The glyoxylate shunt bypasses the oxidative decarboxylation steps of the tricarboxylic acid (TCA) cycle, thereby conserving carbon skeletons for gluconeogenesis and biomass production. In , carbon flux is redirected through the first enzyme of the glyoxylate shunt, isocitrate lyase (ICL), following phosphorylation and inactivation of the TCA cycle enzyme, isocitrate dehydrogenase (ICD), by the kinase/phosphatase, AceK. In contrast, mycobacterial species lack AceK and employ a phosphorylation-insensitive isocitrate dehydrogenase (IDH), which is allosterically activated by the product of ICL activity, glyoxylate. However, expresses IDH, ICD, ICL, and AceK, raising the question of how these enzymes are regulated to ensure proper flux distribution between the competing pathways. Here, we present the structure, kinetics, and regulation of ICL, IDH, and ICD from We found that flux partitioning is coordinated through reciprocal regulation of these enzymes, linking distribution of carbon flux to the availability of the key gluconeogenic precursors, oxaloacetate and pyruvate. Specifically, a greater abundance of these metabolites activated IDH and inhibited ICL, leading to increased TCA cycle flux. Regulation was also exerted through AceK-dependent phosphorylation of ICD; high levels of acetyl-CoA (which would be expected to accumulate when oxaloacetate is limiting) stimulated the kinase activity of AceK, whereas high levels of oxaloacetate stimulated its phosphatase activity. In summary, the TCA cycle-glyoxylate shunt branch point in has a complex enzymology that is profoundly different from those in other species characterized to date. Presumably, this reflects its predilection for consuming fatty acids, especially during infection scenarios.
Over the last two decades, tens of millions of dollars have been invested in understanding virulence in the human pathogen, Pseudomonas aeruginosa. However, the top 'hits' obtained in a recent TnSeq analysis aimed at identifying those genes that are conditionally essential for infection did not include most of the known virulence factors identified in these earlier studies. Instead, it seems that P. aeruginosa faces metabolic challenges in vivo, and unless it can overcome these, it fails to thrive and is cleared from the host. In this review, we look at the kinds of metabolic pathways that the pathogen seems to find essential, and comment on how this knowledge might be therapeutically exploited.
La problématique de l'effondrement des populations d'abeilles dans le monde ne se pose pas en des termes similaires dans les zones tempérées et dans les zones semiarides et arides. Dans ces dernières, l'effondrement est chronique et dû à des régimes de précipitations annuelles très variables qui imposent des fluctuations du niveau des populations d'abeilles. Dans ces conditions, la continuité du rapport homme-abeille ne repose pas tant sur la stabilité de la population ou du stock de pollen et nectar disponible que sur la permanence de la relation, tant pratique que symbolique 1. Une telle distinction de contextes apicoles invite à préciser le régime de relation qui permet sa permanence en dehors de toute stabilité démographique d'un des acteurs de la relation, l'abeille. La question de la domestication des abeilles ne cesse de faire débat depuis des décennies, chez les anthropologues, historiens et archéozoologues notamment, pour faire la part entre l'imaginaire du sens commun, suscité de manière universelle par la « sociabilité » remarquable de ces populations d'insectes, et l'impact des pratiques concrètes des apiculteurs sur ces dernières. Car une question dérange sans cesse du point de vue de la formulation : comment parler de domestication de l'abeille alors que celle-ci vit déjà en société et que son passage à « l'état » domestique ne change en rien son organisation sociale ? Même si la langue française retient la connotation végétale du verbe « cultiver » pour qualifier le champ d'activité lié à l'exploitation des abeilles, ou celui d'« élever » pour désigner l'activité de sélection des reines, certains auteurs imputent à juste titre à ces appellations des points de vue culturels qui ne témoignent pas de la réalité du rapport technique au monde des abeilles, mais des représentations que l'on s'en fait (Marchenay, 1993 ; Tétart 2001).
The glyoxylate shunt bypasses the oxidative decarboxylation steps of the tricarboxylic acid (TCA) cycle, thereby conserving carbon skeletons for biosynthesis. The branchpoint between the TCA cycle and the glyoxylate shunt is therefore widely considered to be one of the most important junctions in the whole of microbial metabolism. In Escherichia coli, AceK-mediated phosphorylation and inactivation of the TCA cycle enzyme, isocitrate dehydrogenase (ICD), is necessary to redirect flux through the first enzyme of the glyoxylate shunt, isocitrate lyase (ICL). In contrast, Mycobacterial species lack AceK and employ a phosphorylation-insensitive isocitrate dehydrogenase (IDH) at the branchpoint. Flux partitioning here is controlled "rheostatically" through cross-activation of IDH by the product of ICL activity, glyoxylate. However, the opportunistic human pathogen, Pseudomonas aeruginosa, expresses IDH, ICD, ICL and AceK. Here, we present the structure, kinetics and regulation of each branchpoint enzyme. We show that flux partitioning is coordinated through reciprocal regulation of the enzymes involved, beautifully linking carbon flux with the availability of key gluconeogenic precursors in a way that cannot be extrapolated from an understanding of the branchpoint enzymes in other organisms.The glyoxylate shunt is an anaplerotic pathway which bypasses the oxidative decarboxylation steps of the TCA cycle, thereby conserving carbon skeletons for gluconeogenesis and biomass production 1 . The mechanisms controlling carbon flux partitioning between the TCA cycle and glyoxylate shunt were largely worked out in the Escherichia coli model in the 1980s. Here, the TCA cycle enzyme, isocitrate dehydrogenase (ICD, encoded by icd), and the glyoxylate shunt enzyme, isocitrate lyase (ICL, encoded by aceA) compete for available isocitrate. ICD has a much lower KM for isocitrate (KM 8 µM 2 ) than ICL (KM 604 µM 3 ), so in order to get significant flux through the glyoxylate shunt, ICD needs to be inactivated. This is accomplished by reversible phosphorylation of ICD on Ser 113, mediated by a dual function kinase/phosphatase, AceK 4 . Phosphoserine 113 is thought to electrostatically repulse isocitrate, thereby preventing substrate binding 5 . Consequently, when the kinase activity of AceK is dominant, ICD becomes phosphorylated and inactivated, allowing carbon flux to be redirected through the glyoxylate shunt. In contrast, when the phosphatase activity of AceK dominates, flux is restored through the TCA cycle. The ratio of kinase:phosphatase activity in AceK is controlled by allosteric regulators 6 .Not all bacteria share the same enzymology as E. coli at the TCA cycle-glyoxylate shunt branchpoint ("TGB"); some bacteria encode a second, AceK-insensitive isocitrate dehydrogenase isozyme, IDH. Genome sequencing efforts have revealed that some species, such as the industrially-important fermenter Corynebacterium glutamicum, contain only idh, whereas others contain only icd (e.g., E. coli). Generally, and as might be expected, there is a...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.