Adenosine triphosphate has two energy-rich bonds

Metabolic energy is generated by the oxidation of carbohydrate, fat, protein, and alcohol. This energy must be harnessed to drive endergonic chemical reactions, membrane transport, and muscle contraction. Nature has solved this task with a simple trick: Exergonic reactions are used for the synthesis of the energy-rich compound adenosine triphosphate (ATP), and the chemical bond energy of ATP drives the endergonic processes. In this sense, ATP serves as the energetic currency of the cell (Fig. 5.1).

Figure 5.1 The function of adenosine triphosphate (ATP) as the “energetic currency” of the cell. ADP, Adenosine diphosphate; P_i, inorganic phosphate.

ATP is a ribonucleotide, one of the precursors for ribonucleic acid (RNA) synthesis. It does not contain a vitamin, and the whole molecule can be synthesized from simple precursors (see Chapter 28). Its most important part is a string of three phosphate residues, bound to carbon 5 of ribose and complexed with a magnesium ion (Figs. 5.2 and 5.3).

Figure 5.2 Sequential hydrolysis of ATP. ΔG⁰′, Standard free energy change; P_i, inorganic phosphate.

Figure 5.3 Magnesium complexes formed by adenosine triphosphate (ATP). Complexes are the actual substrates of ATP-dependent enzymes.

The first phosphate is linked to ribose by a phosphate ester bond, but the two bonds between the phosphates are energy-rich phosphoanhydride bonds. The free energy changes shown in Figure 5.2 apply to standard conditions. The actual free energy change for the hydrolysis of ATP to ADP + inorganic phosphate (P_i) depends on pH, ionic strength, and the concentrations of ATP, ADP, phosphate, and magnesium. It is close to −11 or −12 kcal/mol under “real-cell” conditions. ATP can be hydrolyzed to ADP and phosphate:

or to adenosine monophosphate (AMP) and inorganic pyrophosphate (PP_i):

The PP_i formed in the second reaction still contains an energy-rich phosphoanhydride bond:

PP_i is rapidly hydrolyzed by pyrophosphatases in the cell. Because this removes PP_i from the reaction equilibrium, the cleavage of ATP to AMP + PP_i releases far more energy than the cleavage to ADP + phosphate.

ATP is the phosphate donor in phosphorylation reactions

Table 5.1 lists the most important uses of ATP. Only phosphorylation reactions and the coupling to endergonic reactions are considered here.

Table 5.1 Uses of ATP

Phosphorylation is the covalent attachment of a phosphate group to a substrate, most commonly by the formation of a phosphate ester bond. Assume that the cell is to convert glucose to glucose-6-phosphate, a simple phosphate ester:

One possibility is to synthesize glucose-6-phosphate by reacting free glucose with P_i:

The enzyme glucose-6-phosphatase really exists, but the ΔG⁰′ of the reaction is +3.3 kcal/mol. This translates into an equilibrium constant (K_equ) of about 4 × 10⁻³ L/mol. At an intracellular phosphate concentration of 10 mmol/L, there would be 25,000 molecules of glucose for each molecule of glucose-6-phosphate!

Stay updated, free articles. Join our Telegram channel

Join