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J. Biol. Chem., Vol. 266, Issue 1, 161-167, 01, 1991

Purification and functional characterization of glutamate-1- semialdehyde aminotransferase from Chlamydomonas reinhardtii

D Jahn, MW Chen and D Soll
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511.

The formation of delta-aminolevulinic acid, the first committed precursor of chlorophyll biosynthesis, occurs in the chloroplast of plants and algae by the C5-pathway, a three-step, tRNA-dependent transformation of glutamate. Previously, we reported the purification and characterization of the first two enzymes of this pathway, glutamyl- tRNA synthetase and Glu-tRNA reductase from the green alga Chlamydomonas reinhardtii (Chen, M.-W., Jahn, D., Schon, A., O'Neill, G. P., and Soll, D. (1990) J. Biol. Chem. 265, 4054-4057 and Chen, M.- W., Jahn, D., O'Neill, G. P., and Soll, D. (1990) J. Biol. Chem. 265, 4058-4063). Here we present the purification of the third enzyme of the pathway, the glutamate-1-semialdehyde aminotransferase from C. reinhardtii. The enzyme was purified from the membrane fraction of a whole cell extract employing four different chromatographic separations. The apparent molecular mass of the protein was approximately 43,000 Da as analyzed by denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, by nondenaturing rate zonal sedimentation on glycerol gradients, and by gel filtration. By these criteria, the enzyme in its active form is a monomer of 43,000 Da. In the presence of pyridoxal 5'-phosphate, purified glutamate-1- semialdehyde aminotransferase converts synthetic glutamate 1- semialdehyde to delta-aminolevulinic acid. The enzyme is inhibited by gabaculine and aminooxyacetate, both typical inhibitors of aminotransferases. The purified glutamate-1-semialdehyde aminotransferase successfully reconstitutes the whole C5-pathway in vitro from glutamate in the presence of purified glutamyl-tRNA synthetase, glutamyl-tRNA reductase, Mg2+, ATP, NADPH, tRNA, and pyridoxal 5'-phosphate.
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