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theoretical
14
C-incorporation into compound B by the end of time
interval X, as calculated from Eq.
13.1
,
|Exp
QB
X
¼
absolute difference between the experimental and theoretical
14
C-incorporation of precursor P into compound B during time interval X.
QB
X
|
¼
13.1.2 Source of Oxygen During the Formation
of the Formyl Group of Chl b
Mass spectra of [7-hydroxymethyl]-chlorophyll
b
extracted from leaves greened in
the presence of either
18
O
2
or H
2
18
O
2
revealed that
18
O was incorporated only from
molecular oxygen into the 7-formyl group of Chl
b
(Porra et al.
1993
,
1994
). The
high enrichment using
18
O
2
, and the absence of labeling by H
2
18
O
2
, demonstrated
that molecular oxygen is the sole precursor of the 7-formyl oxygen of chlorophyll
b
in greening maize leaves. This in turn suggested that a mono-oxygenase is
involved in the oxidation of the methyl group to a formyl.
13.2 The Chl b Biosynthetic Pathway
As was mentioned in the previous chapter, the demonstration of metabolic
pathways is a multistep process that involves at least three stages: (a) the detection
and characterization of metabolic intermediates, (b) the demonstration of precursor-
product relationships between putative intermediates, and (c) purification and
characterization of enzymes involved in the metabolic interconversions. In this
chapter I will mainly emphasize the demonstration of precursor-product relation-
ships between various intermediates.
13.2.1 Chlorophyllide b (Chlide b)
Chlorophyllides
b
(Chlides
b
) are the immediate precursors of the
b
chlorophylls.
Chlides
b
are chemically and biochemically heterogeneous. Chemical heterogene-
ity consists in MV and DV substitutions at position 4 of the macrocycle, while
biochemical heterogeneity consists of multiple biosynthetic routes involving MV
and DV Chlides
a
and MV Pchlide
b
precursors (Fig.
13.2
).
13.2.1.1 Monovinyl Chlorophyllides b
The biosynthetic heterogeneity of Chlides
b
is manifested by their biosynthesis via
multiple biosynthetic routes in DV and MV plant species as described below.
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