ADP can be derived from adenosine triphosphate (ATP). It can be interconverted to ATP. In particular, ATP is dephosphorylated by ATPases to produce ADP. ADP in turn may be phosphorylated to become ATP.

What is the source of ADP?

Stored in the chemical bond holding the terminal phosphate compound onto the ATP molecule is the energy derived from the breakdown of sugars. The removal of the terminal phosphate, through the water-mediated reaction called hydrolysis, releases this energy,… (HPO₄²⁻, abbreviated P_i) and adenosine diphosphate (ADP).

How ADP is formed?

When one phosphate group is removed by breaking a phosphoanhydride bond in a process called hydrolysis, energy is released, and ATP is converted to adenosine diphosphate (ADP). Likewise, energy is also released when a phosphate is removed from ADP to form adenosine monophosphate (AMP).