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Abstract
The formation of delta-aminolevulinic acid, the first committed precursor in porphyrin biosynthesis, occurs in certain bacteria and in the chloroplasts of plants and algae in a three-step, tRNA-dependent transformation of glutamate. Glutamyl-tRNA reductase, the second enzyme of this pathway, reduces the activated carboxyl group of glutamyl-tRNA (Glu-tRNA) in the presence of NADPH and releases glutamate 1-semialdehyde (GSA). We have purified Glu-tRNA reductase from Chlamydomonas reinhardtii by employing six different chromatographic separations. The apparent molecular mass of the protein when analyzed under both denaturing (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and nondenaturing conditions (rate zonal sedimentation on glycerol gradients) was 130,000 Da; this indicates that the active enzyme is a monomer. In the presence of NADPH Glu-tRNA reductase catalyzed the reduction to GSA of glutamate acylated to the homologous tRNA. Thus, the reductase alone is sufficient for conversion of Glu-tRNA to GSA. In the absence of NADPH, a stable complex of Glu-tRNA reductase with Glu-tRNA can be isolated.
Highlights
The formation of b-aminolevulinic acid, the first committed precursor in porphyrin biosynthesis, occurs in certain bacteria and in the chloroplasts of plants and algae in a three-step, tRNA-dependent transformation of glutamate
The apparent molecular mass of the protein when analyzed under both denaturing and nondenaturing conditions
In the presence of NADPH Glu-tRNA reductase catalyzed the reduction to glutamate 1-semialdehyde (GSA) of glutamate acylated to the homologous tRNA
Summary
The formation of b-aminolevulinic acid, the first committed precursor in porphyrin biosynthesis, occurs in certain bacteria and in the chloroplasts of plants and algae in a three-step, tRNA-dependent transformation of glutamate. In the presence of NADPH Glu-tRNA reductase catalyzed the reduction to GSA of glutamate acylated to the homologous tRNA. The activated glutamate is reduced by the NADPH-dependent Glu-tRNA reductase to form GSA.
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