The Matrix Reactions

The matrix reactions are shown in figure 26.4, where the reaction steps are numbered to resume where figure 26.3 ended. Most of the matrix reactions constitute a series called the citric acid (Krebs6) cycle. Preceding this, however, are three steps that prepare pyruvic acid to enter the cycle and thus link glycolysis to it.

Step 9. Pyruvic acid is decarboxylated—CO removed and pyruvic acid, a C3 compound, becomes a C2 compound.

Step 10. NAD+ removes hydrogen atoms from the C2 compound (an oxidation reaction) and converts it to an acetyl group (acetic acid).

6Sir Hans Krebs (1900-1981), German biochemist

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Physiology: The Unity of Metabolism Companies, 2003 Form and Function, Third Edition

Chapter 26 Nutrition and Metabolism

Pyruvic acid (C3)

NAD+

Acetyl group (C2)

enzyme A

NAD+

Acetyl group (C2)

enzyme A

NAD+

The Mitochondrial Matrix Reactions

FADH

ADP ATP

Figure 26.4 The Mitochondrial Matrix Reactions. Black circles represent carbon atoms in the carbon skeleton of each molecule. Numbered reaction steps are explained in the text.

FADH

ADP ATP

Figure 26.4 The Mitochondrial Matrix Reactions. Black circles represent carbon atoms in the carbon skeleton of each molecule. Numbered reaction steps are explained in the text.

Step 11. The acetyl group binds to coenzyme A, a derivative of pantothenic acid (a B vitamin). The result is acetyl-coenzyme A (acetyl-CoA). At this stage the C2 remnant of the original glucose molecule is ready to enter the citric acid cycle.

Step 12. At the beginning of the citric acid cycle, CoA hands off the acetyl (C2) group to a C4 compound, oxaloacetic acid. This produces the C6 compound citric acid, for which the cycle is named.

Step 13. Water is removed and the citric acid molecule is reorganized, but it still retains its six carbon atoms.

Step 14. Hydrogen atoms are removed and accepted by NAD+.

Step 15. Another CO2 is removed and the substrate becomes a five-carbon chain.

Steps 16 and 17. Steps 14 and 15 are essentially repeated, generating another free CO2 molecule and leaving a four-carbon chain. No more carbon atoms are removed beyond this point; the substrate remains a series of C4 compounds from here back to the start of the cycle. The three carbon atoms of pyruvic acid have all been removed as CO2 at steps 9, 15, and 17. These decarboxy-lation reactions are the source of most of the CO2 in your breath.

Step 18. Some of the energy in the C4 substrate goes to phosphorylate guanosine diphosphate (GDP) and convert it to guanosine triphosphate (GTP), a molecule similar to ATP. GTP quickly transfers the Pi group to ADP to make ATP. Coenzyme A participates again in this step but is not shown in the figure.

Step 19. Two hydrogen atoms are removed and accepted by the coenzyme FAD.

Step 20. Water is added.

Step 21. A final two hydrogen atoms are removed and transferred to NAD+. This reaction generates oxaloacetic acid, which is available to start the cycle all over again.

It is important to remember that for every glucose molecule that entered glycolysis, all of these matrix reactions occur twice (once for each pyruvic acid). The matrix reactions can be summarized:

2 pyruvate + 6 H2O ^ 6 CO2 + 2 ADP + 2 Pi ^ 2 ATP + 8 NAD+ + 8 H2 ^ 8 NADH + 8 H+ + 2 FAD + 2 H2 ^ 2 FADH2

There is nothing left of the organic matter of the glucose; its carbon atoms have all been carried away as CO2 and exhaled. Although still more of its energy is lost as heat along the way, some is stored in the additional 2 ATP, and most of it, by far, is in the reduced coenzymes— 8 NADH and 2 FADH2 molecules generated by the matrix reactions and 2 NADH generated by glycolysis. These must be oxidized to extract the energy from them.

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Physiology: The Unity of Metabolism Companies, 2003 Form and Function, Third Edition

1000 Part Four Regulation and Maintenance

The citric acid cycle not only oxidizes glucose metabolites but is also a pathway and a source of intermediates for the synthesis of fats and nonessential amino acids. The connections between the citric acid cycle and the metabolism of other nutrients are discussed later.

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  • samwise
    What reaction accurs in the matrix?
    5 years ago

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