Changes in the 31P NMR spectrum of rabbit muscle myosin subfragment 1 ·MgADP with temperature

In pioneering studies on the 31P NMR spectra of MgADP bound to the “molecular motor” myosin subfragment 1 (S1) in the temperature range of 0 to 25 °C, Shriver and Sykes [Biochemistry 20 (1981) 2004–2012/6357–6362; Biochemistry 21 (1982) 3022–3028], proposed that MgADP binds to myosin S1 as a mixture of two interconvertible conformers with different chemical shifts for the β-P resonance of the S1-bound MgADP and that the concentrations of these conformers are related by an equilibrium constant K(T). Their model implied that the weighted average of the chemical shifts of the β-P(MgADP) for S1-bound MgADP asymptotically approaches a high temperature limit. Here, and in our earlier paper [K. Konno, K. Ue, M. Khoroshev, H., Martinez, B.D. Ray, M.F. Morales, Proc. Natl. Acad. Sci. USA 97 (2000) 1461–1466], we report experimental similarities to Shriver and Sykes, but diverge from them (especially at 0 °C) in not finding two distinct peaks and in finding that the average chemical shift does not change with temperature. Our observations can be explained by chemical exchange of β-P(MgADP) of S1-bound MgADP between two nearly energetically equivalent environments.