Andrew L. P. Houseman

Metalloenzyme Active-Site Structure and Function through Multifrequency CW and Pulsed ENDOR

Electron paramagnetic resonance (EPR) techniques have long been major tools in efforts to determine the structure and function of metalloen-zyme active sites (Beinert et al., 1962; Hoff, 1989). Much of the information EPR provides about the composition, structure, and bonding of a paramagnetic metal center is obtained by analysis of hyperfine coupling constants (Abragam and Bleaney, 1970; Atherton, 1973) that arise from interactions between the spin of the unpaired electron(s) and the spins of nuclei associated with the metal center, endogenous ligands, or bound substrate. At the most basic level, the observation of hyperfine coupling and its assignment to one or more nuclei (e.g., 1H, 14N) provide information about the chemical composition of the center. Detailed analysis of these couplings can provide information about its geometry or about substrate binding, as well as deep insights into chemical bonding. In principle these coupling constants can be calculated from splittings in the EPR spectrum. However, as illustrated in Fig. 1, for most metalloproteins these splittings cannot be resolved, and thus the chemical information they carry is lost.