Revisiting a ‘simple’ fungal metabolic pathway reveals redundancy, complexity and diversity

Abstract

Next to D-glucose, the pentoses L-arabinose and Dxylose are the main monosaccharide components of plant cell wall polysaccharides and are therefore of major importance in biotechnological applications that use plant biomass as a substrate. Pentose catabolism is one of the best-studied pathways of primary metabolism of Aspergillus niger, and an initial outline of this pathway with individual enzymes covering each step of the pathway has been previously established. However, although growth on L-arabinose and/or D-xylose of most pentose catabolic pathway (PCP) single deletion mutants of A. niger has been shown to be negatively affected, it was not abolished, suggesting the involvement of additional enzymes. Detailed analysis of the single deletion mutants of the known A. niger PCP genes led to the identification of additional genes involved in the pathway. These results reveal a high level of complexity and redundancy in this pathway, emphasizing the need for a comprehensive understanding of metabolic pathways before entering metabolic engineering of such pathways for the generation of more efficient fungal cell factories.

The Sugar Metabolic Model of Aspergillus niger Can Only Be Reliably Transferred to Fungi of Its Phylum

“…D-glucose is phosphorylated to glucose 6-phosphate by glucokinase [7] and hexokinase [8], which can then be converted into fructose 6-phosphate. The pentoses D-xylose and L-arabinose are converted through the pentose catabolic pathway (PCP) to D-xylulose-5-phosphate, which enters the pentose phosphate pathway (PPP) [9]. Other sugars, like D-galactose, D-mannose, D-galacturonic acid, L-rhamnose and D-gluconic acid, are converted through sugar-specific metabolic pathways [5,10] and eventually all enter glycolysis.…”

“…In a previous study of A. niger NRRL 3 [10], a model for the sugar metabolic pathways of this species has been established, which consists of 155 genes involved in 11 sugar metabolic pathways (glycolysis, PCP, PPP, D-galactose metabolism, D-mannose metabolism, D-gluconate metabolism, D-galacturonic acid catabolism, L-rhamnose metabolism, glycerol metabolism, TCA and glyoxylate cycles). Meanwhile, recent experimental studies have refined this predicted sugar metabolic network and identified new enzymes involved in L-rhamnose catabolism [14] and PCP [9].…”

“…Traditionally, only a single enzyme was assigned to catalyze each reaction in PCP. A recent study revealed the redundancy and complexity of the conversion of pentose sugars in A. niger [9]. L-arabinose goes through the sequential reactions: reduction catalyzed by NADPH-dependent L-arabinose reductase (LarA) and D-xylose reductase (XyrA/XyrB) (EC 1.1.1.21); oxidation catalyzed by L-arabitol-4-dehydrogenase (LadA, EC 1.1.1.12), xylitol dehydrogenase (XdhA) and sorbitol dehydrogenase (SdhA) (EC 1.1.1.12); reduction catalyzed by L-xylulose reductases (LxrA/LxrB, EC 1.1.1.10) [83] and again oxidation (LadA/XdhA/SdhA, EC 1.1.1.9), by which L-arabinose is progressively converted to L-arabitol, L-xylulose, xylitol and Dxylulose (Figure 2).…”