Evaluation of selected benzoquinones, naphthoquinones, and anthraquinones as replacements for phylloquinone in the A1 acceptor site of the photosystem I reaction center

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Abstract
Selected substituted 1,4-benzoquinones, 1,4-naphthoquinones, and 9,10-anthraquinones were investigated as possible replacement quinones in spinach photosystem I (PSI) preparations that had been depleted of endogenous phylloquinone by extraction with hexane/methanol. As a criterion for successful biochemical reconstitution, the restoration of electron transfer was determined by measuring P-430 turnover at room temperature from flash-induced absorbance transients. Restoration of complete electron transfer between A0- and P-430 (terminal iron-sulfur centers, FAFB) was demonstrated by using phylloquinone, 2-methyl-3-decyl-1,4-naphthoqunone, 2-methyl-3-(isoprenyl)2-1,4-naphthoquinone, and 2-methyl-3-(isoprenyl)4-1,4-naphthoquinone. All other quinones tested did not restore P-430 turnover but acted as electron acceptors and oxidized A0-. It is concluded that the specificity of the replacement quinone for interaction with the primary acceptor, A0-, is low but additional structural constraints are required for the quinone occupying the A1 site to donate to the iron-sulfur center, Fx. It is suggested that the 3-phytyl side chain of phylloquinone and the 3-alkyl tails of the three naphthoquinones that restored P-430 turnover may be required for interaction with a hydrophobic domain of the A1 site in the PSI core to promote electron transfer to Fx and then to FAFB.