Identification and characterization of carboxylate transporters in Corynebacterium glutamicum

Transport processes play an important role in cellular fluxes due to the impermeability of the plasma membrane for most substrates. The understanding of carboxylate metabolism is of particular interest for fundamental research, approaching the goal of a complete understanding of metabolic pathways. Biotechnological applications benefit from this knowledge, since production processes can be improved by increasing the yield and reducing the production costs. At the beginning of this work, the transport processes contributing to carboxylate uptake and excretion were largely unknown. Here, several carboxylate transporters were identified and characterized and some putative transporters were suggested, thus promoting the global understanding of carboxylate metabolism in the biotechnologically important organism C. glutamicum. The pyruvate importer MctC was identified and proven to be a high-affinity acetate and propionate carrier. It was shown to be indispensable for pyruvate utilization and to contribute substantially to acetate and propionate uptake under natural conditions of low substrate availability. The activity of MctC was found to depend on the carbon source present in the medium and its transcription was regulated by the major regulators of acetate metabolism RamA and RamB. Furthermore, it was shown to be transcribed in an operon with a small membrane protein encoding gene cgl0832. The characterization of spontaneous mutants, which were able to grow on succinate, fumarate, or L-malate, led to the identification of the transporters DccT and DctA. DccT was proven to be a secondary active, Na+ dependent C4-dicarboxlyate importer. Its presence promoted aerobic growth on succinate, fumarate, L-malate, and oxaloacetate. DctA was also verified to confer dicarboxylate uptake ability. In contrast to DccT, DctA was shown to depend on the electrochemical proton potential and to accept many structurally different carboxylates as substrates, including the C6-tricarboxylate citrate, the C5-dicarboxylate 2-oxoglutarate, the C4-dicarboxlyates succinate, fumarate, L-malate, oxaloacetate, and aspartate, as well as the monocarboxylates glyoxylate and lactate, albeit its substrate affinity was found to be lower than that of DccT. Several different approaches were undertaken in order to identify carboxylate exporters. Although none of them was successful for the identification of a lactate exporter, a putative succinate exporter was suggested by the characterization of site-directed mutants during oxygen deprived conditions. The analysis of gene expression under pyruvate producing conditions as well as phenotypic analysis of site-directed mutants pointed to several putative pyruvate exporters

Membrane transporters in the bioproduction of organic acids: state of the art and future perspectives for industrial applications

Organic acids such as monocarboxylic acids, dicarboxylic acids or even more complex molecules such as sugar acids, have displayed great applicability in the industry as these compounds are used as platform chemicals for polymer, food, agricultural and pharmaceutical sectors. Chemical synthesis of these compounds from petroleum derivatives is currently their major source of production. However, increasing environmental concerns have prompted the production of organic acids by microorganisms. The current trend is the exploitation of industrial biowastes to sustain microbial cell growth and valorize biomass conversion into organic acids. One of the major bottlenecks for the efficient and cost-effective bioproduction is the export of organic acids through the microbial plasma membrane. Membrane transporter proteins are crucial elements for the optimization of substrate import and final product export. Several transporters have been expressed in organic acid-producing species, resulting in increased final product titers in the extracellular medium and higher productivity levels. In this review, the state of the art of plasma membrane transport of organic acids is presented, along with the implications for industrial biotechnology.This work was supported by the strategic programme UID/BIA/04050/2019 funded by Portuguese fundsthrough the FCT I.P., and the projects: PTDC/BIAMIC/5184/2014, funded by national funds through the Fundacao para a Ciencia e Tecnologia (FCT) I.P. and by the European Regional Development Fund (ERDF) through the COMPETE 2020-Programa Operacional Competitividade e Internacionalizacao (POCI), and EcoAgriFood: Innovative green products and processes to promote AgriFood BioEconomy (operacao NORTE-01-0145-FEDER-000009), supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). DR acknowledges FCT for the SFRH/BD/96166/2013 PhD grant. MSS acknowledges the Norte2020 for the UMINHO/BD/25/2016 PhD grant with the reference NORTE-08-5369-FSE-000060. TR acknowledges Yeastdoc European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 764927

Pyruvate: immunonutritional effects on neutrophil intracellular amino or alpha-keto acid profiles and reactive oxygen species production

For the first time the immunonutritional role of pyruvate on neutrophils (PMN), free α-keto and amino acid profiles, important reactive oxygen species (ROS) produced [superoxide anion (O2−), hydrogen peroxide (H2O2)] as well as released myeloperoxidase (MPO) acitivity has been investigated. Exogenous pyruvate significantly increased PMN pyruvate, α-ketoglutarate, asparagine, glutamine, aspartate, glutamate, arginine, citrulline, alanine, glycine and serine in a dose as well as duration of exposure dependent manner. Moreover, increases in O2− formation, H2O2-generation and MPO acitivity in parallel with intracellular pyruvate changes have also been detected. Regarding the interesting findings presented here we believe, that pyruvate fulfils considerably the criteria for a potent immunonutritional molecule in the regulation of the PMN dynamic α-keto and amino acid pools. Moreover it also plays an important role in parallel modulation of the granulocyte-dependent innate immune regulation. Although further research is necessary to clarify pyruvate’s sole therapeutical role in critically ill patients’ immunonutrition, the first scientific successes seem to be very promising

