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. This is due to the two amino acid insert which 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. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. 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. Truncated N-terminus. Truncated N-terminus.