Involvement of an Arginyl Residue in the Catalytic Activity of Myosin Heads

1 Phenylglyoxal reacts rapidly with isolated myosin heads (subfragment 1) and induces two successive and distinguishable effects on their enzymic properties: first, a twofold activation of the Ca 2+ and Mg 2+ ‐dependent ATPases with no effect on the K + ‐ATPase followed by inhibition of the K + , Ca 2+ and actin‐activated Mg 2+ ‐ATPases. A specific protein‐reagent complex is formed during the second phase of the modification reaction ( K i ∼ 5 × 10 −3 M). 2 ADP and ATP with or without cations provide efficient protection only against the loss of ATPase activities, suggesting that the second inhibitory process is occurring at or close to the active site. 3 On the basis of [ 14 C]phenylglyoxal‐labelling experiments and the composition of modified subfragment‐1 derivatives, it is demonstrated that the sequential modification of two reactive arginyl residues is responsible for the observed activation‐inhibition phenomena. Blocking of the first reactive residue produces a shift in the pH/activity curves related to the Ca 2+ and Mg 2+ ‐dependent ATPases with an apparent activation effect. Modification of the second guanidino group does not destroy the affinity of the protein for the nucleotide substrates but does alter the nucleotide binding site as reflected in the inability of Mg 2+ . ATP to dissociate the modified subfragment‐1–actin complex. It is concluded that electrostatic interactions between this positively charged group and the negatively charged ATP and ADP molecules may be critical for the hydrolytic efficiency of myosin heads. 4 After dissociation and separation of the polypeptide constituents of the protein in acetic acid medium, both labelled sites are found to reside in the heavy chain.