Structure of Ceramide-1-Phosphate at the Air-Water Solution Interface in the Absence and Presence of Ca2+

TitleStructure of Ceramide-1-Phosphate at the Air-Water Solution Interface in the Absence and Presence of Ca2+
Publication TypeJournal Article
Year of Publication2009
AuthorsKooijman EE, Vaknin D, Bu W, Joshi L, Kang SW, Gericke A, Mann EK, Kumar S
Journal TitleBiophysical Journal
Date Published03/18
ISBN Number0006-3495
Accession NumberISI:000266376700017
Keywordsbrewster-angle, ceramide 1-phosphate, charged interfaces, cytosolic phospholipase a(2), domains, lipid monolayers, membrane-structure, phase-transitions, phosphatidic-acid, sphingomyelin

Ceramide-1-phosphate, the phosphorylated form of ceramide, gained attention recently due to its diverse intracellular roles, in particular in inflammation mediated-by cPLA(2 alpha). However, surprisingly little is known about the physical chemical properties of this lipid and its potential impact on physiological function. For example, the presence of Ca2+ is indispensable for the interaction of Cer-1-P with the C2 domain of cPLA(2 alpha). We report on the structure and morphology of Cer-1-P in monomolecular layers at the air/water solution interface in the absence and presence of Ca2+ using diverse biophysical techniques, including synchrotron x-ray reflectivity and grazing angle diffraction, to gain insight into the role and function of Cer-1-P in biomembranes. We show that relatively small changes in pH and the presence of monovalent cations dramatically affect the behavior of Cer-1-P. On pure water Cer-1-P forms a solid monolayer despite the negative charge of the phosphomonoester headgroup. In contrast, pH 7.2 buffer yields a considerably less solid-like monolayer, indicating that charge-charge repulsion becomes important at higher pH. Calcium was found to bind strongly to the headgroup of Cer-1-P even in the presence of a 100-fold larger Na+ concentration. Analysis of the x-ray reflectivity data allowed us to estimate how much Ca2+ is bound to the headgroup, similar to 0.5 Ca2+ and similar to 1.0 Ca2+ ions per Cer-1-P molecule for the water and buffer subphase respectively. These results can be qualitatively understood based on the molecular structure of Cer-1-P and the electrostatic/hydrogen-bond interactions of its phosphomonoester headgroup. Biological implications of our results are also discussed.

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