Effect of solution viscosity on intramolecular electron transfer in sulfite oxidase

Our previous studies have shown that the rate constant for intramolecular electron transfer JET) between the heme and molybdenum centers of chicken liver sulfite oxidase varies from similar to20 to 1400 s(-1) depending upon reaction conditions [Pacheco, A., Hazzard. J. T., Tollin, G., and Enemark, J. H. (1999) J. Biol. Inorg. Chem. 4, 390-401]. These two centers are linked by a flexible polypeptide loop, suggesting that conformational changes, which alter the Mo-Fe distance, may play an important role in the observed IET rates. In this study, we have investigated IET in sulfite oxidase using laser flash pliotolysis as a function of solution viscosity, The solution viscosity was varied over the range of 1.0-2.0 cP by addition of either polyethylene glycol 400 or sucrose. In the presence of either viscosogn, an appreciable decrease in the IET rate constant value is observed with an increase in the solvent viscosity. The IET rate constant exhibits a linear dependence on the negative 0.7th power of the viscosity. Steady-state kinetics and EPR experiments are consistent with the interpretation that viscosity, and not other properties of the added viscosogens, is responsible for the dependence of IET rates on the solvent composition, The results are consistent with the role of conformational changes on IET in sulfite oxidase, which help,, to clarify the inconsistency between the large rate constant for IET between the Mo and Fe centers and the long distance (similar to32 Angstrom) between these two metal centers observed in the crystal structure [Kisker, C., Schindelin, H., Pacheco, A., Wehbi, W., Garnett, R. M., Rajagopalan, K. V., Enernark, J. H., and Rees, D. C. (1997) Cell 91, 973-983].