Adenosine is a basic building block of ATP (adenosine triphosphate). ATP is the energy supplier of all cells and also a building block of DNA. Adenosine also functions as a neurotransmitter with a similar molecular structure to caffeine, occupying the same receptors but not stimulating them.
It is also a building block of some coenzymes. In turn, a building block of adenosine is adenine. A derivative of this is the eritadenin contained in shiitake.