Abstract:
The proce-ss ofoxygenic photosynthesis is vital to life on Earth. the central event
in photosynthesis is light induced electron transfer that converts light into energy for
growth. Ofparticular significance is the membrane bound multisubunit protein known as
Photosystem I (PSI). PSI is a reaction centre that is responsible for the transfer of
electrons across the membrane to reduce NADP+ to NADPH. The recent publication ofa
high resolution X-ray structure of PSI has shown new information about the structure, in
particular the electron transfer cofactors, which allows us to study it in more detail.
In PSI, the secondary acceptor is crucial for forward electron transfer. In this
thesis, the effect of removing the native acceptor phylloquinone and replacing it with a
series of structurally related quinones was investigated via transient electron
paramagnetic resonance (EPR) experiments. The orientation of non native quinones in
the binding site and their ability to function in the electron transfer process was
determined.
It was found that PSI will readily accept alkyl naphthoquinones and
anthraquinone. Q band EPR experiments revealed that the non-native quinones are
incorporated into the binding site with the same orientation of the headgroup as in the
native system. X band EPR spectra and deuteration experiments indicate that monosubstituted
naphthoquinones are bound to the Al site with their side group in the position
occupied by the methyl group in native PSI (meta to the hydrogen bonded carbonyl
oxygen). X band EPR experiments show that 2, 3- disubstituted methyl naphthoquinones
are also incorporated into the Al site in the same orientation as phylloquinone, even with
the presence of a halogen- or sulfur-containing side chain in the position normally occupied by the phytyl tail ofphylloquinone. The exception to this is 2-bromo-3-methyl
--.- _. -. - -- - - 4 _._ _ _ - _ _
naphthoquinone which has a poorly resolved spectrum, making determination of the
orientation difficuh.
All of the non-native quinones studied act as efficient electron acceptors.
However, forward electron transfer past the quinone could only be demonstrated for
anthraquinone, which has a more negative midpoint potential than phylloquinone. In the
case of anthraquinone, an increased rate of forward electron transfer compared to native
PSI was found. From these results we can conclude that the rate ofelectron transfer from
Al to Fx in native PSI lies in the normal region ofthe Marcus Curve.
