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Protein-Quinone
Interactions in the Cyanobacterial Photosystem I
Xu, Wu 1,
Johnson, Wade1, Zybailov, Boris2,
Golbeck, John2 and Chitnis, Parag1
Department of Biochemistry, Biophysics and Molecular Biology, Iowa
State University, Ames IA 500111
Department of Biochemistry and Molecular Biology, Pennsylvania State University,
University Park PA 168022
Abstract-
Photosystem I of photosynthetic electron transfer chain contains two
molecules of phylloquinones per reaction center. One or both of these
phylloquinone molecules functions as the very low potential A1 redox
center that transfers electrons from the A0 chlorophyll a molecule to
the FX iron-sulfur cluster. Two phylloquinone molecules are bound to
the PsaA and PsaB core proteins of photosytem I. Highly conserved regions
in PsaA and PsaB (A_686LFSGRGYWQELIE698 and B_686LISWRGYWQELIE698) provide
pi-stacking and H-bonding interactions to the phylloquinone molecules.
These and additional structural interactions between the proteins and
quinones have been proposed to influence the spectroscopic properties
and function of the photosystem I quinones. To identify the functionally
important interactions, we are using two approaches: (1) site-directed
mutants in the photosystem I proteins and (2) in vivo replacement of
the phylloquinone with other quinones in the mutants in phylloquinone
biosynthetic pathway. The disruptive replacement of tryptophenyl residues
in the phylloquinone-binding pocket disallows incorporation of phylloquinone
into photosystem I and destablizes the complex. In contrast, the conservative
mutations influence the EPR characteristics of the phylloquinones to
varying degree. Inactivation of the phylloquinone biosynthetic genes
leads to incorporation of plastoquinone in the A1 site. When the phylloquinone-less
menB mutant cells are grown in the presence of different naphthoquinones,
some are phytylated and are incorporated into photosystem I. Attachment
of phytyl tail is a prerequisite for the incorporation of a quinone
into photosystem I. Thus these experiments demonstrate the role of pi-stacking,
H-bonds and phytyl chain in the assembly and function of the phylloquinones
of photosystem I.
Keywords: photosynthesis,
photosystem, cyanobacteria, mutagenesis
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