Energetics of conformational transition in proteolytically nicked myosin S-1

Force production in muscle contraction results from structural changes in myosin crossbridges. During ATP hydrolysis myosin crossbridges exhibit at least two conformational states referred to as prepower stroke and postpower stroke respectively. Conformational changes in myosin proteolytic fragment S-1, which represent cross-bridges were studied in this research using UV absorption difference spectroscopy and 19F NMR. The heavy chain of S-1 is nicked, but left intact, by trypsin and several other proteases at different sites under controlled conditions. This was used to localize the site for non-denaturational reversible conformational change observed in S-1. Two specific tryptic S-1 fragments nicked at one and two sites have been used here, i.e., 27k-70k and 27k-50k-20k. Differential scanning calorimetric (DSC) studies of chymotryptic S-1 and trypsin treated S-1 indicate that the thermal stability of S-1 is unaffected by nicking. Nucleotide induced conformational changes were monitored with the UV difference absorption of tryptophan residues. The difference spectra induced by ATP differed from that induced by ADP. AMPPNP induced both forms of the spectra by varying temperature. When the 50k-20k junction was intact the non-denaturational structural transition between S1 states was retained