Using Saturation-Recovery EPR To Measure Exchange Couplings in Proteins: Application to Ribonucleotide Reductase

The stable tyrosine radical of ribonucleotide reductase (RNR) from Escherichia coli, Tyr 122 of the B2 subunit, exhibits single-exponential spin-lattice relaxation kinetics for T ≤ 16 K and nonexponential spin-lattice relaxation kinetics at higher temperatures. Saturation-recovery transients of the tyrosine radical are analyzed using a model developed to treat the interaction of two paramagnets in a rigid lattice at a fixed distance apart but with a randon orientation in the static magnetic field. The model describes the spin-lattice relaxation of a radical in proximity to another paramagnetic site in terms of two isotropic or “scalar” rate constants and an orientation-dependent rate constant. The scalar rate constants arise from (1) intrinsic relaxation processes of the radical which exist in the absence of the other paramagnetic site and (2) a scalar-exchange-induced relaxation process arising from orbital overlap between the two paramagnetic sites. The orientation-dependent rate constant arises from a dipole-dipole-induced relaxation process. From simulations of the higher temperature saturation-recovery transients, we conclude that their nonexponential character arises from a dipole-dipole interaction with the diferric center of RNR. The tyrosine radical generated by UV photolysis of l-tyrosine in a borate glass is used as a model for the intrinsic spin-lattice relaxation rate of the tyrosine radical of ribonucleotide reductase. Comparison of the scalar rate constants derived from simulations of the saturation-recovery transients of the tyrosine radical of RNR with the single-exponential rate constants of the model tyrosine radical indicates scalar exchange is also a source of relaxation enhancement of the tyrosine radical of RNR at higher temperatures. We present a new method for determining the exchange coupling of the diferric center based on the temperature dependence of the scalar-exchange and dipolar rate constants. The Fe(III)—Fe(III) exchange coupling is estimated to be -94 ± 7 cm^-1. We also estimate an exchange coupling of |0.0047 · 0.0003 cm^-1 between the diferric center and the tyrosine radical on the basis of the relative contributions of scalar-exchange and dipolar interactions to the spin-lattice relaxation and the distance between the two sites. The source of line broadening of the EPR signal of the tyrosine radical of RNR at temperatures greater than 75 K is discussed as well.