Solution structure of paramagnetic metalloproteins
Paramagnetism causes broadening of the NMR lines and therefore makes difficult the detection of the constraints (NOEs and 3 J) which are necessary for the determination of solution structures. The broadening is due to the fast nuclear relaxation rates, which are induced by the coupling of the nucleus with the unpaired electrons. Nevertheless, an NMR methodology has been developed, allowing the detection of classical constraints. This has allowed us to solve the first solution structure of a paramagnetic metalloprotein in 1994. Since then, several solution structures of paramagnetic proteins have appeared. In addition, para- magnetism has been exploited in order to obtain new, nonclassical constraints. First, the nuclear relaxation has been exploited to obtain metal-proton distances. The position of nuclei with respect to the magnetic susceptibility tensor axes has been determined by the use of pseudocontact shifts. The contact shifts have been used as constraints after discovering or confirming the shift dependence on dihedral angles. Paramagnetic molecules can be strongly magnetically anisotropic and therefore display partial orientation in strong external magnetic fields. These orientational effects result in dipolar contributions to the 15 N- 1 H 1 J coupling. Such contributions can yield powerful structural constraints. In summary, the solution structure of many paramagnetic metalloproteins has been solved and described, and several new strategies have been developed for such solution structure determinations.