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Tuesday 24 June 2008

Definition: Glycerophospholipids are a subtype of phospholipids.

Glycerophospholipids are ubiquitous in nature and are key components of the lipid bilayer of cells, as well as being involved in metabolism and signaling.

Glycerophospholipids may be subdivided into distinct classes, based on the nature of the polar headgroup at the sn-3 position of the glycerol backbone in eukaryotes and eubacteria or the sn-1 position in the case of archaebacteria.

Examples of glycerophospholipids found in biological membranes are:
- phosphatidylcholine (also known as PC or GPCho, and lecithin)
- phosphatidylethanolamine (PE or GPEtn)
- phosphatidylserine (PS or GPSer).

In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as phosphatidylinositols and phosphatidic acids are either precursors of, or are themselves, membrane-derived second messengers.

Typically one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (plasmalogen) glycerophospholipids, as well as dialkylether variants in prokaryotes.

Amphipathic character

The ’head’ is hydrophilic (attracted to water), while the hydrophobic ’tails’ are repelled by water and are forced to aggregate.

The hydrophillic head contains the negatively charged phosphate group, and may contain other polar groups.

The hydrophobic tail usually consists of long fatty acid hydrocarbon chains.

When placed in water, phospholipids form a variety of structures depending on the specific properties of the phospholipid. These specific properties allow phospholipids to play an important role in the phospholipid bilayer.

In biological systems, the phospholipids often occur with other molecules (e.g., proteins, glycolipids, cholesterol) in a bilayer such as a cell membrane.

Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane hydrophilic heads on both sides facing the water.

Such movement can be described by the Fluid Mosaic Model, that describes the membrane as a mosaic of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane.

Cholesterol contributes to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.

Types of phospholipid

- diacylglyceride structures (glycerophospholipid)

  • phosphatidic acid (phosphatidate) (PA)
  • phosphatidylethanolamine (cephalin) (PE)
  • phosphatidylcholine (lecithin) (PC)
  • phosphatidylserine (PS)

- phosphoinositides

  • phosphatidylinositol (PI)
  • phosphatidylinositol phosphate (PIP)
  • phosphatidylinositol bisphosphate (PIP2) and
  • phosphatidylinositol triphosphate (PIP3).

- phosphosphingolipids

  • ceramide phosphorylcholine (sphingomyelin) (SPH)
  • ceramide phosphorylethanolamine (sphingomyelin) (Cer-PE)
  • ceramide phosphorylglycerol

See also