Krebs Cycle 1

The first reaction in the Krebs Cycle is the conversion of pyruvate to acetyl CoA. In reality, this reaction is not really in the Krebs Cycle, but since it is the first reaction that occurs in the mitochondrion and it leads directly into the cycle, it is usually included in discussions of the cycle. In this reaction, pyruvate, a three carbon molecule that is generated in glycolysis and in the metablism of some amino acids, is decarboxylated (a carboxyl group is removed) to the two carbon acetate. The carboxyl group is released as carbon dioxide. This reaction is catalyzed by the enzyme pyruvate dehydrogenase. This reaction is also an oxidation as 2 electrons are removed from pyruvate during the reaction (you should have guessed that this was an oxidation reaction due to the name of the enzyme, all dehydrogenases catalyze oxidation reactions). The two electrons are accepted by NAD and results in the formation of NADH. The oxidation of pyruvate is very exogonic (delta G = -7.5 kcal/mole). Some of the energy released from this reaction is transferred with the electrons to NADH and some is used to energize acetate by adding coenzyme A to acetate thus forming acetyl CoA, the actual product of the reaction.

substrates (reactants )	pyruvate	CoA + NAD
enzyme	pyruvate dehydrogenase
products	acetyl CoA	CO2 + NADH

Items to note about this reaction:

1. Pyruvate is the product of glycolysis, thus it comes from glucose. It may also come from some amino acids.
2. The reaction occurs in the mitochondrion.
3. This is an oxidation reaction which results in the formation of an NADH.
4. One carbon is removed from pyruvate in the form of carbon dioxide (note the yellow carbon in the above figure). This leaves just two carbons remaining from pyruvate.
5. The addition of the coenzyme A to the acetate (see red in the above figure) acts to conserve the energy released from the reaction and to energize the acetate.

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