Is the high propensity of ethanolamine plasmalogens to form non-lamellar lipid structures manifested in the properties of biomembranes?

Plasmalogens are glycerophospholipids characterized by an alk-1'-enylether bond in position sn-1 and an acyl bond in position sn-2. These ubiquitous etherlipids exhibit a different molecular structure as compared to diacyl phospholipids. The most peculiar change is a perpendicular orientation of the sn-2 acyl chain at all segments to the membrane surface. This extended conformation results in an effectively longer aliphatic chain in plasmalogen than in the diacyl analog. Moreover, the lack of the carbonyl oxygen in position sn-1 affects the hydrophilicity of the headgroup and allows stronger intermolecular hydrogen-bonding between the headgroups of the lipid. These properties favour the formation of non-lamellar structures which are expressed in the high affinity of ethanolamine plasmalogen to adopt the inverse hexagonal phase. Such structures may be involved in membrane processes, either temporarily, like in membrane fusion or locally, e.g. to affect the activity of membrane-bound proteins. The predominant distribution of ethanolamine plasmalogens in some cellular membranes like nerve tissues or plasma membranes and their distinctly different properties in model membranes as compared to diacyl phospholipids impose the question, whether these differences are also manifested in the heterogeneous environment of biological membranes. The integration of biophysical studies and biochemical findings clearly indicated that the high propensity of ethanolamine plasmalogen to form non-lamellar structures is reflected in several physiological functions. So far it seems to be evident that ethanolamine plasmalogens play an important role in maintaining the balance between bilayer and non-lamellar phases which is crucial for proper cell function. Furthermore, they are the major phospholipid component of inverse hexagonal phase inclusions in native retina and are able to mediate membrane fusion as demonstrated between neurotransmitter vesicles and presynaptic membranes.