Structure of the Complex between Plastocyanin and Cytochrome f from the Cyanobacterium Nostoc sp. PCC 7119 as Determined by Paramagnetic NMR

Abstract

The complex between cytochrome f and plastocyanin from the cyanobacterium Nostoc has been characterized by NMR spectroscopy. The binding constant is 16 mM?1, and the lifetime of the complex is much less than 10 ms. Intermolecular pseudo-contact shifts observed for the plastocyanin amide nuclei, caused by the heme iron, as well as the chemical-shift perturbation data were used as the sole experimental restraints to determine the ori- entation of plastocyanin relative to cytochrome f with a precision of 1.3 Å. The data show that the hydrophobic patch surrounding tyrosine 1 in cytochrome f docks the hydrophobic patch of plastocyanin. Charge complemen- tarities are found between the rims of the respective recognition sites of cytochrome f and plastocyanin. Sig- nificant differences in the relative orientation of both proteins are found between this complex and those pre- viously reported for plants and Phormidium, indicating that electrostatic and hydrophobic interactions are bal- anced differently in these complexes. In