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Citrate fermentation mechanism in Aspergillus niger (1) 1. By strictly restricting the supply of metal ions such as manganese ions, or by selecting strains tolerant to high concentrations of metal ions like manganese, zinc, and iron, the ability of the cells to redirect sugar metabolism toward protein, fatty acid, and nucleic acid synthesis is reduced. This leads to the accumulation of high levels of ammonium ions within the cells, thereby relieving the feedback inhibition exerted by citrate and ATP on the PFK enzyme and increasing the metabolic flux in the EMP pathway. (2) 2. There is a lateral respiratory chain with high respiratory activity that is sensitive to oxygen, but it does not produce ATP, which leads to a decrease in the ATP concentration within the cell. Thus, it reduced the feedback inhibition of PFK and CS enzymes by ATP, facilitated the smooth operation of the EMP pathway, and increased the biosynthesis of citrate. (3) 3. Propionyl-CoA carboxylase is a constitutive enzyme that is not regulated by metabolic controls; it can continuously supply oxaloacetate. The oxidation and decarboxylation of propionyl-CoA to produce acetyl-CoA and CO maintain equilibrium, ensuring a steady supply of the precursors acetyl-CoA and oxaloacetate. Citrate synthase, in turn, is essentially unregulated or only slightly regulated, which enhances the capacity for citrate synthesis. (4) 4. A-malic dehydrogenase is inhibited by glucose and ammonium ions, which causes the TCA cycle in Aspergillus niger to adopt a \"horseshoe-shaped\" metabolic pattern, thereby weakening the TCA cycle and reducing the intracellular ATP concentration; at the same time, it increases the concentration of a-malic acid. In turn, this feedback-inhibits isocitrate dehydrogenase, reducing the self-degradation of citrate. (5) 5. The cis-aconitic acid hydratase catalyzes the establishment of a balance of citrate:cis-aconitic acid:Isocitrate = 90:3:7; its function is always directed toward the synthesis of citrate, that is, its activity in decomposing citrate is low. Once the citrate concentration rises to a certain level, it inhibits the activity of isocitrate dehydrogenase, thereby further promoting the accumulation of citrate itself and lowering the pH below 2.0. The inactivation of aconitic acid hydratase and isocitrate dehydrogenase facilitates the accumulation of citrate and its elimination from the body.