Acetyl phosphate high energy bond

Fig. 19.7. Some compounds with high-energy bonds. 1,3-bisphosphoglycerate and phosphoenolpyruvate are intermediates of glycolysis. Creatine phosphate is a high-energy phosphate reservoir and shuttle in brain, muscle, and spermatozoa. Acetyl CoA is a precursor of the TCA cycle. The high-energy bonds are shown in blue.

The following abbreviations will be used in this article. ATP = adenosine triphosphate ADP — adenosine diphosphate DPNox oxidized diphosphopyri-dine nucleotide DPNred = reduced diphosphopyridine nucleotide CoA or CoA—SH = coenzyme A CoA—S—COCH3 = acetyl-coenzyme A FAD = flavin adenine dinucleotide R—P = low-energy phosphate bond R P = high-energy bond Pi = inorganic orthophosphate. 

Reaction 41 as written is highly exergonic, since only 16,000 cal. are required for the synthesis of acetoacetate. One mole of acetyl phosphate should furnish sufficient energy to allow the reaction to proceed. Here two high-energy bonds are dissipated, which may seem wasteful from a teleological point of view. However, it is possible that mechanisms exist in the intact tissues for the recovery of this energy by some type of feedback mechanism. 

Acetyl CoA can enter into acetylation reactions either through the carboxyl carbon, as in the formation of acetylsulfanilamide (a so-called head condensation), or through the methyl carbon, as in citrate formation (a so-called tail condensation). The formation of acetoacetate in a mixed bacterial-pigeon liver system involves a head and tail condensation of two molecules of acetyl CoA. In this system, acetoacetate is formed from acetyl phosphate, a bacterial metabolite with a high-energy bond of AF = about 15,000 cal. per mole, by the following reactions ... 

Sources of carbons, reducing equivalents, and energy sources As with fatty acids, all the carbon atoms in cholesterol are provided by acetate, and NADPH provides the reducing equivalents. The pathway is driven by hydrolysis of the high-energy thioester bond of acetyl CoA and the terminal phosphate bond of ATP. 

Fig. 20.18. Pyruvate carboxylase reaction. Pyruvate carboxylase adds a carboxyl group from bicarbonate (which is in equihbrium with CO2) to pyruvate to form oxaloacetate. Biotin is used to activate and transfer the CO2. The energy to form the covalent biotin-C02 complex is provided by the high-energy phosphate bond of ATP, which is cleaved in the reaction. The enzyme is activated by acetyl CoA.

The total energy yield from the oxidation of 1 mole of palmityl CoA to 8 moles of acetyl CoA is therefore 28 moles of ATP 1.5 for each of the 7 FAD(2H), and 2.5 for each of the 7 NADH. To calculate the energy yield from oxidation of 1 mole of palmitate, two ATP need to be subtracted from the total because two high-energy phosphate bonds are cleaved when palmitate is activated to palmityl CoA. 

Starting -with acetyl CoA, what is the approximate yield of high-energy phosphate bonds (net ATP formed) via the glyoxylate cycle ... 

The ligases, as the name implies, catalyse reactions where two molecules are bound together, with the breakdown of high-energy phosphate bonds such as in ATP, which provides the energy for the reaction to take place. The production of acetyl-CoA from acetate by acetyl coenzyme A synthetase is typical ... 

The extended nine-carbon backbone of sialic acids can be constructed from hexose building blocks by aldol addition of a pyruvate unit. Synthetic studies for sialic acid and its modifications have extensively used the catabolic enzyme NeuA, which catalyzes the reversible addition of pyruvate (5) to N-acetyl-D-mannosamine (ManNAc, 4) to form the parent siahc acid NeuSAc (1 Scheme 17.4) [16, 17, 19]. These freely reversible aldol additions have equilibrium constants in favor of cleavage direction [20], which requires that synthetic reactions have to be driven by an excess of one substrate to achieve satisfactory conversions for economic reasons, this usually is 5. In contrast, NeuS utilizes PEP (6) as a high-energy nucleophile, which upon C-C bond formation releases inorganic phosphate and thus renders the addition essentially irreversible [21]. Despite its considerable synthetic potential, NeuS still is an orphan catalyst which so far has been less studied for preparative applications [22]. 

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