution of the diffraction data, 3.2-3.8 Å (12)(13)(14)(15).…”

Photosynthetic generation of oxygen

“…Each oxidation state generated in the oxygen-evolving centre (OEC) is represented as an intermediate of the S-state cycle ( Joliot et al 1969;Kok et al 1970) of which there are five distinct states (S0-S4). In addition to these reactions, side reactions can occur under some conditions including the oxidation of a high-potential cytochrome bound within the PSII core complex (Cyt b559), a b-carotene molecule and a Chl a molecule (Chl Z ; Stewart & Brudvig 1998;Faller et al 2001;Telfer 2002;Tracewell & Brudvig 2003). These side reactions occur on the tens of millisecond time scale and therefore do not compete with the electron transfer pathway leading to water oxidation.…”