The Mechanism of Water‐Proton Relaxation in Enzyme · Paramagnetic‐Ion Complexes

The water proton‐relaxation times, T 1 and T 2 , have been measured in solutions of Mn(II) and ATP in the presence and absence of rabbit muscle phosphofructokinase over a wide range of temperatures and nuclear frequencies, using pulsed nuclear magnetic resonance techniques. The equilibrium between the enzyme and the Mn · ATP complex has also been studied as a function of temperature, and the relaxation rates in the bound complex determined. An analysis of the relaxation rates in both free and enzyme‐bound Mn · ATP sites according to established theories of paramagnetic relaxation has been carried out by means of graphical and computing methods, and yielded estimates of the Mn(II) water co‐ordination number ( q ) and the relevant correlation times with their frequency and temperature dependence. Relaxation in the ternary enzyme complex is dipolar in nature and the correlation time (τ c ) is dominated by the water residence time (τ M ) or the Mn electron spin relaxation time (τ s ) according to frequency. The values obtained for both τ c and τ s are longer than earlier estimates. In the binary Mn · ATP complex the dipolar interactions are dominated by rotation (τ R ) but in addition there is a large scalar interaction governed by both τ M and τ s . The significance of the parameters in the structure of the ternary enzyme complex is discussed, together with the implications of the revised parameters τ c and τ s for earlier mapping studies of the Mn · ATP sites and the spin‐labelled enzyme. The limitations of the analysis have been stressed throughout.