adenosine triphosphate (ATP)

ATP is the nucleotide consisting of the base adenine (red numbers in figure), the sugar ribose (green numbers), and a triphosphate group in an ester linkaage with the 5'-hydroxyl of the ribose. ATP is a major product of metabolism, serving as a ubiquitous carrier of metabolic energy in the form of high phosphoryl transfer potential. This is usually associated in ATP with the phosphoric anhydride bonds within the triphosphate group. As a nucleotide, ATP is a building block for nucleic acid synthesis.

The hydrolysis of ATP is represented in the following chemical equation:

ATP + H₂O --> ADP + P_i

[ Link to Molecule of the Month ATP page - requires CHIME application for proper molecular viewing ]

The origin of the high-energy yield of ATP hydrolysis lies in the fact that it is a phosphoric acid anhydride, an energy-rich compound relative to its hydrolysis products. The hydrolysis products - ADP + P_i - are more stable due to separation of negative charges (see below), better resonance stabilization, and more favorable hydration.

The above hydrolysis reaction is oversimplified in two ways: In a physiological setting, the ATP molecule is actually complexed with a divalent cation - usually Mg²⁺ - an interaction which is stabilized electrostatically by the cation's attraction for the negative charges associated with the phosphate groups of ATP. Omission of the charged nature of ATP was the second oversimplification. Thus, we could more accurately represent the hydrolysis reaction as:

Mg²⁺-ATP^4- + H₂O -->  Mg²⁺-ADP^3- + H⁺.