Structure of a premicellar complex of alkyl sulfates with the interfacial binding surfaces of four subunits of phospholipase A2

The properties of three discrete premicellar complexes (E₁^#, E₂^#, E₃^#) of pig pancreatic group-IB secreted phospholipase A2 (sPLA₂) with monodisperse alkyl sulfates have been characterized Berg, O. G. et al., Biochemistry 43, 7999–8013, 2004]. Here we have solved the 2.7 Å crystal structure of group-IB sPLA₂ complexed with 12 molecules of octyl sulfate (C₈S) in a form consistent with a tetrameric oligomeric that exists during the E₁^# phase of premicellar complexes. The alkyl tails of the C₈S molecules are centered in the middle of the tetrameric cluster of sPLA₂ subunits. Three of the four sPLA₂ subunits also contain a C₈S molecule in the active site pocket. The sulfate oxygen of a C₈S ligand is complexed to the active site calcium in three of the four protein active sites. The interactions of the alkyl sulfate head group with Arg-6 and Lys-10, as well as the backbone amide of Met-20, are analogous to those observed in the previously solved sPLA₂ crystal structures with bound phosphate and sulfate anions. The cluster of three anions found in the present structure is postulated to be the site for nucleating the binding of anionic amphiphiles to the interfacial surface of the protein, and therefore this binding interaction has implications for interfacial activation of the enzyme.