PSEUDOCONTACT SHIFTS USED IN THE RESTRAINT OF THE SOLUTION STRUCTURESOF ELECTRON-TRANSFER COMPLEXES

The geometry of the ferricytochrome b(5)-ferricytochrome c complex has been analysed using long-range interprotein paramagnetic dipolar shif ts. Heteronuclear filtered NMR spectra of samples containing N-15-labe lled cytochrome b(5) in complex with un labelled cytochrome c allowed unambiguous assessment of pseudocontact shifts relative to diamagnetic reference states. Because pseudocontact shifts can be observed for pr otons as much as 20 Angstrom from the paramagnetic centre, this approa ch allows study of electron transfer proteins in fast exchange. Our fi ndings provide the first physical evidence confirming hypotheses prese nted in previous theoretical studies, The absence of certain predicted shifts that are expected based on the best fit to a static model of t he complex suggests that cytochrome b(5) is more dynamic in solution t han in the crystal, in agreement with molecular dynamics simulations.