Enzymic and mechanistic studies on the conversion of glutamate to 5-aminolaevulinate

Higher plants, algae, cyanobacteria and several other photosynthetic and non-photosynthetic bacteria synthesize 5-aminolaevulinate by a tRNA(Glu)-mediated pathway. Glutamate is activated at the alpha-carboxyl by ligation to tRNA(Glu) with an aminoacyl-tRNA synthetase. An NADPH-dependent reductase converts glutamyl-tRNA(Glu) to glutamate 1-semialdehyde, which is finally converted to 5-aminolaevulinate by an aminotransferase. These components are soluble and in plants and algae are located in the chloroplast stroma. In plants and algae the tRNA(Glu) is encoded in chloroplast DNA whereas the enzymes are encoded in nuclear DNA. The tRNA(Glu) has a hypermodified 5-methylaminomethyl-2-thiouridine-pseudouridine-C anticodon and probably plays a role in the light-dark regulation of 5-aminolaevulinate synthesis. Ligation of glutamate to tRNA(Glu) requires ATP and Mg2+ and proceeds via a ternary intermediate. Glutamyl-tRNA(Glu) reduction appears to involve formation of a complex. Glutamate 1-semialdehyde non-enzymically synthesized by reductive ozonolysis from gamma-vinyl GABA is used as substrate by the last enzyme. Glutamate-1-semialdehyde aminotransferase contains pyridoxal phosphate as a prosthetic group. The enzyme is converted to spectrally different forms by treatment with 4,5-diaminovalerate or 4,5-dioxovalerate. The pyridoxamine 5'-phosphate form of the enzyme converts (S)-glutamate 1-semialdehyde to 5-aminolaevulinate via 4,5-diaminovalerate through a bi-bi ping-pong mechanism.