Thermal denaturation and gelation of rubisco: effects of pH and ions.

In order to understand the mechanism of thermal gelation of rubisco, its native and heat denatured states were characterized by absorbance, fluorescence and circular dichroïsm spectroscopies as well as by differential scanning calorimetry in the presence of various salts. It appears that during the denaturation process, divalent anions are released while divalent cations are fixed by the protein, while it is disorganized and while the environment of its aromatic chromophores becomes more hydrophilic. The pH transition of gelation is shifted 1-2 pH units higher than the transition of denaturation temperature which occurs near the isoelectric point of the native molecule. This shift probably corresponds to the breaking of saline bridges within the protein molecule. Finally, a large effect of divalent cations on the phase diagram indicates that a particular denatured state is attained when these cations are in the denaturation medium.