Proline 14 is impairing the electron transfer from plastocyanin to photosystem I in the prochlorophyte Prochloorothrix hollandica

Abstract

Plastocyanin (Pc) from Prochlorothrix hollandica - which occupies a unique divergent branch in the evolutionary tree of oxygen-evolving photosynthetic organisms - 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 first-order rate constant reaches a saturation plateau at high Pc concentration, as is the case with WT Pc. This finding suggests the formation of a transient Pc-PSI complex. Such a complex appears to be hydrophobic in nature as the observed rate constant is independent of ionic strength. The association 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. In fact, replacement of Pro-14 with leucine makes the rate constant for electron transfer with the mutant three times higher than that with the WT molecule. Pc mutants have indeed been used as donor proteins in experiments with PSI isolated from different organisms (cyanobacteria and higher plants); the resulting data reveal that reversion of the "exclusive" Pro-14 of Prochlorothrix Pc to the "standard" leucine enhances the reactivity of Pc towards PSI.