phosphoanhydride

Phosphoanhydride refers to a covalent bond formed between two phosphate groups, creating a phosphorus-oxygen-phosphorus (P–O–P) linkage. These bonds arise from dehydration synthesis between phosphate units and are cleaved by hydrolysis in aqueous environments. The term is used to describe the anhydride-type connections that link phosphate groups within certain molecules.

In biology, the most familiar phosphoanhydride bonds are found in nucleotide triphosphates such as ATP and GTP. The beta–gamma and alpha–beta linkages between successive phosphates are examples of phosphoanhydride bonds. Hydrolysis of these bonds releases energy and yields products such as ADP and inorganic phosphate (Pi) or AMP and pyrophosphate (PPi), depending on the cleavage pattern. These energy-releasing reactions power a wide range of cellular processes, including muscle contraction, active transport, and biosynthesis. Phosphoanhydride bonds also occur in inorganic polyphosphates and in other phosphorylated biomolecules, where they contribute to energy transfer and signaling functions.

The concept of a high-energy phosphate bond refers to the large free-energy change associated with hydrolysis under physiological conditions, rather than an unusually strong bond capable of releasing energy by itself. The energy is often attributed to factors such as electrostatic repulsion among closely spaced phosphate groups, resonance stabilization of the products, and solvation effects. Phosphoanhydride bonds are distinct from phosphoester bonds, which connect phosphate groups to carbon or other non-phosphate moieties, and they form a key category of linkages in energy metabolism and phosphate chemistry.