31P NMR relaxation studies of the activation of the coenzyme phosphate of glycogen phosphorylase. The role of motion of the bound phosphate

Abstract

Spin-lattice and spin-spin relaxation rates (1/T1 and 1/T2) have been determined for the catalytically essential coenzyme phosphate at the active site of glycogen phosphorylase in both activated (R state) and inactive (T state) conformations of the enzyme. Dipolar contributions to 31P relaxation due to exchangeable protons on the phosphate group have been determined by measurement of relaxation rates at different concentrations of H2O and D2O, and field dependence studies have been performed to estimate the contribution of chemical shift anisotropy to the remaining 31P relaxation in D2O. At 109 MHz, dipolar relaxation from exchangeable protons was found to account for 50% of the spin-lattice relaxation for activated phosphorylase in 75% H2O, the remainder being due to chemical shift anisotropy. The spin-lattice relaxation rates in D2O for R-state glycogen phosphorylase are very similar to those measured for other proteins of very different size such as actin (Brauer, M., and B. D. Sykes, 1981, Biochemistry. 20:6767-6775), alkaline phosphatase (Coleman, J. E., I. D. Armitage, J. F. Chlebowski, J. D. Otvos, and A. J. M. S. Uiterkamp, 1979), and phosphoglucomutase (Rhyu, G. I., W. J. Ray, Jr., and J. L. Markley, 1984, Biochemistry. 23:252-260). In inactive (T state) phosphorylase the spin-lattice relaxation rates were almost an order of magnitude slower, while the spin-spin relaxation rates were essentially identical. These results have been analyzed by calculating the theoretically expected 31P relaxation rates in the presence of internal motions that are included in the relaxation calculation using the model-free approach of Lipari and Szabo (1982, J. Am. Chem. Soc. 104:4564-4559). The analysis suggests the coenzyme phosphate is relatively immobilized in the activated enzymic conformation, but in the inactive (Tstate) conformation it is considerably more mobile with a rotational correlation time one to two orders of magnitude smaller. Since the spin-lattice relaxation rate for the active R-state (immobilized) phosphate is similar to that observed in other phosphoenzymes of different size it is suggested that a librational motion on the nanosecond time scale may constitute a common spin-lattice relaxation pathway for phosphates in macromolecules. The consequences of phosphate motion in terms of recent suggestions concerning the environment and the catalytic role of the coenzyme phosphate are discussed.