Abstract
(1) Background: It has been shown that d-galacturonic acid is converted to l-galactonic acid by the basidiomycotic yeast, Cryptococcus diffluens. However, two pathways are hypothesized for the l-galactonic acid conversion process in C. diffluens. One is similar to the conversion process of the filamentous fungi in d-galacturonic acid metabolism and another is the conversion process to l-ascorbic acid, reported in the related yeast, C. laurentii. It is necessary to determine which, if either, process occurs in C. diffluens in order to produce novel value-added products from d-galacturonic acid using yeast strains. (2) Methods: The diethylaminoethy (DEAE)-fractionated enzyme was prepared from the cell-free extract of C. diffluens by the DEAE column chromatography. The l-galactonic acid conversion activity was assayed using DEAE-fractionated enzyme and the converted product was detected and fractionated by high-performance anion-exchange chromatography. Then, the molecular structure was identified by nuclear magnetic resonance analysis. (3) Results: The product showed similar chemical properties to 2-keto-3-deoxy-l-galactonic acid (l-threo-3-deoxy-hexulosonic acid). (4) Conclusions: It is suggested that l-galactonic acid is converted to 2-keto-3-deoxy-l-galactonic acid by dehydratase in C. diffluens. The l-galactonic acid conversion process of C. diffluens is a prioritized pathway, similar to the pathway of ascomycetes.
Highlights
Pectin-rich substrates, which are present in large amounts in citrus peel and sugar beet pulp, are generated in the waste products that result from processing citrus or sugar beet in the food industry [1,2,3]
When cell-free extract (CFE) of C. diffluens OPU-FC11 was reacted with l-galactonic acid (l-GalA), two novel peaks were detected as reaction products (Figure 1)
The elution time of one peak was the same as that of pyruvic acid, but the other peak did not correspond to commercial reagents including d-galacturonic acid (d-GalUA), l-GalA, glycerol, acetate, ascorbic acid, and other various compounds related to d-GalUA metabolism
Summary
Pectin-rich substrates, which are present in large amounts in citrus peel and sugar beet pulp, are generated in the waste products that result from processing citrus or sugar beet in the food industry (for example, following juice extraction) [1,2,3]. This, in turn, enhanced the production of l-GalA using recombinant Saccharomyces cerevisiae containing the gene Cd-GAR1, which encodes galacturonate reductase of C. diffluens [7]. It is not known what is produced following l-GalA conversion in the d-GalUA metabolic pathway of C. diffluens. It has been shown in Fermentation 2019, 5, 73; doi:10.3390/fermentation5030073 www.mdpi.com/journal/fermentation
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