Recruitment of a Foreign Quinone into the A1 Site of Photosystem I

Abstract

Substituted quinones are usually employed as cofactors in electron transport chains, as demonstrated in bacterial reaction centers (1, 2), and in the two photosystems of plants and cyanobacteria (3-5). These quinones comprise a relatively polar ring, which consists of either a benzoquinone (BQ) 1 or a naphthoquinone (NQ) "head group" and a non-polar isoprenoid "tail" of various chain lengths and degrees of saturation. Benzoquinones such as plastoquinone-9 (PQ-9) and ubiquinone-10 function either as fixed or exchangeable electron/proton carriers during photosynthetic and respiratory electron transport. In photosystem II (PS II), PQ-9 functions as a bound one-electron cofactor in the Q A site and as an exchangeable two-electron/ two-proton cofactor in the Q B site. The reduced PQH 2 -9 is displaced from the Q B site, diffuses laterally through the membrane, and becomes oxidized and deprotonated by the cytochrome b 6 f complex. Photosynthetic reaction centers (RCs) of purple bacteria use either ubiquinone-10 (e.g. Rhodobacter sphaeroides) in a similar double role or menaquinone-9 in the Q A site (e.g. Rhodosprillulum viridis). In photosystem I (PS I), phylloquinone (PhQ), a substituted 1,4-NQ with a 20-carbon, largely saturated phytyl tail, functions as a bound one-electron cofactor in the A 1 site. PS I contains two PhQ molecules/P700, but neither of the two quinones functions in a manner equivalent to Q B in the bacterial RC and PS II. Instead, the electron is transferred from the active quinone(s) to soluble ferredoxin via a chain of three bound iron-sulfur centers. Quinones are therefore extremely versatile; they can function as the interface between electron transfer involving organic cofactors and electron transfer involving iron-sulfur clusters (as in PS I), or between pure electron transfer and coupled electron/proton transfer involving a second organic cofactor (as in PS II). Each quinone displays equilibrium binding and redox properties that can be very different for each site of interaction (6), and these properties are conferred largely by the protein environment.To understand the structural determinants that allow quinones to function with a low redox potential in the A 1 site of PS I, we embarked on a project aimed at biological replacement of

Loss of Phylloquinone in Chlamydomonas Affects Plastoquinone Pool Size and Photosystem II Synthesis

“…In Synechocystis sp. PCC 6803 deficient in menD, the ratio of PSI to PSII is significantly reduced relative to wild type (2,3), whereas the opposite is found in C. reinhardtii. In A. thaliana loss of phylloquinone leads to a drastic reduction in PSI and to a significant decrease in plastoquinone (6,7).…”

“…Only a limited number of mutants deficient in the synthesis or assembly of the redox cofactors have been examined. In this respect, mutants of cyanobacteria and land plants deficient in phylloquinone synthesis have been characterized recently (2)(3)(4)(5)(6)(7). Quinones play an important role as redox cofactors in PSII and PSI and as electron carriers between PSII and the cytochrome b 6 f complex.…”

“…By comparison of the genome of the cyanobacterium Synechocystis sp. PCC 6803 with bacterial genes, the enzymes involved in phylloquinone biosynthesis have been identified, and this pathway has been determined in cyanobacteria using reverse genetics (2,3). It involves eight enzymes as follows: menF (isochorismate synthase), menD (2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase), menC (o-succinyl benzoate synthase), menE (O-succinylbenzoic acid-CoA synthase), menB (1,4-dihydroxy-2-naphtoyl-CoA synthase), menH (thioesterase), menA (1,4-dihydroxy-2-naphthoate phytyltransferase), and menG (methyltransferase).…”

“…Loss of phylloquinone synthesis has different effects in cyanobacteria and land plants. In cyanobacteria, insertional inactivation of the menA, -B, -D, and -E genes leads to the complete absence of phylloquinone, which is replaced by plastoquinone-9 in PSI (2,3,16,36). However, inactivation of menG, the last enzyme involved in the phylloquinone synthesis pathway, leads to the accumulation of demethylphylloquinone, which replaces phylloquinone in PSI (5).…”

“…PCC 6803 strain lacking menD, the amount of PSI is lower than in wild-type cells (2,3). We therefore tested whether the level of the different photosynthetic complexes in the menD1 mutant was altered by immunoblotting of total proteins extracted from cells in exponential phase.…”