The Substrate Versatility of Δ¹-Pyrroline-5-carboxylate Reductase (ProC) from Escherichia coli

Δ¹-Pyrroline-5-carboxylate reductase (EC 1.5.1.2; called ProC in most bacteria) is an enzyme of central metabolism that catalyzes the last step of the proline biosynthetic pathways, namely the NADPH-dependent reduction of pyrroline-5-carboxylate (P5C) to L-proline (L-Pro). The enzyme, however, is also active towards other substrates, and these reactions might have physiological relevance. Herein, the substrate versatility of ProC from Escherichia coli was explored as follows. We initially characterized the reverse reaction carried out by ProC, i.e., the formation of P5C from L-Pro. This reaction was easily measurable at pH 10, allowing the determination of the kinetic parameters. Under the same conditions, we then tested the ability of ProC to oxidize a number of L-Pro analogs, confirming that ProC reacts most effectively with analogs containing a simple five-membered ring such as L-thioproline (THP) and 3,4-dehydro-L-proline (DHP). Larger substrates such as L-pipecolate (PIP) reacted with lower efficiency, and the four-membered ring analog, L-azetidine-2-carboxylate (A2C) showed no detectable reactivity and behaved as a weak inhibitor of the ProC reaction. To interpret these results, we built a structural model of ProC and employed this model for a docking analysis of L-Pro and of its analogs. This approach highlighted the presence of a peculiar "three-point interaction", in which the L-Pro carboxylate and amino groups form hydrogen bonds with conserved residues in the binding site, while the substrate ring stacks with the nicotinamide ring of NADP⁺. The L-Pro analogs tried to preserve as much as possible these critical interactions for a correct positioning and a favorable binding. The possibility of an inherent multifunctionality of ProC was further explored by examining the genomic context of the proC gene in a large number of bacterial species.