A Model for the Myosin ATPase Active Site*

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Abstract
Myosin possesses 2 functional -SH groups: one is an AET (S-beta-aminoethylisothiuronium)-binding, fast PCMB(p-chloromercuribenzoate)-reacting site, that binds ATP at the adenine end through Ca2+ or Mg2+; the other is a pyrophosphate- or moniodo-acetate-binding, slow PCMB-reacting site, which binds alpha- and beta-P of ATP via Ca2+, but not Mg2+. ATPase activity of myosin is accelerated by increasing ionic strength. ATPase activity-pH curve also suggests a possible participation of the imidazole group of the histi-dine residue of myosin in binding gamma-P of ATP by ionic force. Kinetic analysis indicates the desorption of the product (ADP) as the rate-determining step. A slight inhibition by ADP is not the case with AET-treated myosin. The ADP molecule tends to remain on myosin molecule in a way similar to ATP binding at 2 -SH group.

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