MODIFIED QUINONE ACCEPTORS IN PHOTOSYSTEM I
Open Access
- Author:
- Zybailov, Boris L
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 04, 2002
- Committee Members:
- John H Golbeck, Committee Chair/Co-Chair
Donald Ashley Bryant, Committee Member
Squire J Booker, Committee Member
A Daniel Jones, Committee Member
Robert Allen Schlegel, Committee Member - Keywords:
- Plastoquinone-9
Phylloquinone
Quinone
Photosystem I - Abstract:
- The function of the A1 acceptor in cyanobacterial Photosystem I (PS I) complex was probed – using EPR and optical spectroscopies – by: i) interrupting phylloquinone biosynthesis; ii) replacing phylloquinone with foreign quinones; iii) introducing point mutations in amino acids within the A1 quinone binding site. Major findings include that: i) interruption of the menA (phytyl transferase) or menB (naphthoate synthase) genes in the phylloquinone biosynthetic pathway results in a functional PS I complex in which plastoquinone-9 replaces phylloquinone in the A1 quinone binding site; ii) forward electron transfer from A1 to FX is endergonic in a PS I complex that contains plastoquinone9; iii) plastoquinone9 is loosely bound in menA and menB mutant PS I complexes, thus it is readily exchanged with both substituted and unsubstituted foreign quinones in vitro; iv) forward electron transfer in cyanobacterial PS I – as determined by studying the sitedirected mutants W697F (PsaA) and W677 (PsaB), S692C (PsaA) and S672 (PsaB), and R694A (PsaA) and R674A (PsaB) – proceeds predominately through the PsaA-branch of cofactors. Backward electron transfer from A1- to P700+ is biphasic, with lifetimes of ca. 10 µs and ca. 300 µs, in a PS I complex that contains plastoquinone-9. The slow kinetic phase disappears upon reduction with dithionite, indicating that the plastoquinone related to the slow phase has been reduced. However, the fast kinetic phase remains, indicating that the plastoquinone related to the fast kinetic phase has not been reduced. Therefore, these two plastoquinones differ in their midpoint potentials. Supplementation of the growth medium of menB mutant cells with a set of unsubstituted quinones causes incorporation of phytylated quinone into the A1 quinone binding site. When the growth medium of menA mutant cells is supplemented in the same way, no such incorporation occurs and the A1 quinone binding site remains occupied by plastoquinone-9. However, when either menA or menB mutant cells are grown with quinones that have long hydrophobic carbon chains (phylloquinone and menaquinone-8), these quinones occupy the A1 quinone binding site at the expense of plastoquinone-9. Therefore, a long hydrophobic carbon chain is required for quinone incorporation into the A1 quinone binding site in vivo.