FIGURE 3.

Stereo diagram of superposition of Ca²⁺-binding loop of PLA₂ enzymes. ScPLA₂ (Protein Data Bank code, 1OZ6; blue), ecarpholin S (red), and myotoxin II (Protein Data Bank code, 1CLP; yellow) were superimposed. Conformational differences of three different Ca²⁺-binding loops are shown. Three residues Gly³⁰-Trp³¹-Gly³² of scPLA₂ bend to form a coordination bond with Ca²⁺, whereas three residues Gly³⁰-Gly³¹-Gly³² of ecarpholin S extend linearly, causing Gly³² carbonyl O to move away from possible Ca²⁺ area (∼5 Å). The whole loop moves closer to Ser⁴⁹ residue, and Nα of Gly³¹ nearly occupies putative Ca²⁺-binding position (1.1 Å away from Gly³¹ N), whereas in myotoxin II, Nɛ of Lys⁴⁹ residue occupies Ca²⁺ position. The side-chain of Ser⁴⁹ residue of ecapholin S is too short to extend into Ca²⁺-binding site (Oγ of Ser⁴⁹ is 3.8 Å away from putative Ca²⁺-binding site). Image was prepared using the program Pymol (59).