Alteration of the structure and function of photosystem I by Pb2+

• Pb2+ modified protein structure in PSI submembrane fractions.
• Binding constant, binding ratio and binding site of Pb2+ were determined.
• An inhibitory site at or near plastocyanine is proposed.

The toxic effects of Pb2+ on photosynthetic electron transport were studied in photosystem I (PSI) submembrane fractions isolated from spinach. Structural and spectroscopic analysis using FTIR, fluorescence and X-ray photoelectron spectroscopy (XPS) showed that Pb2+ binds with proteins via oxygen and nitrogen atoms with an overall binding constant of KPb-PSI = 4.9 × 103 (±0.2) M−1 and the number of bound Pb2+ cation was 0.9 per PSI complex. Pb2+ binding altered the protein conformation indicating a partial protein destabilization. Electron transport and P700 photooxidation/reduction measurements showed that the interaction of Pb2+ cations with PSI produced a donor side limitation of electron transport presumably due to Pb2+ binding to or in the vicinity of plastocyanin.