Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Types of reactions catalyzed by enzymes
Multiple displacement reactions and the coupling of ATP cleavage reactions with endergonic processes
Transfer of phosphate, pyrophosphate, and adenylyl groups from ATP

Many enzymatic reactions, including most of those coupling Biosynthesis to ATP Cleavage, require a combination of Two Types of substitution reactions: substitution at a phosphorus atom followed by substitution at a carbon atom. However, for the group-transfer potential of the ATP molecule to drive an endergonic metabolic process, a coupling mechanism must be in place. Otherwise, cellular ATP Hydrolysis would simply result in the dissipation of heat. A vital component of this coupling mechanism is typically a nucleophilic substitution at a phosphorus atom followed by substitution at a carbon atom.

The First stage in coupling ATP cleavage to another reaction involves The transfer of a portion of the ATP molecule itself to a nucleophile Y. This transfer is generally accomplished via substitution at one of the three phosphorus atoms (Fig. 7-7). Nucleophilic attack may occur at the terminal phosphorus atom (Py) with the displacement of ADP, or at the internal

phosphorus atom (Pa) with the displacement of inorganic pyrophosphate. In the former case, there is formed

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whereas in the latter, a Y-adenylyl derivative (the shorter, non-recommended name Y-adenyl is frequently used). Much less frequently, substitution takes place at the middle phosphorus atom (Pß), resulting in the transfer of a pyrophosphate group to the nucleophile. Even rarer is Substitution at the C-5' atom (Fig. 7-7), as depicted in equation (11-3). If H2O acts as the nucleophile Y in one of these substitution reactions, the resulting hydrolysis tends to go to completion—meaning that all groups of ATP (phosphate, adenylate, or pyrophosphate) are characterized by a high group-transfer potential (Table 3-5). If Y is the —OH group of a standard alcohol, the transfer reaction also goes to completion because a phosphoric acid ester possesses a relatively low group-transfer potential. Consequently, ATP-dependent phosphorylation reactions introduce virtually irreversible steps into metabolic pathways.

FIG. 7-7. Four pathways of ATP cleavage.



Last update: 06/08/2026

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