Carbon fuels acid cycle

In plants, certain invertebrates, and some microorganisms (including E. coli and yeast) acetate can serve both as an energy-rich fuel and as a source of phosphoenolpyruvate for carbohydrate synthesis. In these organisms, enzymes of the glyoxylate cycle catalyze the net conversion of acetate to succinate or other four-carbon intermediates of the citric acid cycle ... 

With the regeneration of oxaloacetate in step 8, the cycle of reactions is completed, and the oxaloacetate condenses with another molecule of acetyl-CoA to commence another turn of the cycle. However, note that there is actually no beginning or end to the citric acid cycle (Fig. 12-2) if any of the intermediates are produced within the mitochondria or gain access to the mitochondria, they can participate in the cycle of reactions but they will always be regenerated. The only actual fuel for the cycle is acetyl-CoA and this is not regenerated its carbon atoms are liberated at steps 3 and 4 as C02. 

The Krebs-citric acid cycle is the final common pathway for the oxidation of fuel molecules amino acids, fatty acids and carbohydrates. Most fuel molecules enter the cycle as a breakdown product, acetyl coenzyme A (acetyl CoA), which reacts with oxaloacetate (a four-carbon compound) to produce citrate (a six-carbon compound), which is then converted in a series of enzyme-catalysed steps back to oxaloacetate. In the process, two molecules of carbon dioxide and four energy-rich molecules are given off, and these latter are the precursors of the energy-rich molecule ATP, which is subsequently formed and which acts as the fuel source for all aerobic organisms. 

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 (Chapter 14). The citric acid cycle includes a series of oxidation-reduction reactions that result in the oxidation of an acetyl group to two molecules of carbon dioxide. 

Figure 17.3. Cellular Respiration. The citric acid cycle constitutes the first stage in cellular respiration, the removal of high-energy electrons from carbon fuels (left). These electrons reduce O2 to generate a proton gradient (middle), which is used to synthesize ATP (right). The reduction of O2 and the synthesis of ATP constitute oxidative phosphorylation.

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, as well as serving as a source of precursors for biosynthesis. [(Above) Chris Warren/Intemational Stock.]... 

Acetyl CoA is the fuel for the citric acid cycle. This important molecule is formed from the breakdown of glycogen (the storage form of glucose), fats, and many amino acids. Indeed, as we will see in Chapter 22. fats contain strings of reduced two-carbon units that are first oxidized to acetyl CoA and then completely oxidized to CO2 by the citric acid cycle. 

Oxidative phosphorylation is the culmination of a series of energy transformations that are called cellular respiration or simply respiration in their entirety. First, carbon fuels are oxidized in the citric acid cycle to yield electrons with high transfer potential. Then, this electron-motive force is converted into a proton-motive force and, finally, the proton-motive force is converted into phosphoryl transfer potential. The conversion of electron-motive force into proton-motive force is carried out by three electron-driven proton pumps—NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and... 

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Acetylcoenzyme A supplies the main fuel for the tricarboxylic acid cycle (see Fig. 5), which is the principal route by which carbohydrate is oxidized to carbon dioxide and water. The effect of one turn of the cycle is the simple oxidation of a molecular unit of acetate. 

Amino acids in excess of those needed for biosynthesis can neither be stored, in contrast with fatty acids and glucose, nor excreted. Rather, surplus amino acids are used as metabolic fuel. The a-amino group is removed, am/ the resulting carbon skeleton is converted into a major metabolic intermediate. Most of the amino groups harvested from surplus amino acids are converted into urea through the urea cycle, whereas their carbon skeletons are transformed into acetyl Co A, acetoacetyl C oA, pyruvate, or one of the intermediates of the citric acid cycle. The principal fate of the carbon skeletons is conversion into glucose and glycogen. 

This reaction, which produces oxaloacetate from pyruvate, provides a connection between the amphibolic citric acid cycle and the anabolism of sugars by gluconeogenesis. On this same topic of carbohydrate anabolism, we should note again that pyruvate cannot be produced from acetyl-GoA in mammals. Because acetyl-GoA is the end product of catabolism of latty acids, we can see that mammals could not exist with fats or acetate as the sole carbon source. The intermediates of carbohydrate metabolism would soon be depleted. Garbohydrates are the principal energy and carbon source in animals (Figure 19.11), and glucose is especially critical in humans because it is the preferred fuel for our brain cells. Plants can carry out the conversion of acetyl-GoA to pyruvate and oxaloacetate, so they can exist without carbohydrates as a carbon source. The conversion of pyruvate to acetyl-GoA does take place in both plants and animals (see Section 19.3). 

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