Lactose and D-galactose catabolism in the filamentous fungusAspergillus nidulans

Abstract

IntroductionThe disaccharide lactose (1,4-0-ß-D-galactopyranosyl-D-glucose) is a byproduct of cheese production accumulating to amounts of 800 000 tons per year worldwide, of which 15 % is used as a carbon source for various microbial fermentations, such as cellulase production by Trichoderma reesei or penicillin production by Penicillium chrysogenum [1]. At present, lactose is the only derepressing carbon source used on industrial scale. Nevertheless, little is known about the regulation of its metabolism in filamentous fungi. Lactose is metabolized slowly, and some important fungi such as A. niger cannot use it at all. A more detailed knowledge on the rate-limiting steps would be helpful to improve its industrial application.We have chosen A. nidulans as an object for investigating how lactose and galactose metabolism are regulated because it has long become a model system for biochemical and genetic research on fungi, and mutants in the lactose-metabolizing pathway of A. nidulans are available [2]. General scheme of lactose and D-galactose catabolism in A. nidulansThe first step in lactose metabolism is its transport by a lactose permease, followed by a hydrolysis to glucose and galactose by an intracellular ß-galactosidase [3]. D-glucose catabolism proceeds via either the glycolytic or the oxidative pentose phosphate pathway, while D-Galactose degradation was known to occur exclusively via the Leloir-pathway. The Leloir pathway is a ubiquitous trait in pro-and eukaryotic cells [4]. It can be used as a catabolic pathway for the degradation of D-galactose as an energy and carbon source while it links as an anabolic pathway the metabolism of carbohydrates, such as the synthesis of lipopolysaccharides, of cell wall components, and of exopolysaccharides for which galactosides are frequently required as building blocks. In cucumber, Leloir pathway was reported to be involved in the sucrose synthesis from stachyose [5]. It involves an ATPdependent galactokinase (EC 2.7.1.6) to form D-galactose 1-phosphate, which is subsequently transferred to UDP-glucose in exchange with D-glucose 1-phosphate by D-galactose 1-phosphate-uridyltransferase (EC 2.7.7.12). The resulting UDP-galactose is a substrate for the reaction catalyzed by UDP-galactose 4-epimerase (EC 5.1.3.2), resulting in UDP-glucose. Regulation of formation of the ß-galactosidase activityWild-type A. nidulans do not exhibit any ß-galactosidase activity on glucose or glycerol, while it is clearly present in mycelia grown on lactose. No extracellular or cellbound ß-galactosidase activity can be detected. The time-profile of activity essentially parallel growth and lactose consumption, and decline after exhaustion of the carbon source. The ß-galactosidase activity of mycelia growing on lactose can further raised by addition of galactose, indicating that enzyme formation is not maximal during growth on lactose [3,6].The formation of ß-galactosidase on lactose or D-galactose, but not on glucose or glycerol suggests that its biosynthesis may be under carbon catabolit...

Utilization of agro-industrial waste for β-galactosidase production under solid state fermentation using halotolerant Aspergillus tubingensis GR1 isolate

“…In addition, we observed that supplementation of organic nitrogen sources to the production medium leads to the enhancement of β-galactosidase production. Fekete et al ( 2008 ) reported that metabolic fate of galactose in A. nidulans proceeds through reductive pathway and it prefers ammonium salts rather than nitrate as nitrogen source. In our present study, it was observed that the degradation of wheat bran resulted in galactose molecules which serve as carbon source and corn steep liquor supply enough ammonium ions and other vitamins for reductive metabolic fate of galactose.…”

l-xylo-3-Hexulose Reductase Is the Missing Link in the Oxidoreductive Pathway for d-Galactose Catabolism in Filamentous Fungi

“…Until recently, A. niger was considered unable to use D-galactose as a carbon source (25,26). In the latest demonstrations of D-galactose utilization in A. niger, the oxidoreductive, but not the Leloir pathway, was shown to be employed, and the D-galactose utilization was enabled with the addition of a small amount of D-xylose (8,12).…”

Microbial Degradation of Lignocellulosic Biomass

“…In filamentous fungi, lactose is cleaved by extracellular β-galactosidase into glucose and galactose. Lactose induction of cellulase genes requires the β-anomer of D-galactose, which can be converted to fructose by an alternative pathway in addition to the Leloir pathway [85]. In this alternative pathway, D-xylose reductase (encoded by xyl1) is the enzyme catalyzing the first step [86].…”