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are not redundant, but may allow the plant to adapt its needs for Chl biosynthesis
according to the prevailing light regime (Su et al.
2001
). In our opinion, adaptation
of Chl biosynthesis to different light conditions, proceeds via multiple and different
Chl biosynthetic routes.
9.3.1 NADPH-Protochlorophyllide a (Photo)
Oxidoreductase A (PORA, or PCR)
As pointed out above, most of the early investigations of the photoreduction of
Pchlide
a
dealt with t-LW-Pchlide H (E650 F657). The notion that the Pchlide
a
H apoprotein of t-LW-Pchlide
a
H (E650 F657) (i.e. PORA) acts as a shuttling
photoenzyme that catalyzes the conversion of Pchlide
a
to Chlide
a
was first
proposed by Sironval et al. (
1967
). In this work the authors reported that
t-LW-Pchlide
a
(E647 F 657), with a red excitation maximum at 647 nm and a
red emission maximum at 657 nm, is photoconverted by light to Chlide
a
(E676
F690). The latter is converted in darkness to Chlide
a
(E682 F697). At this stage of
the reaction, the authors suggested that the apoprotein discharges the newly formed
Chlide
a
(E682 F697) and picks up another Pchlide
a
chromophore that may be
photoconverted to Chlide
a
via a similar cycle. The spectral shifts described by
Sironval et al. were confirmed by Gassman et al
.
(
1968
), and Bonner (
1969
). The
concept of a shuttling Pchlide
a
reductase photoenzyme was also compatible with
the reported conversion of nt-SW-Pchlide
a
(F633) to t-LW-Pchlide
a
(F650)
during the photoreduction process (Gassman
1973
).
A significant step in the understanding of Pchlide
a
photoreduction was achieved
with the realization that NADPH is the hydrogen donor for the reaction (Griffiths
1974
). This was followed by the proposal that the shuttling photoenzyme (POR,
now called PORA), NADPH, and Pchlide
a
formed a photoactive ternary Pchlide
a
-NADPH-enzyme complex with a red absorption maximum at 652 nm (Apel
et al.
1980
). Equally important was the purification of PORA from etiolated barley
(Apel
1981
). The purified enzyme consisted of one polypeptide (Mr 36000) with
two to three bound Pchlide
a
chromophores. It is synthesized in the cytoplasm as a
precursor protein of about 44 kDa. The transit sequence of about 8 kDa is
hydrolyzed when the enzyme is transported into the plastid (Apel
1981
). The size
of PORA reported by various authors depends on the plant species and varies from
33 to 38 kDa (Shulz and Senger
1993
). More recently, pigment-free PORA was
purified from barley etioplasts by solubilization with n-octyl-
-D-glucoside and
chromatography on DEAE-cellulose (Klement et al.
1999
). Using pigment and
protein analysis it was shown that barley etioplasts contained a one-to-one PORA
and Pchlide
a.
The enzyme was twice as active towards MV than toward DV
Pchlide
a
(Klement et al.
1999
).
It has also been demonstrated that during the greening of etiolated tissues a rapid
decline of PORA is observed. For example, after 6 h of continuous illumination,
Β
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