Phospholipid

Phospholipid

Polar group of the molecule, highlighted in red.
The U indicates the uncharged hydrophobic portion of the molecule, highlighted in blue.

Phosphatidyl choline is the major component of lecithin. It is also a source for choline in the synthesis of acetylcholine in cholinergic neurons.

Cell membranes consist of phospholipid bilayers

Phospholipids are a class of lipids and are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from sphingosine instead of glycerol. The first phospholipid identified as such in biological tissues was lecithin, or phosphatidylcholine, in the egg yolk, by Theodore Nicolas Gobley, a French chemist and pharmacist, in 1847.

Amphipathic character

The 'head' of a phospholipid is hydrophilic (attracted to water), while the hydrophobic 'tails' repel water. 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.^[1] Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane with hydrophilic heads on both sides facing the water.

This type of membrane is partially permeable, capable of elastic movement, and has fluid properties, in which embedded proteins (integral or peripheral proteins) and phospholipid molecules are able to move laterally. 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 However, this model has now been superseded, as through the study of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.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

See: 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 (Cer-PE)
• Ceramide phosphorylglycerol

Simulations

Computational simulations of phospholipids are often performed using molecular dynamics with force fields such as GROMOS, CHARMM, or AMBER.

Phospholipid synthesis

Phospholipid synthesis occurs in the cytosol adjacent to ER membrane that is studded with proteins that act in synthesis (GPAT and LPAAT acyl transferases, phosphatase and choline phosphotransferase) and allocation (flippase and floppase). Eventually a vesicle will bud off from the ER containing phospholipids destined for the cytoplasmic cellular membrane on its exterior leaflet and phospholipids destined for the exoplasmic cellular membrane on its inner leaflet.^[2]

In signal transduction

Some types of phospholipid can be split to produce products that function as second messengers in signal transduction. Examples include phosphatidylinositol (4,5)-bisphosphate (PIP₂), that can be split into inositol triphosphate (IP₃) and diacylglycerol (DAG), which both carry out the functions of the G_q type of G protein in response to various stimuli.

Food technology

Phospholipids can also act as an emulsifier, enabling oils to dissolve in water. Phospholipids called lecithin are extracted out of cooking oil and then used as food additives in many things such as bread and can also be purchased separately in a health food store.

Phospholipid derivatives

See table below for an extensive list.

• Natural phospholipid derivates:

  egg PC, egg PG, soy PC, hydrogenated soy PC, sphingomyelin as natural phospholipids.

• Synthetic phospholipid derivates:
  • Phosphatidic acid (DMPA, DPPA, DSPA)
  • Phosphatidylcholine (DDPC, DLPC, DMPC, DPPC, DSPC, DOPC, POPC, DEPC)
  • Phosphatidylglycerol (DMPG, DPPG, DSPG, POPG)
  • Phosphatidylethanolamine (DMPE, DPPE, DSPE DOPE)
  • Phosphatidylserine (DOPS)
  • PEG phospholipid (mPEG-phospholipid, polyglycerin-phospholipid, funcitionalized-phospholipid, terminal activated-phospholipid)

Abbreviations used and chemical information of glycerophospholipids

References

1. ↑ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html. 
2. ↑ Lodish, Harvey; Berk, Krieger, Kaiser, Scott, Bretsher, Ploegh, Matsuaira (2008). Molecular Cell Biology. W.H. Freeman and Company. ISBN 0716776014. http://bcs.whfreeman.com/lodish6e/default.asp?s=&n=&i=&v=&o=&ns=0&uid=0&rau=0. 

• J.M.Berg, J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. viii, 974, [976] (various pages)