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Fig. 7.5 MV Mg-Proto
component of the Mg-Proto pool of higher plants was determined by chemical
derivatization coupled to analytical fluorescence spectroscopy at 77 K (Belanger
and Rebeiz
1982
).
Biosynthesis of MV Mg-Proto in DDV-LDV-LDDV Plant Species
In DDV-LDV-LDDV plant species, MVMg-Proto is formed from DV Mg-Proto by
reduction of the vinyl group to ethyl at position 4 (ring B) of the macrocycle in one
thylakoid environment (Fig.
7.6
) (Kim and Rebeiz
1996
; Kolossov and Rebeiz
2010
). By similarity with DDV-LDV-LDMV plants such as barley (see below), The
reaction is probably catalyzed by a [4-vinyl] Mg-Proto reductase (4VMPR) (Kim
and Rebeiz
1996
).
The presence of this enzyme in DDV-LDV-LDDV plant species such as cucumber
is strongly suggested by the biosynthesis and accumulation of MV Mg-proto during
incubation of etiolated cucumber cotyledons with DV Proto in isolated cucumber
etiochloroplasts (Tripathy and Rebeiz
1986
) and by the metabolism of MVMg-Proto
in organello in DDV-LDV-LDDV Plant species (see below).
Biosynthesis of MV Mg-Proto in DMV-LDV-LDMV Plant Species
In DMV-LDV-LDMV plant species, MV Mg-Proto is formed from DV Mg-Proto
by reduction of the vinyl group to ethyl at position 4 (ring B) of the macrocycle in
one thylakoid locations (Fig.
7.7
), (Kim and Rebeiz
1996
; Kolossov and Rebeiz
2010
). The reaction is catalyzed by a [4-vinyl] Mg-Proto reductase (4VMPR). This
enzyme was detected in isolated barley etiochloroplasts, a DMV-LDV-LDMV, and
appears to be bound to the plastid membranes. A positive response of 4VMPR to
added NADPH has been observed (Kim and Rebeiz
1996
). It is very probable that
4VMPR is distinct from [4-vinyl] Pchlide
a
reductase (4PideR), which converts DV
Pchlide
a
to MV Pchlide
a
(Tripathy and Rebeiz
1988
); from [4-vinyl] Chlide
a
reductase (4VCR), which converts DV Chlide
a
to MV Chlide
a
(Kolossov and
Rebeiz
2001
; Pardo et al.
1980
; Parham and Rebeiz
1992
,
1995
), and from [4-vinyl]
Chl
a
reductase (4VChlR) (Adra and Rebeiz
1998
; Wang et al.
2010
). For example,
Rhodobacter capsulatus
in which the
bchJ
gene which codes for DV Pchlide
a
reductase, has been deleted, accumulates massive amounts of MV Mg-Proto
and its monoester (precursors of Pchlide
a
) in addition to the accumulation of DV
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