Reactions Between b-Aminolevulinic Acid and Protoporphyrin IX - Chlorophyll Biosynthesis and Technological Applications - page 170

Fig. 5.5 (a) Glutamate, (b) glutamate semialdehyde and (c) Hydroxyaminotetrahydropyranone
(HAT) molecules
5.1.2.1 ALA Synthetase
A query for ALA synthetase in Animal cells and lower Plants addressed to the
SwissProt and PIR protein databases via the Biology Workbench, yielded 16 distinct
sequences which are depicted on the LPBP website at http://www.vlpbp.org/greening/
xv/ , Sequenced Enzymes/ALA Synthetase, as well as in Appendix I .
5.1.3 Biosynthetic Heterogeneity of ALA in Higher Plants
In higher plants ALA is formed from glutamic acid (Beale and Castelfranco 1974 )via
three reactions (Kannangara et al. 1984 ). In a first reaction, glutamate- tRNA Glu
ligase catalyzes the ligation of glutamate to tRNA in the presence of ATP and Mg ++ .
In a second reaction, the glutamyl-tRNA complex is converted into a linear glutamate
semialdehyde (GSA) by NADPH:Glu-tRNA(oxido)reductase (also called glutamyl-
tRNA dehydrogenase) (Kannangara et al. 1984 ) or into a cyclic GSA (hydoroxya-
minotetrahydropyranone, HAT for short) (Jordan et al. 1989 ). Finally, GSA amino-
transferase converts GSA to ALA in the presence of vitamin B 6 and pyridoxal
phosphate. These reactions take place in the stroma of the plastid (Fig. 5.5 ).
The understanding of ALA biosynthetic heterogeneity in higher plants is still at a
primitive stage although reports are surfacing in support of that notion. For example
a reported differential inhibition of ALA formation by gabaculine in black pine
( Pinus nigra L.) during seed germination (Drazic and Bogdanovic 2000 ) strongly
suggests that in black pine, ALA is formed at least via two different routes, one of
which is inhibited by gabaculine. Also Averina and coworkers have proposed the
existence of at least two types of Chl biosynthesis centers which differ in their ability
to form ALA (Averina et al. 1993 ; Averina 1998 ). Recently, multiple resonance
excitation energy transfer sites from Proto to various Chl-protein complexes have
been detected in higher green plants (Table 5.1 ), which led to the extension of the
Proto biosynthetic heterogeneity all the way to ALA formation. Therefore, ALA
biosynthesis is proposed to take place in five different thylakoid environments, via
various routes. It is uncertain at this stage whether or not similar reactions in the
biosynthetic routes between ALA and Proto are catalyzed by identical enzymes or
not. In other words it is still uncertain whether the spatial heterogeneity of ALA
formation is accompanied by chemical biosynthetic heterogeneity or not.