Please, carefully, pass the P5C

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Zheng Y, Cabassa-Hourton C, Eubel H, Chevreux G, Lignieres L, Crilat E, Braun H-P, Lebreton S, Savouré A. 2024. Pyrroline-5-carboxylate metabolism protein complex detected in Arabidopsis thaliana leaf mitochondria. Journal of Experimental Botany 75, 917–934.

P5C as an Interface of Proline Interconvertible Amino Acids and Its Role in Regulation of Cell Survival and Apoptosis

Studies of cancer metabolism have focused on the production of energy and the interconversion of carbons between cell cycles. More recently, amino acid metabolism, especially non-essential amino acids (NEAAs), has been investigated, underlining their regulatory role. One of the important mediators in energy production and interconversion of carbons in the cell is Δ¹-pyrroline-5-carboxylate (P5C)—the physiological intracellular intermediate of the interconversion of proline, ornithine, and glutamate. As a central component of these conversions, it links the tricarboxylic acid cycle (TCA), urea cycle (UC), and proline cycle (PC). P5C has a cyclic structure containing a tertiary nitrogen atom (N) and is in tautomeric equilibrium with the open-chain form of L-glutamate-γ-semialdehyde (GSAL). P5C is produced by P5C synthase (P5CS) from glutamate, and ornithine via ornithine δ-amino acid transferase (δOAT). It can also be converted to glutamate by P5C dehydrogenase (P5CDH). P5C is both a direct precursor of proline and a product of its degradation. The conversion of P5C to proline is catalyzed by P5C reductase (PYCR), while proline to P5C by proline dehydrogenase/oxidase (PRODH/POX). P5C-proline-P5C interconversion forms a functional redox couple. Their transformations are accompanied by the transfer of a reducing-oxidizing potential, that affect the NADP+/NADPH ratio and a wide variety of processes, e.g., the synthesis of phosphoribosyl pyrophosphate (PRPP), and purine ribonucleotides, which are crucial for DNA synthesis. This review focuses on the metabolism of P5C in the cell as an interconversion mediator of proline, glutamate, and ornithine and its role in the regulation of survival and death with particular emphasis on the metabolic context.

Incomplete proline catabolism drives premature sperm aging

Infertility is an increasingly common health issue, with rising prevalence in advanced parental age. Environmental stress has established negative effects on reproductive health, however, the impact of altering cellular metabolism and its endogenous reactive oxygen species (ROS) on fertility remains unclear. Here, we demonstrate the loss of proline dehydrogenase, the first committed step in proline catabolism, is relatively benign. In contrast, disruption of alh-6, which facilitates the second step of proline catabolism by converting 1-pyrroline-5-carboxylate (P5C) to glutamate, results in premature reproductive senescence, specifically in males. The premature reproductive senescence in alh-6 mutant males is caused by aberrant ROS homeostasis, which can be countered by genetically limiting the first committed step of proline catabolism that functions upstream of ALH-6 or by pharmacological treatment with antioxidants. Taken together, our work uncovers proline metabolism as a critical component of normal sperm function that can alter the rate of aging in the male reproductive system.

Reactive oxygen species homeostasis and virulence of the fungal pathogen Cryptococcus neoformans requires an intact proline catabolism pathway

Degradation of the multifunctional amino acid proline is associated with mitochondrial oxidative respiration. The two-step oxidation of proline is catalyzed by proline oxidase and Δ(1)-pyrroline-5-carboxylate (P5C) dehydrogenase, which produce P5C and glutamate, respectively. In animal and plant cells, impairment of P5C dehydrogenase activity results in P5C-proline cycling when exogenous proline is supplied via the actions of proline oxidase and P5C reductase (the enzyme that converts P5C to proline). This proline is oxidized by the proline oxidase-FAD complex that delivers electrons to the electron transport chain and to O2, leading to mitochondrial reactive oxygen species (ROS) overproduction. Coupled activity of proline oxidase and P5C dehydrogenase is therefore important for maintaining ROS homeostasis. In the genome of the fungal pathogen Cryptococcus neoformans, there are two paralogs (PUT1 and PUT5) that encode proline oxidases and a single ortholog (PUT2) that encodes P5C dehydrogenase. Transcription of all three catabolic genes is inducible by the presence of proline. However, through the creation of deletion mutants, only Put5 and Put2 were found to be required for proline utilization. The put2Δ mutant also generates excessive mitochondrial superoxide when exposed to proline. Intracellular accumulation of ROS is a critical feature of cell death; consistent with this fact, the put2Δ mutant exhibits a slight, general growth defect. Furthermore, Put2 is required for optimal production of the major cryptococcal virulence factors. During murine infection, the put2Δ mutant was discovered to be avirulent; this is the first report highlighting the importance of P5C dehydrogenase in enabling pathogenesis of a microorganism.