The myosin ATPase mechanism does not require a conformationally sensitive aromatic residue

Summary The effect of nucleotides or calcium ([Ca²⁺]_free=1.0×10^–4 m) or both on the near-u.v. absorption spectrum of myosins purified from two species of scallop,Aequipecten irradians andPlacopecten magellanicus, has been examined. No change in tyrosyl or tryptophanyl absorption was detected.The near-ultraviolet (u.v.) circular dichroism (CD) spectra of myosins from these two species of scallop were examined on a high sensitivity (Jasco J41C) spectropolarimeter.Aequipecten andPlacopecten myosin near-u.v. CD spectra were qualitatively distinct. The near-u.v. CD spectrum ofPlacopecten myosin was not perturbed by addition of a ten-fold molar excess of ADP±Ca²⁺, 0.1m PP_i or 1.0×10^–4m calcium alone. ADP and PP_i did induce small, qualitatively distinct changes in the near-u.v. CD spectrum ofAequipecten myosin but no perturbation was observed with calcium alone.These data, together with those from an earlier study, indicate that the conformation of aromatic residues are not necessarily perturbed in the myosin ATPase mechanism as has previously been suggested.