Biology Reference
In-Depth Information
Fig. 5.6 (a) Schiff base-
enzyme intermediate and
(b) PBG
Fig. 5.7 Conversion of ALA to PBG
In
E. coli
ALA Dehydratase contains two metal binding sites that have been
designated
-site binds preferentially a Zn
2
α
and
β
(Spencer and Jordan
1994
). The
α
+
ion that is essential for catalytic activity. The
-site is exclusively a transition-
metal-ion-binding site thought to be involved in protein conformation.
In animal cells PBG is formed from ALA in the cytoplasm (Rebeiz et al.
1996
).
In plants, PBG synthase is loosely bound to the plastid membranes (Lee et al.
1991
).
Beyond the possibility that in higher plants PBG may contribute to the formation of
Proto in five different environments (Table
5.1
), no specific efforts have been made
to document the nature and extent of PBG biosynthetic heterogeneity in plants.
β
5.2.1 ALA Dehydratase
A query for ALA dehydratase addressed to various protein databases listed in the
Biology Workbench, yielded 30 unique records which are depicted under which are
depicted on the LPBP website at
http://www.vlpbp.org/greening/xv/
,
Sequenced
Enzymes/ ALA Dehydratase, as well as in Appendix
I
.
5.3 Biosynthesis of Uroporphyrinogen III (Urogen III)
Urogen III is the universal precursor of all metabolic tetrapyrroles (Neve and Labbe
1956
). It is the branching point where the biosynthesis of vitamin B
12
diverges from
that of heme and Chl. Its biosynthesis from PBG requires the cooperation of two
enzymes, PBG deaminase (Bogorad
1958a
) and Urogen III synthase also known as
cosynthetase (Bogorad
1958b
). In
E. coli
PBG deaminase is coded for by the gene
hemC
(Thomas and Jordan
1986
). The apoprotein consists of 353 amino acids with
a molecular weight of 34245. The active site contains two constitutive PBG
molecules (dipyrromethane cofactor) attached to the apoprotein by a cysteine
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