Bioenergetics: Phosphoryl group transfers and ATP hydrolysis

Adenosine 5’-triphosphate (ATP) serves as the main source of free energy in living cells. The energy stored by ATP can be liberated through direct hydrolysis or by group transfer. The structure of ATP is shown in Figure 1.

Figure 1 Molecular structure of ATP

Direct hydrolysis of ATP (Equation 1) consists of nucleophilic attack by ${\text{H}}_2\text{O}$‍ at the $\gamma$‍ phosphate position of ATP and cleavage of the $\gamma -\beta$‍ phosphoanhydride bond. Direct hydrolysis of ATP liberates energy mainly in the form of heat. This heat energy can be used to cycle proteins through different conformations (for instance, in muscle contraction).

Group transfer reactions involve the covalent transfer of a portion of the ATP molecule to a substrate. Table 1 classifies the three group transfer reactions that involve ATP by the phosphate position of ATP at which nucleophilic attack occurs.

Table 1 Group transfer reactions involving ATP

Adenylyl transfer has the largest negative standard free energy change and is commonly coupled to biological reactions that have a particularly large positive standard free energy change. One example is the coupling of exergonic adenylylation of fatty acid (Equation 2) and pyrophosphatase-catalyzed pyrophosphate hydrolysis (Equation 3) to endergonic synthesis of fatty acyl-CoA (Equation 4).