The biosynthesis of L-proline in E. coli involves L-glutamic acid being phosphorylated through an ATP driven glutamate 5-kinase resulting in a L-glutamic acid 5-phosphate. This compound is then reduced through a NADPH driven gamma glutamyl phosphate reductase resulting in the release of a phosphate, a NADP and a L-glutamic gamma-semialdehyde. L-glutamic gamma-semialdehyde is dehydrated spontaneously, resulting in a release of water,hydrogen ion and 1-Pyrroline-5-carboxylic acid. The latter compound is reduced by an NADPH driven pyrroline-5-carboxylate reductase which is subsequently reduced to L-proline. L-proline works as a repressor of the pyrroline-5-carboxylate reductase enzyme and glutamate 5-kinase. In E. coli, the biosynthesis of L-proline from L-glutamate is governed by three genetic loci namely proB, proA and proC. The first reaction in the pathway is catalyzed by γ-glutamyl kinase, encoded by proB . The second reaction, NADPH-dependent reduction of γ-glutamyl phosphate to glutamate-5-semialdehyde, in the pathway is catalyzed by glutamate-5-semialdehyde dehydrogenase, encoded by proA . These two enzymes aggregate into a multimeric bi-functional enzyme complex known as γ-glutamyl kinase-GP-reductase multienzyme complex. It is believed that the complex formation serves to protect the highly labile glutamyl phosphate from the hostile nucleophilic and aqueous environment found in the cell . The final step in the pathway, the reduction of pyrroline 5-carboxylate to L-proline, is catalyzed by an NADPH-dependent pyrroline-5-carboxylate reductase encoded by proC . Proline is metabolized by being converted back to L-glutamate, which is further degraded to α-ketoglutarate, an intermediate of the TCA cycle. The process by which proline is turned into L-glutamate starts with L-proline interacting with ubiquinone through a bifunctional protein putA resulting in an ubiquinol, a hydrogen ion and a 1-pyrroline-5-carboxylic acid. The latter compound is then hydrated spontaneously resulting in a L-glutamic gamma-semialdehyde. This compound is then processed by interacting with water through an NAD driven bifunctional protein putA resulting in a hydrogen ion, NADH and L-glutamic acid.

Proline Metabolism References