L-proline is synthesized de novo in the mitochondrion either from L-glutamate, which is the main pathway, or from L-ornithine, a product of L-arginine hydrolysis. Both pathways converge at the level of the intermediate metabolite pyrroline- 5-carboxylic acid (P5C). The last step of L-proline biosynthesis may occur in the cytosol. Besides its role in protein synthesis, L-proline is involved in maintaining osmotic and redox homeostasis and can also be used to synthesize L-glutamate. (Adapted from PMID:34291343.)