Tricarboxylic acids cycle

Acetyl-CoA (with COj released) Acetyl-CoA + recycled Oxaloacetate (plus H2O less CoASH) 

FIGURE 3.4 Tricarboxylic acid cycle with enzymes and supportive reactions shown. (Based on information in Voet and Voet, 1995, Biochemistry, p. 539 Olson, in Devlin, 1986, p. 280.) 

Some other substances entering the tricarboxylic acid cycle are the following  

NADH — nicotinamide adenine dinucleotide, reduced form 

FADH2 — flavin adenine dinucleotide, hydrogen, reduced form 

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Glutamic acid is formed m most organisms from ammonia and a ketoglutaric acid a Ketoglutaric acid is one of the intermediates m the tricarboxylic acid cycle (also called the Krebs cycle) and arises via metabolic breakdown of food sources carbohy drates fats and proteins... 

The 4-phosphopantetheine group of CoA is also utilized (for essentially the same purposes) in acyl carrier proteins (ACPs) involved in fatty acid biosynthesis (see Chapter 25). In acyl carrier proteins, the 4-phosphopantetheine is covalently linked to a serine hydroxyl group. Pantothenic acid is an essential factor for the metabolism of fat, protein, and carbohydrates in the tricarboxylic acid cycle and other pathways. In view of its universal importance in metabolism, it is surprising that pantothenic acid deficiencies are not a more serious problem in humans, but this vitamin is abundant in almost all foods, so that deficiencies are rarely observed. 

Krebs, H. A., 1970. The hi.story of the tricarboxylic acid cycle. Perspectives in Biology and Medicine 14 154—170. 

Tricarboxylic acid cycle (Section 29.7) An alternative name for the citric acid cycle by which acetyl CoA is degraded to CO2. 

Tributyltin hydride, reaction with alkyl halides. 358 Tricarboxylic acid cycle, see Citric acid cycle... 

The citrate cycle is the final common pathway for the oxidation of acetyl-CoA derived from the metabolism of pyruvate, fatty acids, ketone bodies, and amino acids (Krebs, 1943 Greville, 1968). This is sometimes known as the Krebs or tricarboxylic acid cycle. Acetyl-CoA combines with oxaloacetate to form citrate which then undergoes a series of reactions involving the loss of two molecules of CO2 and four dehydrogenation steps. These reactions complete the cycle by regenerating oxaloacetate which can react with another molecule of acetyl-CoA (Figure 4). 

Robinson, J.B. Srere, P.A. (1985). Organization of Krebs tricarboxylic acid cycle enzymes in mitochondria. J. Biol. Chem. 260, 10800-10805. 

Senior, A.E. Shenatt, H.S.A. (1968). Biochemical effects of the hypoglycaemic compound pent-4-enoic acid and related non-hypoglycemic fatty acids. Oxidative phosphorylation and mitochondrial oxidation of pyruvate, 3-hydroxybutyrate and tricarboxylic acid-cycle intermediates. Biochem. J. 110,499-509. 

Purple sulfur bacteria fix carbon dioxide using the Calvin-Benson cycle, but green sulfur bacteria use a completely different pathway, the reverse tricarboxylic acid cycle. Other photosynthetic bacteria use still different pathways for CO2 fixation (Perry and Staley, 1997). 

The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a series of reactions in mitochondria that oxidize acetyl residues (as acetyl-CoA) and reduce coenzymes that upon reoxidation are linked to the formation of ATP. 

Oxidation may take place by a modified tricarboxylic acid cycle in which the production of CO2 is coupled to the synthesis of NADPH and reduced ferredoxin, and the dehydrogenation of succinate to fumarate is coupled to the synthesis of reduced menaquinone. This pathway is used, for example, by Desulfuromonas acetoxidans and in modified form by... 

Both the synthesis of propionate and its metabolism may take place under anaerobic conditions. In Desulfobulbuspropionicum, degradation could plausibly take place by reversal of the steps used for its synthesis from acetate (Stams et al. 1984)—carboxylation of propionate to methylmalonate followed by coenzyme Bi2-mediated rearrangement to succinate, which then enters the tricarboxylic acid cycle. The converse decarboxylation of succinate to propionate has been observed in Propionigenium modestum (Schink and Pfennig 1982),... 

Iron appears to play a significant role in the mechanism of action of aconitase, a constituent enzyme of the tricarboxylic acid cycle (32). 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

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