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NAD+/NADH homeostasis affects metabolic adaptation to hypoxia and secondary metabolite production in filamentous fungi*

Abstract Filamentous fungi are used to produce fermented foods, organic acids, beneficial secondary metabolites and various enzymes. During such processes, these fungi balance cellular NAD+:NADH ratios to adapt to environmental redox stimuli. Cellular NAD(H) status in fungal cells is a trigger of changes in metabolic pathways including those of glycolysis, fermentation, and the production of organic acids, amino acids and secondary metabolites. Under hypoxic conditions, high NADH:NAD+ ratios lead to the inactivation of various dehydrogenases, and the metabolic flow involving NAD+ is down-regulated compared with normoxic conditions. This review provides an overview of the metabolic mechanisms of filamentous fungi under hypoxic conditions that alter the cellular NADH:NAD+ balance. We also discuss the relationship between the intracellular redox balance (NAD/NADH ratio) and the production of beneficial secondary metabolites that arise from repressing the HDAC activity of sirtuin A via Nudix hydrolase A (NdxA)-dependent NAD+ degradation. Fungal metabolic changes in the pathways such as glycolysis, fermentation, and the production of organic acids, amino acids and secondary metabolites are induced by environmental redox stimuli.