P5CS consists of two domains: gamma-glutamyl kinase and gamma-glutamyl phosphate reductase, each of which are used to complete the two steps to create ornithine and proline. The gamma-glutamyl kinase domain employs a 367-residue chain that folds into an N-terminal amino acid kinase domain, responsible for catalysis and proline inhibition, and a C-terminal PUA RNA binding domain. This enzyme is also a tetramer formed by two dimers, and the monomers and dimers are assembled in a manner that allows the amino acid kinase active pockets to be alternatively oriented in the tetramer. The short version (P5CS.short) and the long version (P5CS.long) of PC5S are two isoforms of this enzyme which differs by the addition of two amino acids in the long form and with an extra 6-bp insert following bp+711. This slight difference creates a dramatic difference in how they are affected by the inhibition of ornithine. The ALDH18A1 gene spans 15 kb, is mapped on 10q24.3, and has an exon count of 18.
P5CS catalyzes the phosphorylation- and reduction-conversion of glutamate to Delta-1-pyrroline-5-carboxylate (P5C). This occurs through a process in which glutamate is converted into gamma-glutamyl phosphate in the gamma-glutamyl kinase domain and then the gamma-glutamyl phosphate is the made into gamma-glutamic semi-aldehyde in the gamma-glutamyl phosphate reductase domain. The gamma-glutamic semi-aldehyde is in tautomericequilibrium with P5C and it is the obligatory intermediate in the interconversions of proline, ornithine, and glutamate. The two isoforms (PC5S.short and PC5S.long) are both involved in different activities as well. The short version has high activity in the gut and is a main participant in the biosynthesis of arginine. The long version of PC5S is expressed in various tissues and is significant for its ability to synthesize proline from glutamate. Also, the short version is inhibited by ornithine, whereas the long version is insensitive to the amino acid.
Ornithine and/or arginine are key intermediates for the synthesis of urea, creatine, nitric oxide, polyamines, and protein; while proline is a major component of the connective tissue
proteins, collagen and elastin. Because all three of these amino acids are a part of very significant processes, the presence of P5CS becomes an important regulator which makes sure that none of these three become deficient. Therefore, a lack of P5CS, due to mutations in the ALDH18A1 gene, often leads to neurodegeneration, joint laxity, skin hyperelasticity, bilateral sub capsular cataracts, and a plethora of other complications associated with impaired proline and ornithine synthesis.
^Liu G, Maunoury C, Kamoun P, Aral B (Oct 1996). "Assignment of the human gene encoding the delta 1-pyrroline-5-carboxylate synthetase (P5CS) to 10q24.3 by in situ hybridization". Genomics. 37 (1): 145–6. doi:10.1006/geno.1996.0535. PMID8921385.
^Pérez-Arellano I, Carmona-Álvarez F, Gallego J, Cervera J (Dec 2010). "Molecular mechanisms modulating glutamate kinase activity. Identification of the proline feedback inhibitor binding site". Journal of Molecular Biology. 404 (5): 890–901. doi:10.1016/j.jmb.2010.10.019. PMID20970428.
^Hu CA, Lin WW, Obie C, Valle D (Mar 1999). "Molecular enzymology of mammalian Delta1-pyrroline-5-carboxylate synthase. Alternative splice donor utilization generates isoforms with different sensitivity to ornithine inhibition". The Journal of Biological Chemistry. 274 (10): 6754–62. doi:10.1074/jbc.274.10.6754. PMID10037775.
^ abBaumgartner MR, Hu CA, Almashanu S, Steel G, Obie C, Aral B, Rabier D, Kamoun P, Saudubray JM, Valle D (Nov 2000). "Hyperammonemia with reduced ornithine, citrulline, arginine and proline: a new inborn error caused by a mutation in the gene encoding delta(1)-pyrroline-5-carboxylate synthase". Human Molecular Genetics. 9 (19): 2853–8. doi:10.1093/hmg/9.19.2853. PMID11092761.
^Martinelli D, Häberle J, Rubio V, Giunta C, Hausser I, Carrozzo R, Gougeard N, Marco-Marín C, Goffredo BM, Meschini MC, Bevivino E, Boenzi S, Colafati GS, Brancati F, Baumgartner MR, Dionisi-Vici C (Sep 2012). "Understanding pyrroline-5-carboxylate synthetase deficiency: clinical, molecular, functional, and expression studies, structure-based analysis, and novel therapy with arginine". Journal of Inherited Metabolic Disease. 35 (5): 761–76. doi:10.1007/s10545-011-9411-8. PMID22170564.
Maruyama K, Sugano S (Jan 1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–4. doi:10.1016/0378-1119(94)90802-8. PMID8125298.
Aral B, Schlenzig JS, Liu G, Kamoun P (Mar 1996). "Database cloning human delta 1-pyrroline-5-carboxylate synthetase (P5CS) cDNA: a bifunctional enzyme catalyzing the first 2 steps in proline biosynthesis". Comptes Rendus de l'Académie des Sciences, Série III. 319 (3): 171–8. PMID8761662.
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, Suyama A, Sugano S (Oct 1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene. 200 (1–2): 149–56. doi:10.1016/S0378-1119(97)00411-3. PMID9373149.
Hu CA, Lin WW, Obie C, Valle D (Mar 1999). "Molecular enzymology of mammalian Delta1-pyrroline-5-carboxylate synthase. Alternative splice donor utilization generates isoforms with different sensitivity to ornithine inhibition". The Journal of Biological Chemistry. 274 (10): 6754–62. doi:10.1074/jbc.274.10.6754. PMID10037775.
Baumgartner MR, Hu CA, Almashanu S, Steel G, Obie C, Aral B, Rabier D, Kamoun P, Saudubray JM, Valle D (Nov 2000). "Hyperammonemia with reduced ornithine, citrulline, arginine and proline: a new inborn error caused by a mutation in the gene encoding delta(1)-pyrroline-5-carboxylate synthase". Human Molecular Genetics. 9 (19): 2853–8. doi:10.1093/hmg/9.19.2853. PMID11092761.
Baumgartner MR, Rabier D, Nassogne MC, Dufier JL, Padovani JP, Kamoun P, Valle D, Saudubray JM (Jan 2005). "Delta1-pyrroline-5-carboxylate synthase deficiency: neurodegeneration, cataracts and connective tissue manifestations combined with hyperammonaemia and reduced ornithine, citrulline, arginine and proline". European Journal of Pediatrics. 164 (1): 31–6. doi:10.1007/s00431-004-1545-3. PMID15517380.