Proline 14 is impairing the electron transfer from plastocyanin to Photosystem I in the prochlorophyte Prochlorothrix hollandica

摘要

Plastocyanin (Pc) from the prochlorophyte Prochlorothrix hollandica exhibits some peculiar differences at its north hydrophobic patch as compared with Pc from other organisms. Actually, residues at positions 12 and 14 are tyrosine and proline, respectively, in Prochlorothrix, but they are glycine and leucine in all others. Photosystem I (PSI) reduction by single (Y12G, Y12F, Y12W and P14L) and double (Y12G/P14L) mutants of Prochlorothrix Pc has been investigated by laser flash absorption spectroscopy. With all the mutants, the observed pseudo first-order rate constant reaches a saturation plateau at high Pc concentration, as is the case with wild-type (WT) Pc, suggesting the formation of a transient Pc-PSI complex. The equilibrium constant for complex formation is not significantly altered by mutations, but the electron transfer rate constant is drastically changed, in particular by mutations at position 14. Pc mutants have also been used as donor proteins in experiments with PSI isolated from different organisms (cyanobacteria and higher plants). The experimental data reveal that reversion of the "exclusive" Pro-14 of Prochlorothrix Pc to the "standard" leucine enhances the reactivity of Pc towards PSI.