Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The ATP Cycle and Cellular Bioenergetics
The chemical properties of ATP are well established

Adenosine triphosphate (ATP) and the products of its sequential Hydrolysis steps, adenosine diphosphate (ADP) and adenosine monophosphate (AMP), belong to the nucleotide Class (Fig. 14-2). As recalled (Chapter 3), nucleotide molecules consist of a heterocyclic base (purine or pyrimidine), a five-carbon sugar, and one or more phosphate groups. In ATP, ADP, and AMP molecules, adenine (a purine) serves as the base, while the five-carbon sugar is represented by D-ribose (Fig. 14-2). Various types of NUCLEOTIDES, which are quite numerous, differ from one another in The Nature of their constituent nitrogenous bases and sugars. Nucleotides perform A wide variety of Functions within The Cell, but they are best known as Building Blocks of DNA and RNA molecules, in which they serve as coding elements. ATP, ADP, and AMP (Fig. 14-2) are found in all living forms, where they consistently perform the same universal functions. These compounds are present not only in the Cytosol, but also in Mitochondria and the Cell Nucleus. In normally respiring Cells, ATP accounts for up to 80% or even more of the total amount of all three adenine nucleotides (Table 14-4).

Fig. 14-3. The ATP cycle in cells.

At pH 7.0, ATP and ADP exist as multi-charged anions, ATP^4- and ADP^3-, because all their phosphate groups are almost completely ionized at this pH value. However, in the intracellular fluid, which is characterized by a high content of Mg²⁺ ions, ATP and ADP are present primarily as magnesium complexes, MgATP^2- and MgADP^- (Fig. 14-4). In many enzymatic reactions where ATP participates as a phosphate group donor, its magnesium complex, MgATP^2-, is the active form. The concentration of ATP in cells is maintained at a relatively constant level because The rate of its synthesis approximately balances the rate of its breakdown. Thus, the terminal phosphate groups of ATP molecules undergo continuous turnover during METABOLISM. They are constantly cleaved and replaced by new ones from the cellular inorganic phosphate pool.

Table 14-4. Concentrations of adenine nucleotides, inorganic phosphate, and creatine phosphate (CP) in selected cells, mM

¹⁾ For erythrocytes, cytosolic concentrations are indicated since erythrocytes lack both a nucleus and mitochondria. In all other cases, total cellular content is implied, although we know that, for example, ADP concentrations in the cytosol and mitochondria differ significantly. Creatine phosphate (phosphocreatine) will be discussed later in this chapter.

1.

2.    Fig. 14-4. Complexes of ATP and ADP with Mg²⁺ ions.

ATP has been successfully synthesized in the laboratory, and its Structure and properties have been studied in detail. We also know that it serves as a link between energy-yielding and energy-consuming reactions. This role is based on well-established chemical principles, to the Discussion of which we now turn.