Proline mechanisms of stress survival

Abstract

Significance: The imino acid proline is utilized by different organisms to offset cellular imbalances caused by environmental stress. The wide use in nature of proline as a stress adaptor molecule indicates that proline has a fundamental biological role in stress response. Understanding the mechanisms by which proline enhances abiotic/biotic stress response will facilitate agricultural crop research and improve human health.

Recent advances: It is now recognized that proline metabolism propels cellular signaling processes that promote cellular apoptosis or survival. Studies have shown that proline metabolism influences signaling pathways by increasing reactive oxygen species (ROS) formation in the mitochondria via the electron transport chain. Enhanced ROS production due to proline metabolism has been implicated in the hypersensitive response in plants, lifespan extension in worms, and apoptosis, tumor suppression, and cell survival in animals.

Critical issues: The ability of proline to influence disparate cellular outcomes may be governed by ROS levels generated in the mitochondria. Defining the threshold at which proline metabolic enzyme expression switches from inducing survival pathways to cellular apoptosis would provide molecular insights into cellular redox regulation by proline. Are ROS the only mediators of proline metabolic signaling or are other factors involved?

Future directions: New evidence suggests that proline biosynthesis enzymes interact with redox proteins such as thioredoxin. An important future pursuit will be to identify other interacting partners of proline metabolic enzymes to uncover novel regulatory and signaling networks of cellular stress response.

FIG. 1.

Reactions of the proline metabolic pathway. Proline (Pro) is synthesized from glutamate (Glu) starting with the enzymes glutamate kinase (GK) and γ-glutamyl phosphate reductase (GPR), which in plants and animals are fused together in the bifunctional enzyme P5C synthetase (P5CS). The intermediate, γ-glutamate-semialdehyde (GSA), spontaneously cyclizes to Δ¹-pyrroline-5-carboxylate (P5C), which is then reduced to proline by P5C reductase (P5CR). Alternatively, GSA/P5C can be generated from ornithine and ornithine-δ-aminotransferase (OAT). Proline is oxidized back to glutamate by proline dehydrogenase (PRODH) and P5C dehydrogenase (P5CDH) in the mitochondrion. PRODH couples proline oxidation to the reduction of ubiquinone (CoQ) in the electron transport chain (ETC). In Gram-negative bacteria, PRODH and P5CDH domains are fused together in the PutA protein.

FIG. 2.

Potential functions of proline and proline metabolism in stress protection.

FIG. 3.

Potential reactive oxygen species (ROS) scavenging mechanisms of proline.

FIG. 4.

Proposed mechanisms by which proline metabolism mediates redox homeostasis and energy production via NADP⁺/NADPH. (1) NADP⁺ produced by proline biosynthesis may stimulate the pentose phosphate pathway (PPP), thereby supporting energy production and the biosynthesis of key molecules such as nucleotides. NADPH is utilized for reductive biosynthesis pathways and is critical for glutathione (GSH) and thioredoxin (Trx) antioxidant systems. (2) In plant chloroplasts, NADP⁺ produced from proline biosynthesis may replenish depleted NADP⁺ pools caused by inhibition of the Calvin cycle during stress. Maintaining adequate levels of NADP⁺ for electrons transfer to the ETC would help minimize ROS generation during stress. Dashed line indicates inhibition. GR, glutathione reductase; GPx, glutathione peroxidase; GSSG, oxidized glutathione; GSH, reduced glutathione; TrxR, thioredoxin reductase; Trx-(SH)₂, reduced thioredoxin; Trx-S₂, oxidized thioredoxin; NADP⁺, nicotinamide adenine dinucleotide phosphate; NADPH, nicotinamide adenine dinucleotide phosphate reduced form; Glucose-6-P, glucose-6-phosphate; G6PDH, glucose-6-phosphate dehydrogenase.

FIG. 5.

Formation of γ-glutamylcysteine from the proline biosynthesis pathway. Lack of NADPH dehydrogenase activity in GPR allows cysteine to react with γ-glutamyl phosphate and to generate γ-glutamylcysteine.

FIG. 6.

Proline metabolism and ROS formation. PRODH activity leads to ROS formation in mitochondria by coupling proline oxidation to reduction of the ETC. Increases in PRODH and P5CR activities along with down-regulation of P5CDH are predicted to increase proline-P5C cycling and ROS levels. ROS levels flucuate according to changes in proline metabolism and activate diverse signaling pathways, thereby enabling proline to influence different cellular processes.