ABSTRACT
Phosphorus is an element fundamental to life, and a structural and functional component in all organisms. It is found universally in such vital cell constituents as nucleic acids, nucleotides, phosphoproteins, and phospholipids. It occurs in teichoic and teichuronic acids in the walls of grampositive bacteria, and in phytins (also known as inositol phosphates) in plants. In many types of bacteria and some yeasts it may also be present intracellularly as polyphosphate granules. Simple phosphates (orthophosphate) can form anhydrides with other phosphates, as in organic and inorganic pyrophosphates (see Figure 6.4) and polyphosphate. Phosphate is also capable of forming anhydrides with carboxyl groups of organic acids, with amino groups of amines, and with sulfate (as in adenosine 5′-phosphosulfate). The phosphate anhydride bond serves to store biochemically useful energy. For example, a standard free energy change (∆G°) of –7.3 kcal (30.6 kJ) per mole is associated with the hydrolysis of the terminal anhydride bond of adenosine 5′-triphosphate (ATP), yielding adenosine 5′-diphosphate (ADP) + Pi. Unlike many anhydrides, some of those involving phosphates such as ATP are unusually resistant to hydrolysis in the aqueous environment (Westheimer, 1987). Chemical hydrolysis of these bonds requires 7 min of heating in dilute acid (e.g., 1 N HCl) at the temperature of boiling water (Lehninger, 1970, p. 290). At more neutral pH and physiological temperature, hydrolysis proceeds at an optimal rate only in the presence of appropriate enzymes (e.g., ATPase). The relative resistance of phosphate anhydride bonds to hydrolysis is attributable to the negative charges on the phosphates at neutral pH (Westheimer, 1987). It is the probable reason why ATP got selected in the evolution of life as a repository and universal transfer agent of chemical energy in biological systems.
