It has been reported that the cooperative binding of calcium ions indicated a local conformational change of the human cytosolic phospholipase A 2 (cPLA 2 ) C2 domain (Nalefski et al., (1997) Biochemistry 36, 12011-12018). However its structural evidence is less known (Malmberg et al., (2003) Biochemistry 42, 13227-13240). In this letter, life-time decay and fluorescence quenching techniques were employed to compare the calcium-induced conformational changes. The life-time decay parameters and

more »

... luorescence quenching constant changes were small between the apo-and holo-C2 domains when tryptophan residue was excited at 295 nm. In contrast, the quenching constant change was large, from 0.52 M -1 for the apo-C2 to 8.8 M -1 for the holo-C2 domain, when tyrosine residues were excited at 284 nm. Our results provide new information on amino acid side chain orientation change at calcium binding loop 3, which is necessary for Ca 2+ binding regulated membrane targeting of human cytosolic phospholipase A 2 .