Krebs Citric Acid synthesis

The tricarboxylic acid (TCA) cycle (also known as the citric acid cycle and the Krebs cycle) is a collection of biochemical reactions that oxidize certain organic molecules, generating CO2 and reducing the cofactors NAD and FAD to NADH and FADH2 [147], In turn, NADH and FADH2 donate electrons in the electron transport chain, an important component of oxidative ATP synthesis. The TCA cycle also serves to feed precursors to a number of important biosynthetic pathways, making it a critical hub in metabolism [147] for aerobic organisms. Its ubiquity and importance make it a useful example for the development of a kinetic network model. 

Citrate synthase, one of the enzymes in the series of enzyme-catalyzed reactions known as the Krebs cycle, catalyzes the s3mthesis of citric acid from oxaloacetic acid and acetyl-CoA. If the synthesis is carried out with acetyl-CoA that has radioactive carbon... 

The synthesis of fumarate is a link between the urea cycle and the citric acid cycle. Fumarate is, of course, an intermediate of the citric acid cycle, and it can be converted to oxaloacetate. A transamination reaction can convert oxa-loacetate to aspartate, providing another link between the two cycles (Figure 23.19). In fact, both pathways were discovered by the same person. Flans Krebs. Four high-energy phosphate bonds are required because of the production of pyrophosphate in the conversion of aspartate to argininosuccinate. 

Reaction (a) is the sum of at least two reactions. It is catalyzed by enzyme fractions A and B and also requires diphosphothiamine. Reaction (a) can be coupled with either reaction (b), in which acetylphosphate is formed in the presence of transacetylase, or with reaction (c), which requires the condensing enzyme. In animal tissues, which contain no transacetylase, reaction (a) is coupled with reaction (c). Further discussion of the enzymatic mechanisms involved in the synthesis of citric acid can be found in reviews by Krebs (1949), Ochoa (1951), and Barker (1951). 

Primary metabolites can originate from fundamental processes photosynthesis, glycolysis, and the citric acid cycle (Krebs cycle). They represent biosynthetic intermediates useful as building blocks for the synthesis of secondary metabolites. The latter can be synthesized through a combination of various building blocks (Figure 1.3) ... 

Both cofactors are involved in respiratory electron transfer systems (Figure 5.19), for example, in most redox reactions of the citric acid (Krebs) cycle. NAD is most often involved in degradation (catabolism) of sugars, fats, proteins and ethanol, while NADP is involved mainly in biosynthetic (anabolic) reactions, such as synthesis ofmacromolecules, fatty acids and cholesterol. Dinucleotides, in addition of their activities in redox reactions, participate in post-translational modifications of some proteins and other reactions. 

There is a number of similar procedures starting fntm olefinic diols which by reaction with dinitrogen tetraoxide give citric acid. Sargsyan et al. [11] presented a short account of all known until 1989 synthetic preparations of citric acid. Their paper is based mainly on the patent literature and shows that with an exception of old classical methods, most of other ways to obtain citric acid is characterized by relatively low yield. Evidently, in the context of the Krebs tricarboxylic acid cycle, there is a large number of investigations dealing with enzymatic synthesis of citric acid by condensation of acetate and oxalacetate [12-20]. 

The breakdown of activated acetate itself takes place in the citric acid cycle (Krebs cycle tricarboxylic acid cycle). In this cycle the pathways of protein, fat, and carbohydrate catabolism are united. Furthermore, the cycle provides many of the necessary components for the synthesis of endogenous substances. The citric acid cycle therefore encompasses a large pool of common intermediates, which can be used either for synthesis of new cell material or for degradation to gain energy. The full significance of these interrelationships will be demonstrated later (cf. Chapt. XVIII). 

The citric acid cycle (also known as the tricarboxylic acid cycle, TCA cycle, Krebs cycle) oxidizes acetyl CoA in mitochondria. The cycle produces CO2, NADH and FADH2. The NADH and FADH2 enter oxidative phosphorylation, where they are oxidized to NAD+ and FAD, ready to be used in the citric acid cycle again. The citric acid cycle is also important in some biosynthetic processes such as lipid synthesis, amino acid synthesis, porphyrin synthesis and gluconeogenesis. 

The citric acid cycle is a series of reactions that connects the intermediate acetyl-CoA from the metabolic pathways in stages 1 and 2 with electron transport and the synthesis of ATP in stage 3. As a central pathway in metabolism, the citric acid cycle uses the two-carbon acetyl group in acetyl-CoA to prodnce CO2, NADH + and FADH2. The citric acid cycle is named for the citrate ion from citric acid (C6Hg07), a tricarboxylic acid, which forms in the first reaction. The citric acid cycle is also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, named for H. A. Krebs, who recognized it in 1937 as the major metabolic pathway for the prodnction of energy. 

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