Thunberg condensation

The Thunberg Condensation. An alternate to the tricarboxylic acid cycle has been proposed many times as a mechanism for acetate (or acetyl CoA) oxidation. The suggestion was originally made by Thunberg that two acetates could condense to form succinate, which could be oxidized to acetate and CO2. This Thunberg condensation has been invoked repeatedly to explain various observations, and it is indeed... 

If highly labeled C HaCOOH is converted to succinate, a certain amount of the species would be expected from the Thunberg condensation... 

One of the first persons to study the oxidation of organic compounds by animal tissues was T. Thunberg, who between 1911 and 1920 discovered about 40 organic compounds that could be oxidized by animal tissues. Salts of succinate, fumarate, malate, and citrate were oxidized the fastest. Well aware of Knoop s (3 oxidation theory, Thunberg proposed a cyclic mechanism for oxidation of acetate. Two molecules of this two-carbon compound were supposed to condense (with reduction) to succinate, which was then oxidized as in the citric acid cycle to oxaloacetate. The latter was decarboxylated to pyruvate, which was oxidatively decarboxylated to acetate to complete the cycle. One of the reactions essential for this cycle could not be verified experimentally. It is left to the reader to recognize which one. 

In the early 1900s, Thunberg proposed a cyclic pathway for the oxidation of acetate. In his scheme, two molecules of acetate are condensed, with reduction, to form succinate, which in turn is oxidized to yield oxaloacetate. The decarboxylation of oxaloacetate to pyruvate followed by the oxidative decarboxylation of pyruvate to acetate complete the cycle. Assuming that electron carriers like NAD" and FAD would be part of the scheme, compare the energy liberated by the Thunberg scheme with that liberated by the now-established citric acid cycle. Which of the steps in Thunberg s scheme was not found in subsequent studies ... 

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