Identification and Characterization of the Dicarboxylate Uptake System DccT in Corynebacterium glutamicum▿

Many bacteria can utilize C4-carboxylates as carbon and energy sources. However, Corynebacterium glutamicum ATCC 13032 is not able to use tricarboxylic acid cycle intermediates such as succinate, fumarate, and l-malate as sole carbon sources. Upon prolonged incubation, spontaneous mutants which had gained the ability to grow on succinate, fumarate, and l-malate could be isolated. DNA microarray analysis showed higher mRNA levels of cg0277, which subsequently was named dccT, in the mutants than in the wild type, and transcriptional fusion analysis revealed that a point mutation in the promoter region of dccT was responsible for increased expression. The overexpression of dccT was sufficient to enable the C. glutamicum wild type to grow on succinate, fumarate, and l-malate as the sole carbon sources. Biochemical analyses revealed that DccT, which is a member of the divalent anion/Na+ symporter family, catalyzes the effective uptake of dicarboxylates like succinate, fumarate, l-malate, and likely also oxaloacetate in a sodium-dependent manner

Characterization of the Dicarboxylate Transporter DctA in Corynebacterium glutamicum▿

Transporters of the dicarboxylate amino acid-cation symporter family often mediate uptake of C4-dicarboxylates, such as succinate or l-malate, in bacteria. A member of this family, dicarboxylate transporter A (DctA) from Corynebacterium glutamicum, was characterized to catalyze uptake of the C4-dicarboxylates succinate, fumarate, and l-malate, which was inhibited by oxaloacetate, 2-oxoglutarate, and glyoxylate. DctA activity was not affected by sodium availability but was dependent on the electrochemical proton potential. Efficient growth of C. glutamicum in minimal medium with succinate, fumarate, or l-malate as the sole carbon source required high dctA expression levels due either to a promoter-up mutation identified in a spontaneous mutant or to ectopic overexpression. Mutant analysis indicated that DctA and DccT, a C4-dicarboxylate divalent anion/sodium symporter-type transporter, are the only transporters for succinate, fumarate, and l-malate in C. glutamicum

Identification and Characterization of a Bacterial Transport System for the Uptake of Pyruvate, Propionate, and Acetate in Corynebacterium glutamicum▿

The metabolism of monocarboxylic acids is of central importance for bacteria in their natural habitat as well as during biotechnological production. Although biosynthesis and degradation are well understood, the transport of such compounds is still a matter of discussion. Here we present the identification and characterization of a new transport system in Corynebacterium glutamicum with high affinity for acetate and propionate and with lower affinity for pyruvate. Biochemical analysis of this monocarboxylic acid transporter (MctC) revealed for the first time a quantitative discrimination of passive diffusion and active transport of acetate by bacterial cells. MctC is a secondary transporter and belongs to the class of sodium solute symporters, but it is driven by the electrochemical proton potential. The mctC gene is preceded by and cotranscribed with cg0952, a locus encoding a small membrane protein, and the transcription of the cg0952-mctC operon is under the control of the transcriptional regulators RamA and RamB. Both of these proteins directly bind to the promoter region of the operon; RamA is essential for expression and RamB exerts a slightly negative control on expression of the cg0952-mctC operon. mctC expression is induced in the presence of pyruvate and beneficial under substrate-limiting conditions for C. glutamicum

Design and Development of a qPCR-Based Mitochondrial Analysis Workflow for Medical Laboratories

Mitochondrial DNA (mtDNA) damage is closely associated with typical diseases of aging, such as Alzheimer’s or Parkinson’s disease, and other health conditions, such as infertility. This damage manifests in reduced mitochondrial copy number and deletion mutations in mtDNA. Consequently, the analysis of mitochondrial damage by determining the parameters copy number and deletion ratio using quantitative real-time PCR (qPCR) is of interest for clinical diagnostics. To bring the findings from research into laboratory practice, a suitable and reliable process is needed, which must be thoroughly validated. This process includes the software used for the analysis, which must meet extensive regulatory and process requirements. Existing software does not adequately implement the requirements of laboratories and, in particular, does not provide direct support for the calculation of the aforementioned mtDNA parameters. The paper discusses the development of a new software-based analysis workflow that is designed specifically for laboratories to help with the calculation of mtDNA parameters. The software was developed using the User-Centered Design method and is based on the recently introduced prototype, “PlateFlow”. Initial user tests provide positive feedback. In the future, this workflow could form the basis for validations of mitochondrial tests in medical laboratories