Effect of Pb 2+ on the Kinetic and Spectral Characterization of Ribulose‐1,5‐bisphosphate Carboxylase/Oxygenase

Pb 2+ at various concentrations was added to the purified ribulose‐1,5‐bisphosphate carboxylase/oxygenase (rubisco, E.C.4.1.1.39) in vitro to gain insight into the mechanism of molecular interactions between Pb 2+ and rubisco using spectral methods. It was found that the carboxylase activity of rubisco gradually decreased with increasing concentrations of Pb 2+ . The kinetics constant ( K m ) and v max were 1.74 mol·L −1 and 0.42 mol CO 2 /(mg protein·min), respectively, at a low concentration of Pb 2+ , and 11.82 mol·L −1 and 0.28 mol CO 2 /(mg protein·min), respectively, at a high concentration of Pb 2+ . The spectroscopy assays suggested that the Pb 2+ was determined to be directly bound to rubisco; the binding site of Pb 2+ to rubisco was 1.1 and the binding constants were 8.63×10 4 and 2.18×10 5 L/mol. Based on the analysis of inductively coupled plasma‐mass spectrometry (ICP‐MS) and the circular dichroism (CD) spectra, it was concluded that Pb 2+ replaced Mg 2+ from the catalytic center in rubisco and the binding of Pb 2+ entirely altered the primary conformation of rubisco, implying that the Pb 2+ coordination created a new metal ion‐active site form of rubisco, thus leading to a reduction in the carboxylase activity of rubisco.