Reactions Between b-Aminolevulinic Acid and Protoporphyrin IX - Chlorophyll Biosynthesis and Technological Applications - page 173

Fig. 5.8 (a) Hydroxymethylbilane, (b) uroporphyrinogen I, (c) uroporphyrinogen III
Fig. 5.9 Conversion of PBG to uroporphyrinogen III
residue (Cys-242) (Hart et al. 1987 ; Jordan and Warren 1987 ). In a first step, one
PBG molecule binds to the deaminase. A covalent bond is formed between
the second constitutive PBG molecule and the first PBG substrate, and one mole-
cule of ammonia is released. This first condensation leads to the formation of ring
A of Urogen III. This step is repeated three more times and results in the formation
of an open chain tetrapyrrole which is displaced from the enzyme by water to yield
1-hydroxymethylbilane (HMBL) (also called preuroporphyrinogen) (Fig. 5.8 )
(Battersby et al. 1979 , 1982a , b , 1983 ; Jordan and Seehra 1979 ). Hydroxy-
methylbilane is unstable and in the absence of the cosynthetase cyclises at neutral
pH to yield Urogen I (Fig. 5.8 ). In the presence of the cosynthetase, hydroxy-
methylbilane is rapidly converted into Urogen III (Battersby et al. 1982b ). This
reaction involves inversion of ring D of HMBL and cyclization with the release of
one water molecule. In E. coli the cosynthetase is coded for by the gene hemD
(Jordan et al. 1988 ). The apoprotein consists of 246 amino acids with a molecular
weight of 27766. HemC and hemD occur in tandem and overlap by one base pair.
In animal cells, Urogen III is formed in the cytoplasm (Rebeiz et al. 1996 ). In plant
cells, PGB deaminase and the cosynthetase are loosely bound to the plastid
membranes (Lee et al. 1991 ) (Fig. 5.9 ).
Beyond the possibility that in higher plants Urogen III 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 Urogen III biosynthetic
heterogeneity in plants.