Adenine (A) is a nucleobase (a purine derivative) The others being adenine, guanine, and cytosine. Adenine has a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate (ATP) and the cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), and protein synthesis, as a chemical component of DNA, The shape of adenine is complementary to thymine in DNA.