The role of individual lysine residues in the basic patch on turnip cytochrome f for electrostatic interactions with plastocyanin in vitro

The role of electrostatic interactions in determining the rate of electron transfer between cytochrome f and plastocyanin has been examined in vitro w ith mutants of turnip cytochrome f and mutants of pea and spinach plastocya nins. Mutation of lysine residues Lys58, Lys65 and Lys187 of cytochrome f t o neutral or acidic residues resulted in decreased binding constants and de creased rates of electron transfer to wild-type pea plastocyanin. Interacti on of the cytochrome f mutant K187E with the pea plastocyanin mutant D51K g ave a further decrease in electron transfer rate, indicating that a complem entary charge pair at these positions could not compensate for the decrease d overall charge on the proteins. Similar results were obtained with the in teraction of the cytochrome f mutant K187E with single, double and triple m utants of residues in the acidic patches of spinach plastocyanin. These res ults suggest that the lysine residues of the basic patch on cytochrome f ar e predominantly involved in long-range electrostatic interactions with plas tocyanin. However, analysis of the data using thermodynamic cycles provided evidence for the interaction of Lys187 of cytochrome f with Asp51, Asp42 a nd Glu43 of plastocyanin in the complex, in agreement with a structural mod el of a cytochrome f-plastocyanin complex determined by NMR.