An NMR study elucidating the binding of Mg(II) and Mn(II) to spinach plastocyanin. Regulation of the binding of plastocyanin to subunit PsaF of photosystem I

In this work we address the question whether light-induced changes in the Mg(II) content in the chloroplast lumen can modulate the electron donation to photosystem I, in particular the electrostatic interaction between plastocyanin (Pc) and the photosystem 1 subunit PsaF. For this, we have used 2D NMR spectroscopy to study the binding of Mg(II) ions and the isolated luminal domain of PsaF to (15)N-labelled Pc. From the chemical-shift perturbations in the (1)H-(15)N HSQC spectra, dissociation constants of (4.9 ± 1.7) mM and (1.4 ± 0.2) mM were determined for the Pc-Mg(II) and Pc-PsaF complexes, respectively. In both cases, significant chemical-shift changes were observed for Pc backbone amide groups belonging to the two acidic patches, residues 42-45 and 59-61. In addition, competitive effects were observed upon the addition of Mg(II) ions to the Pc-PsaF complex, further strengthening that Mg(II) and PsaF bind to the same region on Pc. To structurally elucidate the Mg(II) binding site we have utilized Mn(II) as a paramagnetic analogue of Mg(II). The paramagnetic relaxation enhancement induced by Mn(II) results in line broadening in the Pc HSQC spectra which can be used to estimate distances between the bound ion and the affected nuclear spins. The calculations suggest a location of the bound Mn(II) ion close to Glu43 in the lower acidic patch, and most likely in the form of a hexaquo complex embedded within the hydration shell of Pc. The results presented here suggest a specific binding site for Mg(II) that may regulate the binding of Pc to photosystem 1 in vivo.