The metal cofactor determines the thermal stability in cupredoxins, but how the redox state of copper modulates their melting points remains unknown. The metal coordination environment is highly conserved in cyanobacterial plastocyanins. However, the oxidised form is more stable than the reduced one in thermophilic Phormidium, but the opposite occurs in mesophilic Synechocystis. We have performed neutral amino-acid substitutions at loops of Phormidium plastocyanin far from the copper site. Notably, mutation P49G/G50P confers a redox-dependent thermal stability similar to that of the mesophilic plastocyanin. Moreover, X-ray absorption spectroscopy reveals that P49G/G50P mutation makes the electron density distribution at the oxidised copper site shift towards that of Synechocystis plastocyanin.
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