The creation of L-proline in E. coli starts with 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 an NADPH driven gamma glutamyl phosphate reductase resulting in the release of a phosphate, an 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 then reduced to L-proline. L-proline works as a repressor of the pyrroline-5-carboxylate reductase enzyme and glutamate 5-kinase. Three genetic loci, proA, proB and proC control the biosynthesis of L-proline in E. coli.The pathway begins with a reaction that is catalyzed by γ-glutamyl kinase, which is encoded by proB. Next, NADPH-dependent reduction of γ-glutamyl phosphate to glutamate-5-semialdehyde, occurs through catalyzation by glutamate-5-semialdehyde dehydrogenase, encoded by proA. Following this, both enzymes join together in a multimeric bi-functional enzyme complex called γ-glutamyl kinase-GP-reductase multienzyme complex. This formation is thought to protect the highly labile glutamyl phosphate from the antagonistic nucleophilic and aqueous environment found in the cell. Finally, NADPH-dependent pyrroline-5-carboxylate reductase encoded by proC catalyzes the reduction of pyrroline 5-carboxylate into L-proline. Proline is metabolized in E. coli by returning to the form of L-glutamate, which is then degraded to α-ketoglutarate,which serves as an intermediary of the TCA cycle. Interestingly enough, L-glutamate, the obligate intermediate of the proline degradation pathway, is not able to serve as an outright source of carbon and energy for E. coli, because the rate at which glutamate transport supplies exogenous glutamate is not adequate. 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