Biology Reference
In-Depth Information
Fig. 11.6 (a) DV and (b) MV Pchlide
a
esters
esterified DV biosynthetic route (Fig.
11.2
route 17) and one fully esterified MV
biosynthetic route (Fig.
11.2
, route 16) are considered to contribute to the formation
of Pchlide
a
E. It should be emphasized however, that this hypothesis is based on
detection of putative intermediates and structural similarities, but has not been
confirmed by demonstration of precursor-product relationships between the Mpde
and Pchlide
a
E components of the two putative routes.
11.1.2.2 Metabolism of Pchlide a E
Biosynthetically and functionally, the Pchlide
a
ester pool is the least understood
pool of the Chl biosynthetic pathway. The full extent of its biological function is still
unclear and is surrounded by controversy. At one time, on the basis of its structural
similarity to MV Chl
a
, it was assumed to be the immediate major photoprecursor of
MV Chl
a
(Granick
1950
; Koski
1950
; Smith
1948
). This hypothesis lost its appeal
when (Wolff and Price
1957
) demonstrated that the major immediate precursors of
MV Chl
a
were MV Pchlide
a
and MV Chlide
a
. Thus for a while Pchlide
a
ester
floated as a tetrapyrrole pool without any defined function. However by 1973,
several laboratories had reported that Pchlide
a
ester was probably partially photo-
convertible to Chl
a
[for a review of this early work, see Rebeiz and Castelfranco
(
1973
)]. Some post-1973 work about the photoconvertibility of Pchlide
a
ester to
Chl
a
is described below.
Also, the contribution of Pchlide a E to the greening process is suggested by the
pattern of Pchlide
a
E formation under natural photoperiodic greening conditions.
Since Pchlide
a
E was observed to accumulate noticeably during the first four dark
cycles of the photoperiod, it was suggested by Cohen
et al.
that it may well
contribute to Chl
a
biosynthesis and accumulation at the onset of light (i.e. at
dawn) during the first few days of photoperiodic greening (Cohen et al.
1977
).
Although the level of Pchlide
a
E dropped after the fourth dark cycle, it was always
Search WWH ::
Custom Search