CRYSTALLINE RIBONUCLEASE-A LOSES FUNCTION BELOW THE DYNAMIC TRANSITION AT 220-K

WHEN the dynamic properties of many different proteins are plotted as a function of temperature, biphasic behaviour is observed, with a broad transition centred around 220 K. Atomic mean-square displacements from X-ray crystallography 1 and Mossbauer scattering 2-3 show this behaviour, as do electron transfer rates 4 and dynamic information from inelastic neutron scattering 5. Molecular dynamics simulations over a range of temperatures also exhibit a transition at about 220 K: high-temperature atomic fluctuations are dominated by anharmonic collective motions of bonded and nonbonded groups of atoms, but below 220 K the predominant dynamic behaviour is harmonic vibration of individual atoms 6. Here we show by high-resolution X-ray diffraction that crystalline ribonuclease A does not bind substrate or inhibitor at 212 K but will bind either rapidly at 228 K. Once bound at the higher temperature, inhibitor cannot be washed off after the enzyme is cooled to below the transition temperature. These results suggest that enzyme flexibility is required for catalytic function.