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In-Depth Information
9.2.4 Effect of Environment on the Photoconversion
The rate of photoconversion expressed as a percentage of the photoconvertible
protochlorophyllide was found to be independent of the initial concentration of
the holochrome and was not influenced by the viscosity of the medium (Boardman
1962
). This led to the proposal that the photoconversion did not involve a collision
process between independent protein molecules or between a protein molecule and a
hydrogen donor molecule. Instead, Boardman (
1962
) proposed a restricted collision
process between the photo-activated Pchl molecule and the hydrogen donor. How-
ever since the rate of phototransformation was temperature-dependent, it seemed
likely that the hydrogenation involved some vibrational or rotational movement of
that part of the protein molecule in close proximity to the Pchlide
a
chromophore.
9.2.5 Photoconversion Kinetics
While
Smith and Benitez
(
1954
), opted for a bimolecular reaction with respect to
within molecular groups, the true kinetics of the photoconversion was first order,
which is still compatible with the restricted collision hypothesis (Boardman
1962
).
9.3 The Multiple Light-Dependent Pchlide
a
Oxidoreductases (PORs)
It has been proposed that at least four different POR isozymes may be present in
plants (Dehesh et al.
1986
; Ikeuchi and Murakami
1982
). In
Arabidopsis thaliana
and Barley, two different genes with about 75 % homology, PorA and PorB, have
been shown to code for two different Pchlide
a
oxidoreductases, namely PORA and
PORB (Armstrong et al.
1995
; Holtorf et al.
1995
). PORA is synthesized in the dark
and constitutes the bulk of the crystalline prolamellar body of etioplasts. However
the transcription of its gene is turned off in the light and the enzyme is rapidly
degraded by a light-induced protease (Reinbothe et al.
1995
,
1999
). On the other
hand, the PorB gene is transcribed in darkness and in the light, and the transcripts
are translated continuously into the enzyme which is responsible for the bulk of Chl
a
biosynthesis and accumulation in daylight. More recently, a gene that encodes a
third POR in
Arabidopsis thaliana
has been reported (Oosawa et al.
2000
). The
Protein has been named PORC. This enzyme is expressed only during the light
phase of the photoperiod. Since PORA, B and C respond to light differently (see
below), it has been suggested that the function of the three PORs of
Arabidopsis
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