Activation energy Active acetate

Hisatsune and co-workers [290—299] have made extensive kinetic studies of the decomposition of various ions in alkali halide discs. Widths and frequencies of IR absorption bands are an indication of the extent to which a reactant ion forms a solid solution with the matrix halide. Sodium acetate was much less soluble in KBr than in KI but the activation energy for acetate breakdown in the latter matrix was the larger [297]. Shifts in frequency, indicating changes in symmetry, have been reported for oxalate [294] and formate [300] ions dispersed in KBr. 

By using activated malonyl groups in the synthesis of fatty acids and activated acetate in their degradation, the cell makes both processes energetically favorable, although one is effectively the reversal of the other. The extra energy required to make fatty acid synthesis favorable is provided by the ATP used to synthesize malonyl-CoA from acetyl-CoA and HCO3 (Fig. 21-1). 

The oxidation by 02 of [H] (either as NADH or FADH, succinate or directly as active acetic acid) occurs via a sequence of electron transfer steps between redox components being incorporated (more or less deeply) into the inner mitochondrial membrane. As is shown in Fig. 11 in the above mentioned electron transport chain, referred to as respiratory chain, the electronic energy is successively decreased step by step. Oxygen is introduced only into the last step of the chemical events in the respiratory chain. Nature has developed a special enzyme system, the cytochrome oxidase, for the realization of oxygen reduction to water. It is assumed that this enzyme system provides the indispensible cooperation of four electrons and protons, respectively, which is required for oxygen reduction ... 

In 1951, Feodor Lynen (1911-1979) and his coworker E. Reichert demon-started that S-acetyl coenzyme A is a more generally implicated form of active acetate than acetyl phosphate that was recognized in this role by Fritz Lipmann in 1940. The thiol ester character of 5-acetyl Co A called the attention of Th. Wieland to energy-rich S-acyl compounds as promising intermediates for the formation of the peptide bond. In 1951, the same year when the isolation of 5-acetyl CoA was published [3], Wieland and his coworkers described [4] the preparation of thiophenyl esters of benzyloxycarbonyl-amino acids and benzyloxycarbonyl-peptides and their application in the synthesis of blocked peptides ... 

Many anaerobic microorganisms can use CO or CO2 as a sole source of carbon and CO and/or H2 for the generation of energy [1]. Thus, acetogens generate acetyl coenzyme A, an activated acetic acid that serves as a universal precursor for the generation of biomass (see below), and acetic acid from... 

In addition, a small amount of the thioester CH3COSCH3 was also formed, which was proposed to act as an activated acetic acid intermediate in the mechanism of Eq. 7, in analogy with the formation of acetyl-CoA catalyzed by ACS (Eq. 1). In the absence of Ni no reaction was observe indicating that Fe alone does not afford carbon flxation imder the reported conditions. Table 1 shows the free energies of these and other selected reactions. 

It was evident from early studies that in the presence of CoASH, ATP could in some way be used to activate acetate so that it will acetylate sulfanilamide (7) and choline (8). Chou and Lipmann (9) succeeded in partially purifying the enzyme(s) responsible for this activation from pigeon-liver extracts and they concluded that the phosphate bond energy of ATP is utilized to bring about the synthesis of acetyl coenzyme A (acetyl-SCoA) however, the mechanism of this activation remained obscure. The nature of the over-all process was further elucidated through the experiments of Lipmann et al. (10) who demonstrated that ATP, CoASH, and acetate react to form acetyl-SCoA, AMP, and inorganic pyrophosphate (P-P) in stoichiometric amounts [reaction (4)]. They also demonstrated that the reaction is freely reversible. More recently, the studies of Jones et al. (11) have indicated that the mechanism of this conversion is as follows ... 

Of analogous metabolic significance are cyclic chains of transformations ending in regeneration of catalytically active components, for example, the Krebs cycle as a whole here OA accepts PU (or an active acetate residue) by condensing with it to isocitrate, and is reconstituted in a chain of reactions, the net result of which is the complete oxidation of PU. Important energy-absorbing synthetic steps are associated with the cyclic... 

Looking back at this period of my work on acetyl-CoA I realize that most of the biochemists working on active acetate were prejudiced by the idea that active acetate could only be some kind of a reactive but also labile phosphate derivative. The idea of the energy-rich phosphate bond was dominating the biochemical thinking. With the discovery of the thiolester bond in acetyl CoA the energy-rich phosphate bond had lost its unique role in metabolic reactions, and it was to be expected that still other energy-rich bonds would be discovered. Indeed, this has happened in the meantime. 

Coenzyme A can form high-energy bonds with acetic acid via its sulfhydryl group to yield acetyl-coenzyme A (acetyl-CoA), also referred to as activated acetate. Through a variety of biochemical sequences, CoA can also be converted to a number of other acyl-CoA derivatives, such as malonyl-CoA, methylmalo-... 

HS-CoA Coenzyme A. HS-CoA accepts the acetyl residue with its sulphydryl (HS) group. The acetyl residue linked to CoA through an energy-rich thiol ester bond is known as acetyl CoA or active acetate. This active acetate is a key substance in metabolism, to which we shall often have need to return. We are indebted to Lynen for a major part of our knowledge of this nodal point of metabolism. 

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). 

In the citrate cycle, water is taken up and one molecule of activated acetate is broken down into 2 CO2 and hydrogen which is bound to the coenzymes (4- [2 H]). The combined steps yield a relatively small decrease of free energy AF° = — 25... 

Under standard conditions this corresponds to a total amount of energy of 12 X 7 = 84 kcal/mole of activated acetate or an eflSciency of about 40% of the theoretically available free energy. 

Energy Balance of the Aerobic Carbohydrate Breakdown. Up to the formation of pyruvate 1 mole of ATP and 1 of NADHa arise from each triose. Another mole of NAD is reduced during the oxidative decarboxylation of pyruvate to acetyl-CoA. Up to the formation of acetyl-CoA (employing the respiratory chain) 1 - - 2 X 3 = 7 ATP are stored. Complete oxidation of active acetate in the citrate cycle yields another 12 moles of ATP per triose, i.e. a total of 19 ATP per mole of triose or 38 per mole of glucose. 

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