Purines are heterocyclic aromatic compounds composed of linked pyrimidine and imidazole rings. In mammals, purines are most commonly expressed in DNA and RNA (including the purines adenine and guanine), as well as single-molecule nucleotides (adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), cyclic AMP, and to a lesser extent guanosine triphosphate (GTP) and cyclic guanosine monophosphate (cGMP). Purines are also key elements of the following energy metabolism molecules: reduced nicotinamide adenine dinucleotide, nicotinamide adenine dinucleotide phosphate (NADPH), and coenzyme Q. Purines can also act as direct neurotransmitters; for example, adenosine may interact with receptors to modulate cardiovascular and central nervous system (CNS) function.