Aspergillus niger acidogenic metabolism: A biased view from the C and N interface

Abstract

Aspergillus niger is the major citric acid producer and this has matured into an industrial process through fermentation. Despite a century of research to elucidate the underlying mechanisms of acidogenesis, clear and definitive answers have eluded us. The voluminous literature ranges from fungal physiology to genomics. Much of it focuses on carbon stoichiometry between glucose and citric acid and has overlooked the extant close interactions between the fungal carbon and nitrogen metabolism. Because the regulation at the carbon-nitrogen interface is important, several approaches were taken to discern the role of A. niger nitrogen metabolism in acidogenesis. An enhanced GABA shunt operates while the carbon flux exits TCA cycle at citrate. The NADP-glutamate dehydrogenase bridges TCA cycle and GABA shunt. The allosteric response of this enzyme to 2-oxoglutarate could control the carbon flux at the interface of carbon-nitrogen metabolism. While Mn[II] limitation favours citric acid production, arginase (metalloenzyme with bound Mn[II]) does not directly affect the fungal citrate overflow. The acidogenic growth appears to mimic nitrogen starvation and is associated with an extended autophagic state of A. niger. The atg strains are compromised in citrate overflow thereby directly implicating autophagy in the process. Autophagy-associated movement of relevant enzymes during fermentation suggests that the entire citrate synthesis machinery could be operating insidethe fungal vacuole.