Inhibition of oxidative phosphorylation for enhancing citric acid production by Aspergillus niger

One of the reasons we as a society have been able to make it to the point we have is because of our ability to produce things on a massive scale. For better or for worse, the ability to mass-produce the things we need has made them cheaper and more widely available, allowing for increased enjoyment and human productivity. One such thing that is infrequently thought about is citric acid. It has a wide variety of uses, largely in food production and preservation, and thanks to some breakthroughs made at the beginning of the 20^th century, this substance can be produced on a large scale. It is mainly done through the fermentation of the filamentous fungus Aspergillus niger. This is the most economical and widely-used method for the production of citric acid; it accounts for nearly 90 percent of the world's production. However, for as much as modern science has been able to harness this process, there are still some aspects of it that are not understood, and these are of particular interest to the biochemical community at large. For example, when the concentration of extracellular citric acid accumulated by A. niger reaches 150–200 g L−1 under suitable conditions, a "metabolic overflow" occurs, and this occurrence has been made the model for investigating why cells overproduce organic acids from the tricarbonyte acid (TCA) cycle.

Several theories have been advanced and studied to try and understand this occurrence. For example, glycolytic pathways have been heavily scrutinized, looking at the way hexokinase, phosphofructokinase, and pyruvate kinase interact with the glycolytic metabolism. The study by Wang et al. from the Tianjin University of Science and Technology in Tianjin, China sought to better understand this metabolic overflow by comparing two different strains of A. Niger. They inferred that the differences in yields of these two strains might come from the difference in energy metabolism, and to measure this they looked at the changes in intercellular ATP, NADH, and NADH/NAD⁺. To do this, the research team used the Amplite Fluorimetric Total NAD and NADH Assay Kit. Unlike other NAD and NADH Assay kits, this one does not measure NAD/NADH absorption at 340nm, as this can cause interference and decreased sensitivity. Instead, it uses specific enzymes to recognize NAD and NADH in an enzyme cycling reaction. There is no need to purify NAD/NADH from the sampling mix, and this increases accuracy.

At the end of the study, Wang's team concluded that excess ATP has a strong inhibitory effect on citric acid accumulation and that strengthening NADH oxidation and reducing the concentration of intracellular ATP can accelerate glycolysis to enhance citric acid yield. This provides insight into this process of metabolic overflow, helping researchers better understand what causes the overproduction of these organic acids. This research would not have been possible if it weren't for the accuracy the Amplite Fluorimetric Total NAD and NADH Assay Kit provides in measuring NAD/NADH. Because researchers were able to carefully monitor this enzyme, they were able to make valid and useful conclusions that stand to significantly contribute to the understanding of this phenomenon.

References

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1. Wang, Lu, et al. "Inhibition of oxidative phosphorylation for enhancing citric acid production by Aspergillus niger." Microbial cell factories 14.1 (2015): 7.