Dioxide Assimilation in Microorganisms

The assimilation of CO2 by bacteria was first demonstrated by Wood and Werkman who observed that during the fermentation of glycerol by propionic acid bacteria there was an over-all uptake of COj and that the amount of product (succinic acid) formed corresponded to the CO which was taken up. Since pyruvic acid was known to be present it was proposed that pyruvic acid condenses with CO to produce oxalacetic acid (Fig. 3, Wood-Werkman reaction) which is subsequently converted to succinic acid. 

In 1941, Krampitz and Werkman separated an enzyme preparation (oxalacetate /3-carboxylase) from Micrococcus lysodeikiicus which decar-boxylates oxalacetic acid to form pyruvic acid. Cocarboxylase was not required for this reaction. When oxalacetate, the enzyme and magnesium ions were incubated in the presence of until approximately half of the oxalacetate disappeared, was found in the remaining oxalacetate. The isotope was found exclusively in the carboxyl group adjacent to the methylene group. Since the reaction medium contained only oxalacetate and CO2, and since the only products formed were pyruvic acid and COs, it was concluded that this reaction was reversible. 

The subsequent conversion of oxalacetate to succinate is believed to occur by way of malate and fumarate since Krebs and Eggleston have demonstrated that the following reactions occur reversibly in propionic acid bacteria. 

HOOC CHj CO COOH HOOC-CH, CHOH COOH HOOC CH CH COOH 

In accordance with these reactions it has been found that isotopic CO2 is assimilated by propionic acid bacteria to produce among other products, succinic acid containing isotopic carbon localized in the carboxyl groups. In general, it has been observed that those organisms which form succinic, propionic, or lactic acids are able to fix isotopic COj in the carboxyl positions. 

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