Adenosine 5′-triphosphate (ATP) binds to a great number of proteins to elicit a wide variety of effects, including energy production and molecular signalling. Proteins have evolved different strategies to specifically recognize ATP, utilizing different ways of binding the phosphoryl moieties as well as the adenine base. The most common, conserved sequence and structural motif for binding ATP is the Walker-A motif, or P-loop, found in many different protein structural families. Greater variation in the sequence of the P-loop is being recognized, as more ATP-binding proteins are being structurally and functionally characterized. In contrast to the P-loop, recognition of the adenine base often makes use of conserved structural motifs of main-chain atoms via hydrogen-bonding interactions, or side-chains in stacking interactions, without a definitive amino acid sequence pattern. Key concepts:Key concepts: * A major class of ATP-binding proteins are those that contain a P-loop or Walker-A motif. * P-loops or glycine-rich loops function by binding the phosphoryl groups of ATP. * Several sequence variations on the Walker-A motif are now known and have been functionally characterized. * The Walker-B motif contains a conserved acidic residue (Glu/Asp) that functions to bind directly or indirectly a metal ion important in catalysis. * Adenine-binding does not occur through specific sequence motifs, but rather uses a conserved pattern of polar and nonpolar interactions within a structural motif. * Both main-chain hydrogen bonding and aromatic residue stacking contribute to adenine-binding by proteins.