The role of one-carbon compounds in citric acid biosynthesis

The role of carbon dioxide, formic acid, and methanol in the formation of citric acid from sucrose by Aspergillus niger has been investigated. It was shown that labeled carbon dioxide enters the citric acid molecule via known reactions of CO2-fixation and the tricarboxylic acid cycle; the label was found predominantly in the C-1 and C-6 positions. In addition, some labeling was discovered in the C-2 and C-5 carbons, possibly due to reversals in the cycle. Formic acid appeared to be oxidized to carbon dioxide before being incorporated into citric acid. Methanol was not utilized appreciably by A. niger for organic acid synthesis under the conditions of the experiment.