Alternative Pathways for the Catabolism of L-Proline and its Analogues in Bacteria

Abstract

The amino acid L-proline can be incorporated into proteins and peptides but it can also be degraded as a carbon, nitrogen, and energy source.  In aerobic bacteria, L-proline catabolism most often involves the membrane-associated FAD-containing enzyme L-proline dehydrogenase which donates electrons directly to the electron transport chain.  In anaerobic bacteria, L-proline catabolism most often involves the Stickland fermentation reactions in which it is first converted to D-proline by proline racemase and then reduced to 5-aminovaleric acid by the selenium-containing enzyme D-proline reductase.  This review article summarizes the alternative pathways found in various bacteria which also allow the catabolism of L-proline.  These include nonspecific L-amnio acid oxidases and deaminases in some aerobic bacteria, L-proline dehydrogenases in anaerobic iron-reducing bacteria, reversible L-Δ1-pyrroline-5-carboxylate (P5C) reductases in anaerobic lactic acid bacteria, and D-amino acid dehydrogenases in various aerobic and anaerobic bacteria. The article also discusses the catabolism of the L-proline analogues L-thiaproline, L-azetidine-2-carboxylate, and 3,4-dehydro-DL-proline.  Because the information currently available about many of these processes is quite limited, amino acid sequence analysis was used in some cases to identify possible proteins and to suggest directions for further research. The study of L-proline catabolism illustrates the metabolic diversity of prokaryotic microorganisms and emphasizes the importance of studying more than the core biochemical pathways.