In biochemistry, an ether lipid refers to any lipid in which the lipid "tail" group is attached to the glycerol backbone via an ether bond at any position. In contrast, conventional glycerophospholipids and triglycerides are triesters. Structural types include: Based on whether the sn-1 lipid is unsaturated next to the ether linkage, they can be further divided into alkenyl-acylphospholipids ("plasmenylphospholipid", 1-0-alk-1’-enyl-2-acyl-sn-glycerol) and alkyl-acylphospholipids ("plasmanylphospholipid"). This class of lipids have important roles in human cell signaling and structure. The term "plasmalogen" can refer to any ether lipid with a vinyl ether linkage, i.e. ones with a carbon-carbon double bond next to the ether linkage. Without specification it generally refers to alkenyl-acylphospholipids, but "neutral plasmalogens" (alkenyldiacylglycerols) and "diplasmalogens" (dialkenylphospholipids) also exist. The prototypical plasmalogen is platelet-activating factor.

In eukaryotes

Biosynthesis

The formation of the ether bond in mammals requires two enzymes, dihydroxyacetonephosphate acyltransferase (DHAPAT) and alkyldihydroxyacetonephosphate synthase (ADAPS), that reside in the peroxisome. Accordingly, peroxisomal defects often lead to impairment of ether-lipid production. Monoalkylglycerol ethers (MAGEs) are also generated from 2-acetyl MAGEs (precursors of PAF) by KIAA1363.

Functions

Structural

Plasmalogens as well as some 1-O-alkyl lipids are ubiquitous and sometimes major parts of the cell membranes in mammals. The glycosylphosphatidylinositol anchor of mammalian proteins generally consist of an 1-O-alkyl lipid.

Second messenger

Differences between the catabolism of ether glycerophospholipids by specific phospholipases enzymes might be involved in the generation of lipid second messenger systems such as prostaglandins and arachidonic acid that are important in signal transduction. Ether lipids can also act directly in cell signaling, as the platelet-activating factor is an ether lipid signaling molecule that is involved in leukocyte function in the mammalian immune system.

Antioxidant

Another possible function of the plasmalogen ether lipids is as antioxidants, as protective effects against oxidative stress have been demonstrated in cell culture and these lipids might therefore play a role in serum lipoprotein metabolism. This antioxidant activity comes from the enol ether double bond being targeted by a variety of reactive oxygen species.

Synthetic ether lipid analogs

Synthetic ether lipid analogs have cytostatic and cytotoxic properties, probably by disrupting membrane structure and acting as inhibitors of enzymes within signal transmission pathways, such as protein kinase C and phospholipase C. A toxic ether lipid analogue miltefosine has recently been introduced as an oral treatment for the tropical disease leishmaniasis, which is caused by leishmania, a protozoal parasite with a particularly high ether lipid content in its membranes.

In archaea

The cell membrane of archaea consist mostly of ether phospholipids. These lipids have a flipped chirality compared to bacterial and eukaryotic membranes, a conundrum known as the "lipid divide". The "tail" groups are also not simply n-alkyl groups, but highly methylated chains made up of saturated isoprenoid units (e.g. phytanyl). Among different groups of archaea, diverse modifications on the basic archaeol backbone have emerged.

In bacteria

Ether phospholipids are major parts of the cell membrane in anaerobic bacteria. These lipids can be variously 1-O-alkyl, 2-O-alkyl, or 1,2-O-dialkyl. Some groups have, like archaea, evolved tetraether lipids.

In prokaryotes

Some ether lipids found in marine animals are S-batyl alcohol, S-chimyl alcohol, and S-selachyl alcohol.