PLASTOCYANIN - STRUCTURE AND FUNCTION

The aim of this review is to analyze the current state of knowledge concerning the blue copper protein plastocyanin (PC) focusing on its interactions with its reaction partners cytochrome f and P700. Plastocyanin is a 10 kD blue copper protein which is located in the lumen of the thylakoid where it functions as a mobile electron carrier shuttling electrons from cytochrome f to P700 in Photosystem I. PC is a typical beta-barrel protein containing a single copper center which is ligated to two histidines, a methionine and a cysteine in a distorted tetrahedral geometry. PC has two potential binding sites for reaction partners. Site 1 consists of the H87 ligand to the copper and Site 2 consists of Y83 which is surrounded by two clusters of negative charges which are highly conserved in higher plant PCs. The interaction of PC with cytochrome f has been studied extensively. It is electrostatic in nature with negative charges on PC interacting with positive charges on cytochrome f Evidence from cross-linking, chemical modification, kinetics and site-directed mutagenesis studies implicate Site 2 as the binding site for Cytf The interaction is thought to occur in two stages: an initial diffusional approach guided by electrostatic interactions, followed by more precise docking to form a final electron transfer complex. Due to the multisubunit nature of the Photosystem I complex, the evidence is not as clear for the binding site for P700. However, a small positively-charged subunit (Subunit III) of Photosystem I has been implicated in PC binding. Also, both chemical modification and site-directed mutagenesis experiments have suggested that PC interacts with P700 via Site 1.