Transport mechanism, fumaric acid

Transport of fumaric acid outside of the fungal cell is poorly understood. It is important to understand how and at what conditions fumaric acid is transported out of the cell. The ambient pH is one of the main factors found to influence the excretion of organic acids in filamentous fungi and there are different hypothesis on the mechanisms involved (overflow metabolism hypothesis, charge balance hypothesis, and aggressive acidification hypothesis all detailed in Vrabl et al., 2012). 

De Vries W, Aleem MTH, Hemrika-Wagner A and Stouthamer AH (1977) The fimctioning of cytochrome 6 in the electron transport to fumarate in Propionibacterium freudenreichii and P. pentosaceum. Arch Microbiol 112 271-276 Delwiche EA (1948) Mechanism of propionic acid formation by Propionibacterium pentosaceum. J Bacteriol 56 811-820... 

The transport mechanism of fumaric acid in Rhizopus strains has not been studied yet. Increasing the number or activity of the dicarboxylic acid transporters could lower the intracellular fumarate concentration and could therefore have a positive effect on the production yield. 

The efficiency of an enzyme can be reduced or can even become negligible in the presence of certain substances, known as inhibitors. Many inhibitors have structural resemblances with the substrates and compete with them for the formation of complexes with the enzyme. This is the case of the inactivation of cytochrome c oxidase by the cyanide ion, which blocks the mitochondrial electron-transport chain to oxygen. Similarly, the inactivation of the succinate dehydrogenase by malonate involves its inhibition of the conversion of succinate to fumarate in the citric acid cycle. In the latter case, the mechanism for competitive inhibition is... 

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