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Calcium binding by phosphatidylserine headgroups. Deuterium NMR study
Biophysical Journal
|July 1, 1991
Summary
Calcium binding to phosphatidylserine (POPS) membranes induces dehydration and phase changes. In mixed POPC/POPS membranes, POPS remains in a liquid-crystalline state even with calcium, indicating reduced cochleate formation.
Area of Science:
- Biochemistry
- Membrane Biophysics
- Nuclear Magnetic Resonance Spectroscopy
Background:
- Phosphatidylserine (POPS) is a key anionic phospholipid in cell membranes.
- Calcium ions are known to interact with anionic lipids, potentially altering membrane structure.
- Understanding these interactions is crucial for cell signaling and membrane dynamics.
Purpose of the Study:
- To investigate the effect of calcium binding on the structure and phase behavior of POPS-containing membranes.
- To determine the conditions under which calcium-induced dehydration and cochleate formation occur.
- To examine the influence of calcium on POPS headgroup conformation in mixed lipid bilayers.
Main Methods:
- Deuterium magnetic resonance (2H NMR) spectroscopy was employed.
- Experiments were conducted on pure POPS bilayers and mixed POPC/POPS (5:1 m:m) bilayers.
- Varying concentrations of calcium chloride (CaCl2) were added to the lipid systems.
Main Results:
- In pure POPS bilayers, calcium addition induced a transition from a liquid-crystalline phase to a dehydrated cochleate phase, observable as distinct spectral components.
- A Ca2+ to POPS molar ratio of 0.5 nearly eliminated the liquid-crystalline POPS signal, indicating significant cochleate formation.
- In mixed POPC/POPS bilayers, even high calcium concentrations (Ca2+ to POPS ratio > 50) did not induce substantial cochleate formation, with POPS remaining largely in the liquid-crystalline state (<5% cochleate).
- Deuterium NMR data revealed a temperature-induced conformational change in the serine headgroup of POPS, independent of calcium presence in mixed bilayers.
Conclusions:
- Calcium binding promotes the dehydration and precipitation of POPS into cochleate structures in pure POPS membranes.
- The presence of zwitterionic lipids like POPC significantly inhibits calcium-induced cochleate formation, stabilizing POPS in a liquid-crystalline state.
- The serine headgroup exhibits temperature-dependent conformational flexibility, a property not directly modulated by calcium in mixed lipid environments.