Tricarboxylic acid cycle originally proposed form

When the oxidation of substantial amounts of pyruvate was blocked by malonate, stoichiometric amounts of pyruvate would react if either oxaloacetate or its precursors, malate or fumarate, was added. None of the precursors to succinate in the chain was effective in this regard. This strongly supported Krebs hypothesis that oxidation of pyruvate involved its condensation with oxaloacetate and the subsequent series of conversions in the chain described by Krebs. The Krebs cycle in its originally proposed form is shown in figure 13.3. It is also known as the tricarboxylic acid (TCA) cycle or the citrate cycle. 

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 The suggestion was originally made by Thunberg that two acetates could condense to form succinate, which could be oxidized to acetate and CO2.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... 

The cycle was originally proposed as a mechanism for the terminal oxidation of carbohydrate. It was always obvious that it must also apply to parts of the protein molecule because several amino acids yield members of the cycle directly—glutamic acid, aspartic acid, and alanine—or indirectly—histidine, proline, arginine, and others. Work carried out during the last decade with the help of specially prepared tissue extracts and of isotopes has produced conclusive evidence in support of the conception that the tricarboxylic acid cycle is also the terminal mechanism of the oxidation of fatty acids and ketone bodies. These substances all form the same derivative of acetic acid—acetyl coenzyme A— which can condense with oxalacetate to form citrate. The pathway leading from various... 

Original tricarboxylic acid (TCA) cycle proposed by Krebs. This cycle is also called the citrate cycle, or the Krebs cycle. To start the cycle in operation, pyruvate loses one of its carbons and condenses with a four-carbon dicarboxylic acid, oxaloacetic acid, to form a six-carbon tricarboxylic acid, citrate. In one turning of the cycle, two carbons are lost as C02, thus returning the citrate to oxaloacetate. The conversion blocked by malonate is indicated by a red bar. 

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See also in sourсe #XX -- [ , ]

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