Use of Nmr Relaxation Measurements to Derive the Binding Site of Plastocyanin in Complexes with Cytochrome-f and C

1 Introduction Plastocyanin (PC) is a small (M r 10 500), 'blue' copper protein which transfers electrons from cyto-chrome / to the primary electron donor of photosystem I (P700) in the photosynthetic electron transport chain. It reacts rapidly with cytochrome / in vitro and also with several other cytochromes, although somewhat more slowly [1].. The crystal structures of both oxidized and reduced poplar plastocyanin have been determined and show that redox changes cause only small changes at the Cu site, leaving the structure of the rest of the molecule essentially unchanged [2,3]. The Cu atom and its ligands (His-37, Cys-84, His-87 and Met-92) are located in a hydrophobic pocket near one end of the molecule (the "northern" end) such that only the imidazole ring of His-87 (the northern histidine) is accessible to solvent (Fig 1). Studies with small molecules, such as [ F e (C N) 6 ] 3 " and [Co(phen)3]3+, have identified two reaction sites on plastocyanin; one close to the copper ligand His-87 at the northern hydrophobic patch and the other close to the more remote Tyr-83 at the eastern acidic patch [4-8]. Chemical modification of the acidic amino acid residues, inhibitory effects of small molecules and ionic strength