Ionization of Reactive Lysyl Residue of Myosin Subfragment 1

The ε-NH2 groups of lysyl residues of myosin subfragment 1 belong to two classes on the basis of their reaction with 2,4,6-trinitrobenzenesulfonate: one reactive lysyl residue and 82 slow-reacting lysyl residues. The trinitrophenylation of the reactive lysyl residue is accompanied by a sharp decrease in the K+(EDTA)-activated ATPase of myosin subfragment 1. The rate of trinitrophenylation of this group was followed as an increase in Ai45 or as a decrease in K+(EDTA)-activated ATPase at various pHs between 6.5 and 10. The second-order rate constant obtained by these methods sharply increased with pH, plateauing at about pH 9.7. A typical dissociation curve with pK = 9.0 was obtained by plotting the pH dependence of the rate constant. For this reactive lysyl, the pK value was low and the maximal rate of trinitrophenylation was high in comparison to the corresponding quantities of the slow-reacting lysyls of myosin subfragment 1 and of a model compound, Na-carbobenzoxy- L-lysine. The pH dependence of the trinitrophenylation of lysyl residues of myosin subfragment 1 was anomalous. The pK value and maximal rate of trinitrophenylation of poly-L-lysine resembled those of the reactive lysyl residue. The presence of an aromatic moiety in the model compound was found to promote trinitrophenylation. It is suggested that the anomalous behavior of the reactive lysyl residue is caused by a vicinal positive charge and by other neighboring groups.