Citric acid cycle overview

Figure 29.1 An overview of catabolic pathways for the degradation of food and the production of biochemical energy. The ultimate products of food catabolism are C02 and H2O, with the energy released in the citric acid cycle used to drive the endergonic synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) plus phosphate ion, HOPO32-.

FIGURE 22-9 Overview of amino acid biosynthesis. The carbon skeleton precursors derive from three sources glycolysis (pink), the citric acid cycle (blue), and the pentose phosphate pathway (purple). 

Systems of biochemical reactions like glycolysis, the citric acid cycle, and larger and smaller sequential and cyclic sets of enzyme-catalyzed reactions present challenges to make calculations and to obtain an overview. The calculations of equilibrium compositions for these systems of reactions are different from equilibrium calculations on chemical reactions because additional constraints, which arise from the enzyme mechanisms, must be taken into account. These additional constraints are taken into account when the stoichiometric number matrix is used in the equilibrium calculation via the program equcalcrx, but they must be explicitly written out when the conservation matrix is used with the program equcalcc. The stoichiometric number matrix for a system of reactions can also be used to calculate net reactions and pathways. 

Figure 17.2. Overview of the Citric Acid Cycle. The citric acid cycle oxidizes two-carbon units, producing two molecules of CO2, one molecule of GTP, and high-energy electrons in the form of NADH and FADH2.

Conceptual Insights, Overview of Carbohydrate and Fatty Acid Metabolism. View this media module to gain a "bigger picture" understanding of the roles of the pentose phosphate pathway in the context of other metabolic pathways (glycolysis, citric acid cycle, glycogen and fatty acid metabolism). 

Overview of Anabolism The Citric Acid Cycle as a Source of Biosynthetic Intermediates... 

Overview of Pyruvate Oxidation and the Citric Acid Cycle The Three Stages of Respiration (Figure 14.2)... 

The citric acid cycle has two other common names. One is the Krebs cycle, after Sir Hans Krebs, who first investigated the pathway (work for which he received a Nobel Prize in 1953). The other name is the tricarboxylic acid cycle (or TCA cycle), from the fact that some of the molecules involved are acids with three carboxyl groups. We shall start our discussion with a general overview of the pathway and then go on to discuss specific reactions. 

Goenzymes are introduced in this chapter and are discussed in later chapters in the context of the reactions in which they play a role. Chapter 16 discusses carbohydrates. Chapter 17 begins the overview of the metabohc pathways by discussing glycolysis. Glycogen metabolism, gluconeogenesis, and the pentose phosphate pathway (Chapter 18) provide bases for treating control mechanisms in carbohydrate metabolism. Discussion of the citric acid cycle is followed by the electron transport chain and oxidative phosphorylation in Chapters 19 and 20. The catabolic and anabolic aspects of lipid metabohsm are dealt with in Chapter 21. In Chapter 22, photosynthesis rounds out the discussion of carbohydrate metabolism. Chapter... 

Through the reactions of the citric acid cycle, the carbon atoms of the acetyl group of acetyl-CoA are oxidized to carbon dioxide. There are four separate oxidations in the cycle, three involving NAD and one involving FAD. Figure 21.3 gives an overview of the cycle, showing the four steps. 

Figure 5.17 An overview of the citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle).

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