Ionization of reactive lysyl residue of myosin subfragment 1

Abstract

The .epsilon.-NH2 groups of lysyl residues of rabbit skeletal muscle myosin subfragment 1 belong to 2 classes on the basis of their reaction with 2,4,6-trinitrobenzenesulfonate: 1 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 A345 or as a decrease in K+(EDTA)-activated ATPase at various pHs between 6.5-10. The 2nd-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, N.alpha.-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 promoted trinitrophenylation. The anomalous behavior of the reactive lysyl residue apparently is caused by a vicinal positive charge and by other neighboring groups.