Text Preview
When oxygen is available, the end products of glycolysis, the pyruvate molecules, are transported to the mitochondria where they are converted to acetyl-CoA. Other metabolites, such as fatty acids, also form acetyl-CoA. The CoA in acetyl-CoA refers to coenzyme A, a cofactor derived from the vitamin pantothenic acid. The role of coenzyme A is to carry acetyl residues in certain enzymatic reactions.
The energy of metabolite oxidation drives acetyl-CoA formation. Acetyl-CoA is a high-energy compound like ATP. Pyruvate has three carbon atoms. One is removed as carbon dioxide during acetyl-CoA formation. The reaction is an oxidation reaction with a large, negative free energy change.
Let’s see how well you understand this reaction.
The formation of acetyl-CoA from pyruvate is an oxidation reaction because:
Coenzyme A is added.
the free energy change is negative.
NADH is made.
Enter your answer and click Submit to see if you’re right
Sorry, the correct answer is c.
Yes, c is the correct answer.
Oxidation and reduction reactions are always coupled. Whenever a coenzyme like NAD is reduced, another molecule must be oxidized. In this reaction, pyruvate is oxidized.
The TCA cycle consists of eight enzyme-catalyzed reactions. It starts with the entry of acetyl-CoA. In the first reaction, acetyl-CoA combines with the four-carbon molecule oxaloacetate to form the six-carbon molecule citrate. Citrate, or citric acid, is the tricarboxylic acid for which the TCA cycle is named. You may have heard of citric acid. It’s what gives acidic foods like lemons their characteristic sour taste. This reaction is irreversible with a large, negative free energy change. Like ATP in glycolysis, acetyl-CoA provides the energy that drives Reaction 1.
In the first part of reaction 2, a molecule of water is removed. This part of the reaction is a dehydration reaction. Then a molecule of water is added back at a different location in the molecule to form an isomer of citrate.
Reaction 3 is an oxidation-reduction reaction. Note how NAD is reduced and carbon dioxide is formed.
Reaction 4 is another oxidation-reduction reaction. As in reaction 3, NADH and carbon dioxide are formed. In addition, some of the oxidation energy is saved in the formation of the high-energy compound, succinyl-CoA.
The energy stored in succinyl-CoA drives the formation of GTP in reaction 5. GTP is energetically equivalent to ATP. When GTP donates a phosphate group to ADP, ATP is formed.
Reaction 6 is an oxidation-reduction reaction. It differs from the other oxidation-reduction reactions in several ways. It is the only reaction in the TCA cycle that takes place in the inner mitochondrial membrane. No carbon is lost, and a different coenzyme, FAD, is reduced to FADH2. Reaction 6 is catalyzed within the protein complex called Complex II. In addition to being part of the TCA cycle, Complex II is also part of the electron transport chain.
In reaction 7, a molecule of water is added to the substrate. This hydration reaction prepares the substrate for the next reaction.
In reaction 8, oxaloacetate is regenerated and NADH is formed, so reaction 8 is another oxidation-reduction reaction.
The TCA cycle starts when one molecule of acetyl-CoA, derived from pyruvate, is added to oxaloacetate and the cycle ends when the oxaloacetate is regenerated. This is one “turn” of the TCA cycle. Two carbon atoms enter each turn of the TCA cycle as part of acetyl-CoA and two carbons leave each turn as carbon dioxide. This is necessary for the regeneration of the starting reactant, oxaloacetate.
Next, we’ll focus on these carbon atoms. We’ll see if the carbon atoms that leave the TCA cycle are the same carbon atoms that entered it.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education