Identification of a calcium binding site in Staphylococcus hyicus lipase: generation of calcium-independent variants.

In this study we have identified the presence of a high-affinity binding site for calcium in the lipase from Staphylococcus hyicus. By means of isothermal titration calorimetry we showed that the enzyme binds one calcium per molecule of enzyme with a dissociation constant of 55 microM. The residual activity of the apoenzyme compared to the activity in the presence of calcium ions varies from 65% at 10 degreesC to nearly zero at 40 degreesC. On the basis of primary sequence alignment with other staphylococcal lipases and the lipases from Bacillus thermocatenulatus and from Pseudomonas glumae in combination with site-directed mutagenesis, aspartates 354 and 357 could be identified as calcium ligands. Kinetic measurements with the D357E variant showed that replacement of Asp357 by a glutamate decreased the affinity for calcium ions 30-fold. Introduction of a lysine, an asparagine, or an alanine at position 357 and of a lysine or an asparagine at position 354 resulted in calcium-independent variants. Isothermal titration calorimetry confirmed the loss of calcium binding. Although the D357K, D357N, and D357A variants did not bind calcium, at room temperature they were nearly as active as wild-type lipase in the presence of calcium, but at elevated temperatures these calcium-independent lipases showed a reduced activity. Over the whole temperature range the activities of the D354K and D354N variants are significantly lower than wild-type enzyme in the presence of calcium and are comparable to the activity of the wild-type apoenzyme. Our results show that binding of calcium is important for the structural stabilization of staphylococcal lipases (and possibly other lipases) and that it is possible to engineer calcium-independent variants on the basis of limited structural homology with another lipase.