Role of the calcium ion and the disulfide bond in the Burkholderia glumae lipase

The role of the Ca²⁺ ion that is present in the structure of Burkholderia glumae lipase was investigated. Previously, we demonstrated that the denatured lipase could be refolded in vitro into an active enzyme in the absence of calcium. Thus, an essential role for the ion in catalytic activity or in protein folding can be excluded. Therefore, a possible role of the Ca²⁺ ion in stabilizing the enzyme was considered. Chelation of the Ca²⁺ ion by EDTA severely reduced the enzyme activity and increased its protease sensitivity, however, only at elevated temperatures. Furthermore, EDTA induced unfolding of the lipase in the presence of urea. From these results, it appeared that the Ca²⁺ ion in B. glumae lipase fulfils a structural role by stabilizing the enzyme under denaturing conditions. In contrast, calcium appears to play an additional role in the Pseudomonas aeruginosa lipase, since, unlike B. glumae lipase, in vitro refolding of this enzyme was strictly dependent on calcium. Besides the role of the Ca²⁺ ion, also the role of the disulfide bond in B. glumae lipase was studied. Incubation of the native enzyme with dithiothreitol reduced the enzyme activity and increased its protease sensitivity at elevated temperatures. Therefore, the disulfide bond, like calcium, appears to stabilize the enzyme under detrimental conditions.