Dehydrogenase Catalyzes the Oxidation of Malate to Oxaloacetate

Malate Dehydrogenase Catalyzes the Oxidation of Malate to Oxaloacetate 

The final oxidation step of the cycle involves the conversion of malate to oxaloacetate by malate dehydrogenase. 

Some early applications of isotopic tracer techniques to the TCA cycle led to a new generalization concerning the stereochemistry of interaction between enzymes and certain types of substrate. 

In the early 1940s, before the discovery of 14C and when acetyl-coenzyme A was unknown, two research groups used the stable isotope 13C and mass spectrometers to study carbon flow in the TCA cycle. Pyruvate and 13C02 were added to pigeon liver preparations to form carboxy-labeled oxaloacetate. Malonate was added to stop the TCA cycle at succinate. The expected result was that half of the 13C would be found in succinate and the other half in C02 (fig. 13.10). 

in 1948, Ogston suggested that citrate was not necessarily excluded by the isotopic evidence, because the two —CH2—COO arms might actually not be equivalent when citrate was the substrate for an enzymic reaction. He pointed out that if the substrate were attached to the enzyme at three points, its orientation would be fixed by those attachments, and it would be impossible for the two identical arms to exchange positions. Thus, only one of them occupy the position that allowed it to participate in the reaction (fig. 13.11). 

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Succinate Dehydrogenase Catalyzes the Oxidation of Succinate to Fumarate Fumarase Catalyzes the Addition of Water to Fumarate to Form Malate Malate Dehydrogenase Catalyzes the Oxidation of Malate to Oxaloacetate... 

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