Role of the cofactor calcium in the activation of outer membrane phospholipase A

Iban Ubarretxena-Belandia, Jan Willem P. Boots, Hubertus M. Verheij, Niek Dekker

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


The enzymatic activity of the outer membrane phospholipase A (OMPLA), an integral membrane protein of Escherichia coli, is regulated by dimerization for which the cofactor Ca2+ is required. In this study, the interaction of Ca2+ with OMPLA was characterized, with an emphasis on the role of the cofactor in the activation process and dimerization. Kinetic experiments were done in which the enzyme was solubilized in mixed micelles of substrate and different detergents. It appeared that the affinity of OMPLA for Ca2+ was high (12 μM) if alkylphosphocholines were used as detergent, moderate (62 μM) if sulfobetaines were used, and very low (24 mM) if alkylpolyoxyethylene glycols were used. These results show that there is a strong modulation of the calcium binding properties of OMPLA by the lipid environment. In the presence of hexadecylphosphocholine micelles, the affinity of OMPLA for Ca2+ was determined by three direct binding techniques. Using gel filtration, it appeared that OMPLA has one high-affinity site (K(d) ≃ 36 μM) and a second site with moderate affinity (K(d) ≃ 358 μM). Sulfonylated-OMPLA, in which the active site serine had been covalently modified with hexadecanesulfonylfluoride, was used as a mimic for the acyl- enzyme intermediate. In gel filtration experiments, this sulfonylated-OMPLA displayed binding of two Ca2+ per enzyme monomer both with similar high affinity (K(d) ≃ 48 μM), indicative of a strong synergistic effect of active site occupation and the affinity of the second Ca2+ binding site. Isothermal titration calorimetric measurements confirmed only the presence of a high-affinity Ca2+ binding site, whereas in fluorescence experiments only the binding of the second Ca2+ could be observed. Chemical cross-linking was applied to investigate which of the two Ca2+ sites is involved in dimerization. OMPLA was monomeric in the absence of Ca2+, whereas already at low Ca2+ concentrations the enzyme was convened to its dimeric form. Therefore, we suggest that the first Ca2+ plays a role in the stabilization of the dimeric state of the enzyme. The role of the second Ca2+ and the observed synergy between active site occupancy and Ca2+ affinity are discussed.

Original languageEnglish
Pages (from-to)16011-16018
Number of pages8
Issue number45
StatePublished - 10 Nov 1998
Externally publishedYes


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