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

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.