Figure 4. Arrangement of the water molecule expected to initiate nucleophilic attack on the γ-phosphate of ATP.

The arrangement of non-deep-sea fish actins (A) and deep-sea fish actins (B). Green spheres show water molecules expected to be nucleophilic water for ATP hydrolysis. Red spheres indicate the water molecules coordinated to Mg²⁺ and those bridging the expected nucleophilic water and H161 with hydrogen bonds. Black dotted lines show typical hydrogen bonds formed during the MD simulation. Angle θ and distance d _Nu are defined by O^β-P^γ-O^w and P^γ-O^w, respectively, where O^w represents the oxygen of the expected nucleophilic water (see Figure 1C for the definition of the other atoms).