Three surface residues of plastocyanin from Prochlorothrix hollandica have been modified by site-directed mutagenesis. Changes have been made in methionine 33, located in the hydrophobic patch of the copper protein, and in arginine 86 and proline 53, both located in the eastern hydrophilic area. The reactivity toward photosystem I of single mutants M33N, P53A, P53E, R86Q, R86E, and the double mutant M33N/P14L has been studied by laser flash absorption spectroscopy. All the mutations yield increased reactivity of plastocyanin toward photosystem I as compared with wild type plastocyanin, thus indicating that in Prochlorothrix electron donation to photosystem I is not optimized. The most drastic increases in the intracomplex electron transfer rate are obtained with mutants in methionine 33, whereas replacing arginine 86 only modestly affects the plastocyanin-photosystem I equilibrium constant for complex formation. Mutations at position 53 also promote major changes in the association of plastocyanin with photosystem I, yielding a change from a mechanism involving complex formation to a simpler collisional interaction. Molecular dynamics calculations indicate that mutations at position 33 promote changes in the H-bond network around the copper center. The comparative kinetic analysis of the reactivity of Prochlorothrix plastocyanin mutants toward photosystem I from other cyanobacteria reveals that mutations M33N, P53A, and P53E result in enhanced general reactivity.