Positions of two homologous phospholipases in the lipid bilayer. (A) position of porcine pancreatic sPLA2 calculated by PPM, and (B) experimentally defined arrangement of human pancreatic sPLA2. (a homology model, 1ysk PDB entry) studied by ATR FTIR spectroscopy . Trp residues identified as penetrating the non-polar environment by fluorescence quenching are colored red. Ca2+ ions are shown as balls colored magenta. Hydrocarbon core boundary at the extracellular side is indicated by red dots. The layer of lipid phosphates ("P") is shown by gold dots (at 5 Å outside the hydrocarbon boundary). The center of membrane is indicated by grey dots (at 15 Å inside the boundary). The obtained orientations are quite similar, but the model of human sPLA2 (B) penetrates slightly deeper and with a slightly different (by ~10°) tilt into the membrane interior. Therefore, N23, N24, N117, which were localized outside hydrophobic boundaries by our method (A), appeared to be immersed into the hydrophobic slab in the experimentally-derived position of the protein (B).