Relevance of metal ions for lipase stability: structural rearrangements induced in the Burkholderia glumae lipase by calcium depletion.

We have studied the accessibility of the structural calcium ion in the Burkholderia glumae lipase and the consequences of its removal on the protein conformation by different biophysical techniques (circular dichroism, fluorimetry, and mass spectrometry) and by molecular-dynamics simulations. We show that, in the native protein, calcium is not accessible unless specific flexible loops are displaced, for example, by a temperature increase. Such movements concern the whole calcium-binding pocket and particularly the environment of the coordinating aspartate residue 241. As a consequence of metal depletion the protein unfolds irreversibly and undergoes aggregation. The removal of the metal ion causes major structural transitions and leads to an increase in beta-structure, in particular in protein regions that are largely unstructured in the native protein and encompass the calcium coordination residues.