type	Journal Article
authors	Hoober, J. K.; Kahn, A.; Ash, D. E.; Gough, S.; Kannangara, C. G.
title	Biosynthesis of delta-aminolevulinate in greening barley leaves. IX. Structure of the substrate, mode of gabaculine inhibition, and the catalytic mechanism of glutamate 1-semialdehyde aminotransferase
journal	Carlsberg Res Commun
Activity	5.4.3.8
Family	5.4.3.8
ui	BdagblS
year	(1988)
volume	53
number	1
pages	11-25
keywords	Aminolevulinic Acid/*biosynthesis
abstract	Glutamic acid 1-semialdehyde hydrochloride was synthesized and purified. Its prior structural characterization was extended and confirmed by 1H NMR spectroscopy and chemical analyses. In aqueous solution at pH 1 to 2 glutamic acid 1-semialdehyde exists in a stable hydrated form, but at pH 8.0 it has a half-life of 3 to 4 min. Spontaneous degradation of the material at pH 8.0 generated some undefined condensation products, but coincidentally a significant amount isomerized to 5-aminolevulinate. At pH 6.8 to 7.0, glutamate 1-semialdehyde is sufficiently stable to permit routine and reproducible assay for glutamate 1-semialdehyde aminotransferase activity. Only about 20% of the enzyme extracted from chloroplasts was sensitive to inactivation by gabaculine with no pretreatment. However, when the enzyme was exposed to 5-aminolevulinate, levulinate or 4,5-dioxovalerate in the absence of glutamate 1-semialdehyde, it was completely inactivated by gabaculine; 4,6-dioxoheptanoate had no effect on the enzyme. These results lead to the hypothesis that the aminotransferase exists in the chloroplast in a complex with pyridoxamine phosphate, which must be converted to the pyridoxal form before it can form a stable adduct with gabaculine. We propose that the enzyme catalyzes the conversion of glutamate 1-semialdehyde to 5-aminolevulinate via 4,5-diaminovalerate.
last changed	2008/05/21 16:26