ATPbound

ATPbound refers to the state of a protein, enzyme, or molecular machine when ATP is bound at its nucleotide-binding site. This binding is often coordinated with a divalent metal ion, typically Mg2+, which stabilizes the ATP complex. The term distinguishes ATP-bound forms from nucleotide-free (apo) states and from forms bound to ADP or inorganic phosphate after ATP hydrolysis.

In many biological systems, ATP binding induces conformational changes that enable function. For enzymes, ATP binding positions catalytic residues and substrates for phosphate transfer or bond formation. In motor proteins and mechanical machines, the ATP-bound state often primes the structure for substrate interaction, conformational cycling, or force generation, while hydrolysis to ADP and Pi drives subsequent steps and resets the cycle. The activity of many ATPases and motor proteins depends on transitions between ATP-bound, ADP-bound, and nucleotide-free states.

Structurally, the ATP-bound state is associated with characteristic arrangements of nucleotide-binding domains and hinge regions, reflecting a “closed” conformation in many proteins. Researchers frequently use ATP analogs, such as AMPPNP or ATPγS, to stabilize the ATP-bound configuration for structural methods like X-ray crystallography or cryo-electron microscopy, enabling study of the pre-hydrolysis form.

Examples and relevance span diverse systems, including P-loop NTPases, ABC transporters, chaperones such as Hsp70, and motor proteins like myosin and kinesin. Understanding ATPbound states is essential for elucidating energy transduction, substrate affinity modulation, transporter mechanics, and allosteric regulation across cellular processes.