Metabolic engineering of the fungal D-galacturonate pathway

Joosu Kuivanen

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    Industrial biotechnology is one of the enabling technologies for biorefineries, where biomass is converted into value-added products. In addition to biofuels, several platform and fine chemicals can be produced from biomass using biotechnological routes taking advantage of metabolic pathways in the cell. Some of these metabolic pathways exist naturally in the cells that are used as production hosts. However, many of the desired chemical products are not naturally produced by the cellular metabolism. Consequently, genetic engineering is needed to redirect the cellular metabolism towards a product of interest. In this thesis, one of these metabolic pathways – the catabolic D-galacturonate pathway in filamentous fungi – was engineered and redirected to desired end products. D-Galacturonic acid is the main monomer of pectin, which is a common heteropolysaccharide in certain biomasses. Two examples of pectin-rich biomasses are citrus processing waste and sugar beet pulp from agro-industry. These residual biomasses are often poorly utilised. Biotechnological production of L-galactonic acid, a potential platform chemical, was demonstrated in this thesis for first time. The production was obtained in Aspergillus niger and Hypocrea jecorina (Trichoderma reesei) strains by deleting the second gene, encoding a dehydratase, from the fungal D-galacturonate pathway. Overexpression of the first gene, encoding a D-galacturonate reductase, in the pathway improved the initial production rate in A. niger. In addition, production at low pH resulted in higher productivity and titres in cultivations with the engineered A. niger strains. Final titres between 7 and 9 g L-galactonic acid l-1 and product yields close to 100% were observed from pure D-galacturonic acid with both of the production hosts. In addition to L-galactonic acid production from pure D-galacturonic acid, a consolidated bioprocess from citrus processing waste, a pectin-rich biomass, to L-galactonic acid was investigated using the engineered strains of A. niger. Two different bioprocess types, submerged and solid state fermentation, were compared. As a result, similar final titres and product yields were observed to those obtained in the process from pure D-galacturonic acid. The highest product yield, approaching 90% of the theoretical maximum, was achieved in the solid state fermentation. The second reaction in the fungal D-galacturonate pathway is dehydration of L-galactonic acid by the action of an L-galactonate dehydratase. Deletion of the gene gaaB encoding this enzyme in A. niger is crucial for L-galactonic acid production. Despite the deletion of gaaB, product yields (L-galactonic acid per consumed D-galacturonic acid) have remained below the theoretical maximum. In addition, catabolisation of mucic acid, an industrially potential dicarboxylic acid that can be produced via an engineered D-galacturonate pathway, has been observed in the earlier studies. We hypothesised that catabolisation of L-galactonic acid and mucic acid may be due to other dehydratases. For these reasons, all the five putative dehydratase-encoding genes from A. niger were expressed in yeast and the resulting enzymes were characterised. The current study revealed the substrate specifities for four of the studied dehydratases, whereas one of the putative dehydratases was apparently not in fact an active dehydratase. In addition to GaaB, two dehydratases with activity towards D-galactonic acid and one with activity towards L-rhamnonic acid were identified. GaaB was the only dehydratase with activity towards L-galactonic acid. Although GaaB has broad substrate specificity, neither it nor any other dehydratase showed activity towards mucic acid or its lactone. In summary, undesired L-galactonic acid or mucic acid catabolisation was not explained by these dehydratases. L-Galactonate-5-dehydrogenase, a bacterial enzyme oxidising L-galactonic acid to D-tagaturonic acid, was also studied in this thesis. This enzyme activity has been demonstrated earlier from crude extract of Escherichia coli. Later on, the corresponding gene encoding the enzyme was suggested to be yjjN, although without characterisation of the enzyme. In this work, it was shown that yjjN does indeed encode an L-galactonate-5-dehydrogenase. The Km and kcat for L-galactonic acid were 19.5 mM and 0.51 s-1, respectively. In addition, the YjjN enzyme was applied in a colorimetric assay for L-galactonic and L-gulonic acids with detection limits of 1.65 μM and 10 μM, respectively. L-Galactonic acid can be lactonised and further oxidised to L-ascorbic acid (vitamin C) via chemical or biochemical routes. Synthetic L-ascorbic acid is widely used as a nutrient and preservative in several industries. Currently, it is produced in a process combining chemical and biochemical steps. In this thesis, an A. niger strain was engineered for direct conversion of D-galacturonic acid to L-ascorbic acid. In addition to the deletion of gaaB, two heterologous genes, encoding L-galactono-1,4-lactone lactonase and L-galactono-1,4-lactone dehydrogenase from a plant biosynthetic L-ascorbic acid pathway, were introduced into A. niger. In addition, a gene encoding an unspecific L-gulono-1,4-lactone lactonase from a mammalian biosynthetic L-ascorbic acid pathway was tested instead of the plant lactonase. The lactonase enzyme activity was not observed in any of the engineered A. niger strains. However, the resulting strains were capable of L-ascorbic acid production from pure D-galacturonic acid or citrus processing waste with final titres up to 170 mg l-1. Pectin-rich biomass has potential as a raw material for the production of renewable chemicals. This thesis presents new ways to utilise this residual biomass by using industrial biotechnology. In addition, the thesis broadens basic understanding of the fungal catabolic D-galacturonate pathway and how it can be engineered for production of useful chemicals.
    Translated title of the contributionMikrosienten D-galakturonaattireitin metabolian muokkaus
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Frey, Alexander, Supervising Professor
    • Richard, Peter, Thesis Advisor
    Publisher
    Print ISBNs978-951-38-8340-9
    Electronic ISBNs978-951-38-8341-6
    Publication statusPublished - 2015
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • filamentous fungi
    • Aspergillus niger
    • pectin
    • D-galacturonic acid
    • L-galactonic acid
    • L-ascorbic acid
    • metabolic engineering

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