Fig. 2

Reactant state structure of triphosphate. a ATP bound to myosin (based on the 1VOM crystal structure). Only the triphosphate moiety (labeled α,β,γ) of ATP is depicted. Two water molecules that coordinate Mg²⁺ are not shown. Thin dotted lines show hydrogen bonds shorter than 2.8 Å between the heavy atoms. b Methyl triphosphate (β- and γ-protonated), after energy minimization in vacuum. The Mg²⁺ hexa-coordination is depicted with dotted lines. c First step of the sequential mechanism: breaking of the P_γ–O_βγ bond to form a stable P_γO₃ ⁻ metaphosphate. d Second step of the sequential mechanism: lysis of water W_a and simultaneous attack of the metaphosphate