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

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          ABSTRACT

          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.

          Abstract

          Transporter protein expression enables the optimization of microbial cell factories for the bioproduction of organic acids.

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          Most cited references118

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          Microbial production of organic acids: expanding the markets.

          Microbial production of organic acids is a promising approach for obtaining building-block chemicals from renewable carbon sources. Although some acids have been produced for some time and in-depth knowledge of these microbial production processes has been gained, further microbial production processes seem to be feasible, but large-scale production has not yet been possible. Citric, lactic and succinic acid production exemplify three processes in different stages of industrial development. Although the questions being addressed by current research on these processes are diverging, a comparison is helpful for understanding microbial organic acid production in general. In this article, through analysis of the current advances in production of these acids, we present guidelines for future developments in this fast-moving field.
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            Engineering of yeast hexose transporters to transport D-xylose without inhibition by D-glucose.

            All known D-xylose transporters are competitively inhibited by D-glucose, which is one of the major reasons hampering simultaneous fermentation of D-glucose and D-xylose, two primary sugars present in lignocellulosic biomass. We have set up a yeast growth-based screening system for mutant D-xylose transporters that are insensitive to the presence of D-glucose. All of the identified variants had a mutation at either a conserved asparagine residue in transmembrane helix 8 or a threonine residue in transmembrane helix 5. According to a homology model of the yeast hexose transporter Gal2 deduced from the crystal structure of the D-xylose transporter XylE from Escherichia coli, both residues are found in the same region of the protein and are positioned slightly to the extracellular side of the central sugar-binding pocket. Therefore, it is likely that alterations sterically prevent D-glucose but not D-xylose from entering the pocket. In contrast, changing amino acids that are supposed to directly interact with the C6 hydroxymethyl group of D-glucose negatively affected transport of both D-glucose and D-xylose. Determination of kinetic properties of the mutant transporters revealed that Gal2-N376F had the highest affinity for D-xylose, along with a moderate transport velocity, and had completely lost the ability to transport hexoses. These transporter versions should prove valuable for glucose-xylose cofermentation in lignocellulosic hydrolysates by Saccharomyces cerevisiae and other biotechnologically relevant organisms. Moreover, our data contribute to the mechanistic understanding of sugar transport because the decisive role of the conserved asparagine residue for determining sugar specificity has not been recognized before.
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              An overview of biorefinery-derived platform chemicals from a cellulose and hemicellulose biorefinery

              Until recently, most of energy and industrially produced chemicals were derived from fossil fuel-based resources. This along with the continued depletion of finite fossil resources and their attributed adverse environmental impacts, alternatively sourced and more sustainable resources are being pursued as feedstock replacements. Thus, biomass has been identified as an alternate renewable and more sustainable resource as a means to reduce this sector's dependence on fossil fuel-based resources and to alleviate their environmental impacts. As such, lignocellulosic biomass has been further identified and demonstrated as an abundant renewable resource for the production of biofuels, platform chemicals, and their respective value-added products. This review article provides an overview of the techniques developed for the valorization of biomass in the production of platform chemicals within a biorefinery, and the status for commercialization.
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                Author and article information

                Contributors
                Journal
                FEMS Microbiol Lett
                FEMS Microbiol. Lett
                femsle
                FEMS Microbiology Letters
                Oxford University Press
                0378-1097
                1574-6968
                18 July 2020
                August 2020
                18 July 2020
                : 367
                : 15
                : fnaa118
                Affiliations
                Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho , Campus de Gualtar, Braga 4710-057, Portugal
                Author notes
                Corresponding author: Centro de Biologia Molecular e Ambiental (CBMA), Dep. Biologia, Universidade do Minho, Campus de Gualtar, Braga 4710-057, Portugal. Tel: +00351253601519; Fax: +00351253604319; E-mail: ijoao@ 123456bio.uminho.pt
                Author information
                http://orcid.org/0000-0001-8431-1567
                http://orcid.org/0000-0003-1622-4888
                https://orcid.org/0000-0003-1754-9254
                https://orcid.org/0000-0002-0143-9758
                Article
                fnaa118
                10.1093/femsle/fnaa118
                7419537
                32681640
                c6d17e35-bdf1-4c29-92d7-4fe169a53425
                © FEMS 2020.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 31 October 2019
                : 17 July 2020
                Page count
                Pages: 15
                Funding
                Funded by: Portuguese funds;
                Award ID: UID/BIA/04050/2019
                Funded by: national funds;
                Award ID: PTDC/BIAMIC/5184/2014
                Funded by: European Regional Development Fund, DOI 10.13039/501100008530;
                Award ID: SFRH/BD/96166/2013
                Funded by: Horizon 2020;
                Award ID: 764927
                Categories
                Minireview
                Biotechnology and Synthetic Biology
                AcademicSubjects/SCI01150
                Mini Review

                Microbiology & Virology
                industrial biotechnology,cell factories,carboxylic acids,transporter proteins,permease

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