Phosphoryl-group transfer thermodynamics

ATP is not a high-energy compound in comparison with many other biological compounds. The functions of ATP depend on its having a AG value for hydrolysis that is intermediate in value compared with AG values for hydrolysis of other phosphate esters. Thus, ATP and ADP can act as a donor-acceptor pair for phosphoryl-group transfer. In many cases the free energy of ATP hydrolysis is used to support reactions that would otherwise be thermodynamically unfavorable. This usually occurs via phosphorylation of one of the reactants in an otherwise unfavorable reaction. 

This phosphotransferase [EC 2.7.2.1] catalyzes the thermodynamically favored phosphorylation of ADP to form ATP Aeq = [ATP][acetate]/ [acetyl phosphate] [ADP] = 3000). GDP is also an effective phosphoryl group acceptor. This enzyme is easily cold-denatured, and one must use glycerol to maintain full catalytic activity. Initial kinetic evidence, as well as borohydride reduction experiments, suggested the formation of an enzyme-bound acyl-phosphate intermediate, but later kinetic and stereochemicaT data indicate that the kinetic mechanism is sequential and that there is direct in-line phosphoryl transfer. Incidental generation of a metaphosphate anion during catalysis may explain the formation of an enzyme-bound acyl-phosphate. Acetate kinase is ideally suited for the regeneration of ATP or GTP from ADP or GDP, respectively. 

The hydrolysis of a thioester is thermodynamically more favorable than thai of an oxygen ester because the electrons of the C=0 bond cannot form resonance structures with the C—S bond that are as stable as those that they can form with the C—O bond. Consequently, acetyl CoA has a high acetyl group-transfer potential because transfer of the acetyl group is exergonic. Acetyl CoA carries an activated acetyl group, just as ATP carries an activated phosphoryl group. 

Thus, the y-phosphate group of ATP is transferred to some group of the protein and the enzyme phosphoryl intermediate is attacked by the selenide generating the only known direct phosphorus-selenium bond in biology. The ADP, which stays complexed to the enzyme is concomitantly hydrolyzed to AMP, and thus thermodynamically drives the overall reaction. Sulfide does not serve as a substrate even at high concentrations, so SPS is one of the few enzymes that can discriminate between its Se substrate and the sulfur homolog. Recent evidence also indicates that the actual substrate is not free selenide but rather some selenium species generated from selenocysteine via the activity of one of the selenocysteine lyases present in the cell. ... 

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