Bifunctional enzyme that converts glutamate to glutamate 5-semialdehyde, an intermediate in the biosynthesis of proline, ornithine and arginine. ATP + L-glutamate = ADP + L-glutamyl 5-phosphate L-glutamate 5-semialdehyde + NADP(+) + phosphate = H(+) + L-glutamyl 5-phosphate + NADPH Isoform Short: Inhibited by L-ornithine with a Ki of approximately 0.25 mm. Isoform Long: Insensitive to ornithine inhibition. Thus, the two amino acid insert in the long isoform abolishes feedback inhibition of P5CS activity by L-ornithine. Amino-acid biosynthesis; L-proline biosynthesis; L-glutamate 5-semialdehyde from L-glutamate: step 1/2. Amino-acid biosynthesis; L-proline biosynthesis; L-glutamate 5-semialdehyde from L-glutamate: step 2/2. Homohexamer or homotetramer. In the N-terminal section; belongs to the glutamate 5-kinase family. In the C-terminal section; belongs to the gamma-glutamyl phosphate reductase family.