Lipid headgroup superlattice modulates the activity of surface-acting cholesterol oxidase in ternary phospholipid/cholesterol bilayers

The relationship between the molecular organization of lipid headgroups and the activity of surface-acting enzyme was examined using a bacterial cholesterol oxidase (COD) as a model. The initial rate of cholesterol oxidation by COD in fluid state 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1- palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers was measured as a function of POPE-to-phospholipid mole ratio (XPE) and cholesterol-to-lipid mole ratio ( XCHOL )a t 37°C. At XPE ) 0, the COD activity changed abruptly at XCHOL 0.40, whereas major activity peaks were detected at XPE 0.18, 0.32, 0.50, 0.64, and 0.73 when XCHOL was fixed to 0.33 or 0.40. At a fixed XCHOL of 0.50, the COD activity increased progressively with PE content and exhibited small peaks or kinks at XPE 0.40, 0.50, 0.58, 0.69, and 0.81. When XPE and XCHOL were systematically varied within a narrow 2-D lipid composition window, an onset of COD activity at XCHOL 0.40 and the elimination of the activity peak at XPE 0.64 for XCHOL >0.40 were clearly observed. Except for XPE 0.40 and 0.58, the observed critical PE mole ratios agree closely ((0.03) with those predicted by a headgroup superlattice model (Virtanen, J.A., et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4964-4969; Cannon, B., et al. (2006) J. Phys. Chem. B 110, 6339-6350), which proposes that lipids with headgroups of different sizes tend to adopt regular, superlattice-like distributions at discrete and predictable compositions in fluid lipid bilayers. Our results indicate that headgroup superlattice domains exist in lipid bilayers and that they may play a crucial role in modulating the activity of enzymes acting on the cell membrane surface. The lateral organization of phospholipid and cholesterol in cellular membranes and its effect on protein structure and function are thought to play a significant role in the pathogenesis and designing of therapeutic interventions for various diseases such as Alzheimer's, atherosclerosis, and bacterial or viral infections ( 1, 2). For example, a membrane- surface-acting ‚-amyloid protein, a primary factor in Alzhe- imer's disease, has been shown to interact with membrane cholesterol (3-5). Cholesterol enriched lipid domains in the plasma membrane of vascular smooth muscle cells (SMC 1 )