In order to maintain living processes, all organisms must obtain supplies of free energy from their environment. Autotrophic organisms utilize simple exergonic processes; for example, the energy of sunlight (green plants), the reaction Fe²⁺ → Fe³⁺ (some bacteria). On the other hand, heterotrophic organ isms obtain free energy by coupling their metabolism to the breakdown of complex organic molecules in their environment. In all these organisms, ATP plays a central role in the transference of free energy from the exergonic to the endergonic processes. ATP is a nucleotide consisting of the nucleoside adenosine (adenine linked to ribose) and three phosphate groups. In its reactions in the cell, it functions as the Mg²⁺ complex (see Figure 1).

Fig1. Adenosine triphosphate (ATP) is shown as the magnesium complex.

The Intermediate Value for the Free Energy of Hydrolysis of ATP Has Important Bioenergetic Significance

 The standard free energy of hydrolysis of a number of biochemically important phosphates is shown in Table 1. An estimate of the comparative tendency of each of the phosphate groups to transfer to a suitable acceptor may be obtained from the ΔG^0′ of hydrolysis at 37°C. This is termed the group trans fer potential. The value for the hydrolysis of the terminal phosphate of ATP (when ATP is converted to ADP + Pi) divides the list into two groups. Low-energy phosphates, having a low group transfer potential, exemplified by the ester phosphates found in the intermediates of glycolysis, have G^0′ values smaller than that of ATP, while in high-energy phosphates, with a more negative G^0′, the value is higher than that of ATP. The components of this latter group, including ATP, are usually anhydrides (eg, the 1-phosphate of 1,3-bisphosphoglycerate), enol phosphates (eg, phosphoenolpyruvate), and phosphoguanidines (eg, creatine phosphate, arginine phosphate).

Table1. Standard Free Energy of Hydrolysis of Some Organophosphates of Biochemical Importance

The symbol ~ P indicates that the group attached to the bond, on transfer to an appropriate acceptor, results in trans fer of the larger quantity of free energy. Thus, ATP has a high group transfer potential, whereas the phosphate in adenosine monophosphate (AMP) is of the low-energy type since it is a normal ester link (Figure 2). In energy transfer reactions, ATP may be converted to ADP and P_i or, in reactions requiring a greater energy input, to AMP + PP_i (see Table 1).

Fig2. Structure of ATP, ADP, and AMP showing the position and the number of high-energy phosphates (~P  ).

The intermediate position of ATP allows it to play an important role in energy transfer. The high free-energy change on hydrolysis of ATP is not in itself caused by the breaking of the P-O bond linking the terminal phosphate to the molecule (see Figure 2), in fact, energy is needed to bring this about. It is the consequences of this bond breakage that cause net energy to be released. Firstly, there is strong electro static repulsion between the negatively charged oxygen atoms in the adjacent phosphate groups of ATP (see Figure 2), which destabilizes the molecule and makes the removal of one phosphate group energetically favorable. Secondly, the ortho phosphate produced is greatly stabilized by the formation of resonance hybrids in which the three negative charges are shared between the four oxygen atoms. Overall, therefore, the products of hydrolysis, ADP and orthophosphate, are more stable, and so lower in energy, than ATP (Figure 3). Other “high-energy compounds” are thiol esters involving coenzyme A (eg, acetyl-CoA), acyl carrier protein, amino acid esters involved in protein synthesis, S-adenosylmethionine (active methionine), uridine diphosphate glucose (UDPGlc), and 5-phosphoribosyl-1-pyrophosphate (PRPP).

Fig3. The free-energy change on hydrolysis of ATP to ADP. Initially, energy input is required to break the terminal P-O bond. However, the breaking of the bond relieves the strong electrostatic repulsion between the negatively charged oxygen atoms in the adjacent phosphate groups of ATP, making the removal of one phosphate group energetically favorable. In addition, the orthophosphate released is greatly stabilized by the formation of resonance hybrids in which the three negative charges are shared between the four oxygen atoms. These effects more than compensate for the initial energy input and result in the high free-energy change seen when ATP is hydrolyzed to ADP.