Harvesting Electrons from the Cycle

343

  • 19.1 The Citric Acid Cycle Consists of Two Stages

  • 19.2 Stage One Oxidizes Two Carbon Atoms to Gather Energy-Rich Electrons

  • 19.3 Stage Two Regenerates Oxaloacetate and Harvests Energy-Rich Electrons

  • 19.4 The Citric Acid Cycle Is Regulated

  • 19.5 The Glyoxylate Cycle Enables Plants and Bacteria to Convert Fats into Carbohydrates

Roundabouts, or traffic circles, function as hubs to facilitate traffic flow. The citric acid cycle is the biochemical hub of the cell, oxidizing carbon fuels, usually in the form of acetyl CoA, and serving as a source of precursors for biosynthesis.

In Chapter 18, we learned how glucose is metabolized to acetyl CoA. This reaction, catalyzed by the pyruvate dehydrogenase complex, is the irreversible link between glycolysis and the citric acid cycle. Here, we will learn a common fate of acetyl CoA when oxygen is present and energy is required: the complete combustion of the acetyl group by the citric acid cycle.

What is the function of the citric acid cycle in transforming fuel molecules into ATP? Recall that fuel molecules are carbon compounds that are capable of being oxidized—of losing electrons. These fuel molecules are first processed to acetyl CoA, the actual fuel for the citric acid cycle. The citric acid cycle includes a series of oxidation–reduction reactions that ultimately result in the oxidation of the acetyl group to two molecules of carbon dioxide. These oxidations generate high-transfer-potential or high-energy electrons that will be used to power the synthesis of ATP. The function of the citric acid cycle is the harvesting of high-energy electrons from carbon fuels.

We begin with an examination of the cycle itself, noting that it consists of two parts: one part oxidizes carbon atoms to CO2 and the other regenerates oxaloacetate. We then see how this metabolic hub is regulated, and we end with an examination of the glyoxylate cycle, a cycle unique to plants and some microorganisms that uses reactions of the citric acid cycle.

344