Ether lipid

In

• Ether phospholipids: phospholipids are known to have ether-linked "tails" instead of the usual ester linkage.[1]
  • Ether on sn-1, ester on sn-2: "ether lipids" in the context of bacteria and eukaryotes refer to this class of lipids. Compared to the usual 1,2-diacyl-sn-glycerol (DAG), the sn-1 linkage is replaced with an ester bond.^[1][2][3]

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.^[4]

• • Ether on sn-2 and sn-3: this class with flipped chirality on the phosphate connection is called an "archaeal ether lipid". With few (if any) exceptions, it is only found among archaea. The part excluding the phoshphate group is known as archaeol.^[5][6]
• Ether analogues of triglycerides: 1-alkyldiacyl-sn-glycerols (alkyldiacylglycerols) are found in significant proportions in marine animals.^[5]
• Other ether lipids: a number of other lipids not belonging to any of the classes above contain the ether linkage. For example, seminolipid, a vital part of the testes and sperm cells, has a ether linkage.^[1]

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.^[1] The prototypical plasmalogen is platelet-activating factor.^[7]

In eukaryotes

Biosynthesis

The formation of the ether bond in mammals requires two enzymes,

Plasmalogens as well as some 1-O-alkyl lipids are ubiquitous and sometimes major parts of the cell membranes in mammals.^[9] The glycosylphosphatidylinositol anchor of mammalian proteins generally consist of an 1-O-alkyl lipid.^[1]

Second messenger

Differences between the

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.^[12] This antioxidant activity comes from the enol ether double bond being targeted by a variety of reactive oxygen species.^[13]

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

Among different groups of archaea, diverse modifications on the basic archaeol backbone have emerged.

• The two tails can be linked together, forming a macrocyclic lipid.^[15]
• Bipolar macrocyclic tetraether lipids (
  covalently linked 'bilayers'.^[16][15]
  • Some such covelant bilayers feature crosslinks between the two chains, giving an H-shaped molecule.[15]
  • Crenarchaeol is a tetraether backbone with cyclopentane and cyclohexane rings on the cross-linked "tail"s.^[15]
• Some lipids replace the glycerol backbone with four-carbon polyols (tetriols).^[15]

In bacteria

Ether phospholipids are major parts of the cell membrane in anaerobic bacteria.^[1] These lipids can be variously 1-O-alkyl, 2-O-alkyl, or 1,2-O-dialkyl. Some groups have, like archaea, evolved tetraether lipids.^[17]

In prokaryotes

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

See also

• Membrane lipid
• Glycerol dialkyl glycerol tetraether

References