The alkaline transition of blue copper proteins, Cucumis sativus plastocyanin and Pseudomonas aeruginosa azurin

Autoreduction of Cucumis sativus plastocyanin and Pseudomonas aeruginosa azurin took place at alkaline pHs, having been accompanied by the decrease in the intensities of the charge transfer band, Cys-S−(π) → Cu(II) at 597 and 626 nm, and the Cu(II)-EPR signals with small AII values of 6.5 × 10−3 and 5.3 × 10−3 cm−1 for plastocyanin and azurin, respectively. Further, an extra Cu(II)-EPR signal with a large AII value of 21 × 10−3 cm−1 also reversibly emerged with increasing pH. Plastocyanin and azurin are in an equilibrium of the three forms at alkaline pHs.