Citric acid cycle energy yield

Fats, carbohydrates, and proteins are metabolized in the body to yield acetyl CoA, which is further degraded in the citric acid cycle to yield two molecules of CO2 plus a large amount of energy. The energy output of the various steps in the citric acid cycle is coupled to the electron-transport chain, a series of enzyme-catalyzed reactions whose ultimate purpose is to synthesize adenosine triphosphate (ATP). 

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... 

STAGE 3 Acetyl CoA is oxidized in the citric acid cycle to yield CO2 and energy. 

Four of the B vitamins are essential in the citric acid cycle and therefore in energy-yielding metabolism (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor in the a-ketoglutarate dehydrogenase complex and in succinate dehydrogenase (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD),... 

In most tissues, where the primary role of the citric acid cycle is in energy-yielding metabohsm, respiratory control via the respiratory chain and oxidative phosphorylation regulates citric acid cycle activity (Chapter 14). Thus, activity is immediately dependent on the supply of NAD, which in turn, because of the tight couphng between oxidation and phosphorylation, is dependent on the availabihty of ADP and hence, ulti-... 

As noted earlier, although the citric acid cycle is central to energy-yielding metabolism its role is not limited to energy conservation. Four- and five-carbon intermediates of the cycle serve as precursors for a wide variety of products. To replace intermediates removed for this purpose, cells employ anaplerotic (replenishing) reactions, which are described below. 

Eugene Kennedy and Albert Lehninger showed in 1948 that, in eulcaiyotes, the entire set of reactions of the citric acid cycle takes place in mitochondria. Isolated mitochondria were found to contain not only all the enzymes and coenzymes required for the citric acid cycle, but also all the enzymes and proteins necessaiy for the last stage of respiration—electron transfer and ATP synthesis by oxidative phosphoiylation. As we shall see in later chapters, mitochondria also contain the enzymes for the oxidation of fatty acids and some amino acids to acetyl-CoA, and the oxidative degradation of other amino acids to a-ketoglutarate, succinyl-CoA, or oxaloacetate. Thus, in nonphotosynthetic eulcaiyotes, the mitochondrion is the site of most energy-yielding... 

In examining the eight successive reaction steps of the citric acid cycle, we place special emphasis on the chemical transformations taking place as citrate formed from acetyl-CoA and oxaloacetate is oxidized to yield C02 and the energy of this oxidation is conserved in the reduced coenzymes NADH and FADH2. 

The same intermediates of glycolysis and the citric acid cycle that activate isocitrate dehydrogenase are allosteric inhibitors of isocitrate lyase. When energy-yielding metabolism is sufficiently fast to keep the concentrations of glycolytic and citric acid cycle intermediates low, isocitrate dehydrogenase is inactivated, the inhibition of isocitrate lyase is relieved, and isocitrate flows into the glyoxylate pathway, to be used in the biosynthesis of carbohydrates, amino acids, and other cellular components. 

Energy Yield from the Citric Acid Cycle The reaction catalyzed by succinyl-CoA synthetase produces the high-energy compound GTP. How is the free energy contained in GTP incorporated into the cellular ATP pool ... 

In the second stage of fatty acid oxidation, the acetyl-CoA is oxidized to C02 in the citric acid cycle. A large fraction of the theoretical yield of free energy from fatty acid oxidation is recovered as ATP by oxidative phosphorylation, the final stage of the oxidative pathway. 

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