Group Transfer Potentials

The concept of group transfer potential is not particularly novel. Other kinds of transfer (of hydrogen ions and electrons, for example) are commonly... 

The free energy change, which we henceforth call the group transfer potential, is given by... 

Even this set of equations represents an approximation, because ATP, ADP, and Pi all exist in solutions as a mixture of ionic species. This problem is discussed in a later section. For now, it is enough to note that the free energy changes listed in Table 3.3 are the group transfer potentials observed for transfers to water. 

Several additional points should be made. First, although oxygen esters usually have lower group-transfer potentials than thiol esters, the O—acyl bonds in acylcarnitines have high group-transfer potentials, and the transesterification reactions mediated by the acyl transferases have equilibrium constants close to 1. Second, note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis (Chapter 25), and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate, as well as some amino acids. 

Figure 30-17. Formation of S-adenosylmethionine. -CHj represents the high group transfer potential of "active methionine."...

Nucleoside Triphosphates Have High Group Transfer Potential... 

Nucleoside triphosphates have high group transfer potential and participate in covalent bond syntheses. The cyclic phosphodiesters cAMP and cGMP function as intracellular second messengers. 

Acyl-coenzyme A compounds, which, because of their high acetyl group transfer potential represent carriers of activated acyl groups in biological systems, are prepared from the corresponding carboxylic acid, CDI, and coenzyme a.[4h5],[177],[182],[183]... 

In the second step, the amino acid is bonded to the 2 or 3 -OH function of ribose in the form of an ester, via transfer to the tRNA and the terminal adenosine residue of the tRNA. This ester formation gives a higher-energy bond (or to be more exact, a bond with a high group transfer potential) with a AG°-value of -29 kJ/mol. 

Some examples of cofactors are collected in Table 2 and include inorganic ions as iron(II), magnesium(II), calcium(II), zinc(II), etc compounds of high group transfer potential such as ATP and GPT involved in energy coupling with cells ... 

As discussed on p. 16, the group transfer potential can be expressed quantitatively as the change in free enthalpy (AG) during hydrolysis of the compound concerned. This is an arbitrary determination, but it provides important indications of the chemical energy stored in such a group. In the case of acetyl-CoA, the reaction to be considered is ... 

Inorganic polyphosphate, present in all cells, may serve as a reservoir of phosphoryl groups with high group transfer potential. 

In the second glycolytic reaction that generates a compound with high phosphoryl group transfer potential, enolase promotes reversible removal of a molecule of water from 2-phosphoglycerate to yield phosphoenolpyruvate (PEP) ... 

The biosynthesis of fatty acids such as palmitate thus requires acetyl-CoA and the input of chemical energy7 in two forms the group transfer potential of ATP and the reducing power of NADPH. The ATP is required to attach C02 to acetyl-CoA to make malonyl-CoA the NADPH is required to reduce the double bonds. We return to the sources of acetyl-CoA and NADPH soon, but first let s consider the structure of the remarkable enzyme complex that catalyzes the synthesis of fatty acids. 

From the foregoing considerations we see that complexing with Mg2+ somewhat decreases the group transfer potential of the phospho group of ATP. Furthermore, changes in the concentration of free Mg2+ with time and between different regions of a cell may... 

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