Dynamic interaction of plastocyanin with the cytochrome bf complex

The interaction between plastocyanin and the intact cytochrome bf complex, both from spinach, has been studied by stopped-flow kinetics with mutant pl astocyanin to elucidate the site of electron transfer and the docking regio ns of the molecule. Mutation of Tyr-83 to Arg or Leu provides no evidence f or a second electron transfer path via Tyr-83 of plastocyanin, which has be en proposed to be the site of electron transfer from cytochrome f, The data found with mutations of acidic residues indicate that both conserved negat ive patches are essential for the binding of plastocyanin to the intact cyt ochrome bf complex. Replacing Ala-90 and Gly-10 at the flat hydrophobic sur face of plastocyanin by larger residues slowed down and accelerated, respec tively, the rate of electron transfer as compared with wild-type plastocyan in, These opposing effects reveal that the hydrophobic region around the el ectron transfer site at His-87 is divided up into two regions, of which onl y that with Ala-90 contributes to the attachment to the cytochrome bf compl ex. These binding sites of plastocyanin are substantially different from th ose interacting with photosystem I. It appears that each of the two binding regions of plastocyanin is split into halves, which are used in different combinations in the molecular recognition at the two membrane complexes.