Adenosine triphosphate (ATP) plays an essential role in energy transfer within the cell. In the form of NAD, adenine participates in multiple redox reactions. Phosphorylation and ATP‐hydrolysis reactions have key roles in signal transduction and regulation of many proteins, especially enzymes. In each cell, proteins with many different functions use adenine and its derivatives as ligands; adenine, of course, is present in DNA and RNA. We show that an adenine binding motif, which differs according to the backbone chain direction of a loop that binds adenine (and in one variant by the participation of an aspartate side‐chain), is common to many proteins; it was found from an analysis of all adenylate‐containing protein structures from the Protein Data Bank. Indeed, 224 protein–ligand complexes (86 different proteins) from a total of 645 protein structure files bind ATP, CoA, NAD, NADP, FAD, or other adenine‐containing ligands, and use the same structural elements to recognize adenine, regardless of whether the ligand is a coenzyme, cofactor, substrate, or an allosteric effector. The common adenine‐binding motif shown in this study is simple to construct. It uses only (1) backbone polar interactions that are not dependent on the protein sequence or particular properties of amino acid side‐chains, and (2) nonspecific hydrophobic interactions. This is probably why so many different proteins with different functions use this motif to bind an adenylate‐containing ligand. The adenylate‐binding motif reported is present in “ancient proteins” common to all living organisms, suggesting that adenine‐containing ligands and the common motif for binding them were exploited very early in evolution. The geometry of adenine binding by this motif mimics almost exactly the geometry of adenine base‐pairing seen in DNA and RNA. Proteins 2001;44:282–291. © 2001 Wiley‐Liss, Inc.