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firmly bound. Substrate specificity was lax since in addition to Mg-Proto, zinc proto,
calcium Proto, Mg-mesoporphyrin and Mg-deuteroporphyrin also acted as substrates.
S-adenosyl homocysteine and S-adenosylethionine inhibited the reaction competi-
tively. The enzyme has also been detected in corn (
Zea mays
) chloroplasts (Radmer
and Bogorad
1967
). A 1600-fold purification of the
R. spheroides
enzyme was
achieved by affinity chromatography (Hinchigeri et al.
1984
). The purified enzyme
exhibited an equilibrium-ordered sequential Bi Bi mechanism with Mg-Proto as the
obligatory first substrate, and SAM as the second substrate. The nucleotide sequence
of the
R. capsulatus
enzyme has been reported (Bollivar and Bauer
1992
).
Originally, in
R. capsulatus
,SAMMTwasbelievedtobecodedforbythe
bchH
gene, while the
bchM
gene was believed to code for a polypeptide involved in the
formation of the cyclopentanone ring (ring E) of Pchlide
a
(Bauer et al.
1993
). Later on,
the
bchM
gene of
R. capsulatus
was expressed in
E. coli
and the gene product was
subsequently demonstrated by enzymatic analysis to catalyze methylation of Mg-proto
to form Mpe (Bollivar et al.
1994
). Activity required the substrates Mg-proto and
S-adenosyl-L-methionine. To our knowledge, no higher plant SAMMT gene has been
isolated. A query for SAMMT addressed to the various protein databases listed in the
Biology Workbench, yielded 3 unique records which are depicted on the VLPBP
proto MT
”. These sequences can be viewed and used for sequence similarity searches
or other manipulations using the Biology Workbench.
The metabolic function of Mpe as a precursor of Pchlide
a
was demonstrated by
conversion of exogenous [
14
C]-Mpe and unlabeled Mpe to [
14
C]-Pchlide
a
, and Pchlide
a
respectively, in organello (Mattheis and Rebeiz
1977
). Pchlide
a
is the immediate
precursor of Chlide
a.
In this undertaking, an in organello system capable of the
converting
14
C-ALA to
14
C- Pchlide
a
,
14
C-Pchlide ester
a
and
14
C-Chl
a
and
b
(Rebeiz and Castelfranco,
1971a
,
b
), and capable of the net conversion of exogenous
ALA to Mg-Protoporphyrins and Pchlide
a
(Rebeiz et al.
1975
) was used.
7.2.1 Biosynthetic Heterogeneity of the Mpe Pool
Whenmore powerful fluorescence spectroscopic techniques were used to reinvestigate
the chemical nature of the Mpe pool of plants it was found to be chemically heteroge-
neous and to consist of DV and monovinyl (MV) components (Belanger and Rebeiz
1982
). Substrate amounts of MV Mpe are now routinely prepared by incubation of
etiolated barley leaves with ALA and Dpy (Rebeiz
2002
).
The proportion of DV to MV Mpe biosynthesis depended upon the greening
group affiliation of plants, the plants species, and pretreatment of plant tissues. For
example cucumber cotyledons a DDV-LDV-LDDV plant tissue(Abd-El-Mageed
et al.
1997
), pretreated with 2,2
0
-dipyridyl (Dpy) accumulate more DV than MV
Mpe in darkness. On the other hands, more MV Mpe was formed in DMV-LDV-
LDMV plants such as etiolated corn or barley.
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