Coordination of the ADP-Mg ²⁺ and the nucleotide-binding site differences between ADP-bound and rigor. (A) View of the nucleotide-binding pocket and the placement of the Mg²⁺·ADP within it. The density of the nucleotide from the single particle focused refinement is shown in grey. Side chains that are involved in the coordination and binding of the nucleotide are displayed around the ADP. (B) Ligplot diagram detailing the interaction between the myosin nucleotide-binding site and nucleotide. Hydrogen bonds are shown in green, while hydrophobic contacts are shown by red semi-circles. The orientation of the nucleotide is similar to A. (C) Comparison of the nucleotide-binding site in the rigor complex (orchid) versus the Mg²⁺·ADP-bound complex (gold). The changes of each part of the nucleotide pocket are displayed including the movement of the HF helix and P-loop, switch I, the purine-binding loop, the N-terminus, and loop 673–686. These comparisons were created by aligning the actin filament of the helical models and overlaying the single particle S1 models on their helical counterparts using the matchmaker command in ChimeraX.