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Transient EPR
Studies of Photosystem I in Quinone Biosynthetic Pathway Mutants
van der Est, Art1
Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, Canada
L2S 3A11
Abstract-
The quinone acceptors both type I and type II photosynthetic reaction
centers play a pivotal role in the light induced electron transfer in
these systems. Although, the quinones found in the various reaction
centers are structurally very similar, the rates of electron transfer
and nature of the subsequent acceptors vary greatly. From a large number
of studies, it has become clear that the properties of the quinones
and their function as electron acceptors are influenced by protein-cofactor
interactions. The nature of the quinone binding and the structural features
of the quinone and its binding site, which are most important for its
function are still not clearly understood. We have been studying these
relationships using transient electron spin resonance spectroscopy to
investigate Photosystem I, in which the native phylloquinone has been
replaced by a foreign quinone. A very elegant way of exchanging the
quinone is to manipulate the biosynthesis of phylloquinone genetically.
Results will be presented for several such biosynthetic pathway mutants.
Interruption of the menA or menB gene inhibits the biosynthesis
of phylloquinone. In these mutants Photosystem I incorporates plastoquinone
from Photosystem II and is still able to function albeit with a greatly
reduced rate of electron transfer. Interruption of the menG gene
which codes for a methyl transferase, results in Photosystem I containing-2-phytylnaphthoquinone
instead of phylloquinone. Our results show that the rates of both forward
and back electron transfer in the mutants depend crucially on the structure
of the quinone and suggest that the forward rate decreases and back
reaction rate increases as the midpoint potential of the quinone becomes
less negative compared to phylloquinone.
Keywords:
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