Phospholipids

Lipids which contain a phosphate moiety are grouped together as phospholipids. This group of lipids is derived from phosphatidic acid, a molecule with glycerol esterified to two fatty acids and one molecule of phosphoric acid. The fatty acids most commonly found in phosphatidic acid are palmitic and stearic acids in their saturated forms. The further esterification of phosphatidic acid with a low-molecular-weight alcohol (i.e. ethanolamine, choline, serine, or inositol) gives rise to phospholipids.

structure_phosphatidic_acid
Phosphatidic Acid


Propanol Based Phospholipids

Just as described for propanol based lipids, fatty acids can be linked to simpler phosphate containing alcohol structures.


Glycerol Based Phospholipids

Like glycerol based lipids, these glycerophospholipds have a glycerol backbone. In addition the glycerol is attached to a phosphate group as explained above. The figure below reveals the structures of the most commonly found phospholipids in nature.

Glycerophospholipids
 
Common Glycerophospholipids


The main reason phospholipids are of such great interest is their presence in membranes. The glycerophospholipds are amphipathic molecules with polar heads and (usually) long nonpolar tails.

Phosphatidate
Simplest structure of a phospholipid, phosphatidate 

The phosphatidate pictured above is usually an intermediate for the biosynthesis of more complex glycerophospholipids. In the more complex forms, the phosphate is esterified to both glycerol and another alcohol (see table above). Generally these more complex molecules are the phosphatidylethanolamines (cephalin), phosphatidylserines, and the phosphatidylcholines (lecithin).


Ethanolamine Glycerophospholipids

Selection of Biosynth's Ethanolamine Glycerophospholipids
Cat. No. - Product Name
D-3250 - 1,2-Di-O-hexadecyl-sn-glycero-3-phosphoethanolamine
D-5679 - 1,2-Dimyristoyl-sn-glycero-3-phosphoethanolamin
D-3244 - 1,2-Di-O-hexadecyl-sn-glycero-3-phospho-(N-methyl)ethanolamin
D-6067 - 1,3-Dipalmitoyl-glycero-2-phospho-(N-methyl)ethanolamin
D-6057 - 1,2-Dipalmitoyl-sn-glycero-3-phospho-(N,N-dimethyl)ethanolamine
H-1832 - 1,2-Hexadecyliden-rac-glycero-3-phospho-(N-N-dimethyl)ethanolamin


Serine Glycerophospholipids

Biosynth's Serine Glycerophospholipids
Cat.No. - Product Name
D-3260 - 1,2-Di-O-hexadecyl-sn-glycero-3-phosphoserin
D-6069 - 1,2-Dipalmitoyl-sn-glycero-3-phosphoserin

Choline Glycerophospholipids

Glycerophospholipids (also called phosphoglycerides) are the structural components of lipid bilayers. Even though triacylglycerols are more abundantly found in mammals based purely on weight, they lack amphipathic properties, and therefore, cannot form bilayers. This lipid bilayer is the basis of all cell membrane structures, and acts as a relatively impermeable barrier to the passage of most water soluble molecules. Protein molecules embedded in this bilayer can act as transporters for molecules that would normally not be able to pass through the membrane. Differences in the lengths and saturation of the fatty acid chains influence the fluidity of the membrane (fluid mosaic model) as does the proximity of the phospholipids to each other.

Bilayers form spontaneously among phospholipids molecules in aqueous environments due to their amphipathic nature. When surrounded by water on all sides, they aggregate to protect their hydrophobic tails in the interior of the bilayer or micelle. The bilayer is actually the more favorable structure of the two since it protects the tails on all sides from interaction with water. These bilayer structures are conveniently employed in nature to partition individual cells from one another as well as provide a way to partition organelles within the cells themselves. The fluidity of the structure is also advantageous to its functions within and among the cells.

As mentioned, the fluidity of a specific bilayer can vary. The type of phopholipid bilayer surrounding a specific cell or cell compartment (i.e. lysosome, Golgi, or ER) is governed by its function and by its surrounding temperature. The lipid bilayer is usually not just phopholipids, but contains cholesterol and glycolipids, in addition to the proteins already mentioned. Cholesterol boosts the permeability barrier properties of a bilayer. This means that the bilayers are more rigid in areas where there is lots of cholesterol. In addition, cholesterol functions as a way to prevent phase transition, by preventing the crystallization of hydrocarbon chains. The table below lists the different composition of phospholipids plasma membranes (PM).

 
Table: Approximate Lipid Composition of Various Cell Membranes (%)

LipidLiver-
PM
Erythrocyte
PM
Mitochondrion
PM
E. coli
PM
 Cholesterol   17 23 
 Phosphatidyl-ethanolamine    7 18  35  70 
 Phosphatidylserine  4 trace 
 Phosphatidyl-choline  24 17  39 
 Sphingomyelin  19 18 
 Glycolipids  7 trace 
 Others    22 13  21  30 



A selection from Biosynth's Choline Glycerophospholipids
Cat.No. - Product Name
D-1422 - 1,2-Diarachinoyl-sn-glycero-3-phosphocholine
D-3235 - 1,2-Di-O-hexadecyl-sn-glycero-3-phosphocholine
D-5667 - 1,2-Dimyristoyl-sn-glycero-3-phosphocholine
D-7850 - 1,2-Distearoyl-sn-glycero-3-phosphocholine
G-4205 - sn-Glycero-3-phosphocholine
O-0320 - 1-O-Octadecyl-2-O-methyl-sn-glycero-3-phosphocholine



Other Phospholipids

Phosphate, or Phosphorous, is a non-metallic element and after calcium, the most abundant mineral in the body. Accordingly, phosphates are extremely important in human metabolism. Phosphate salts, such as sodium phosphate, are involved in acid-base balance. The rest of the body's phosphates are found in a variety of organic forms, including the phospholipids, which help form cell membranes.

Selection of Biosynth's Phosphate Salts
Cat.No. - Product Name
D-3220 - 1,2,-Di-O-hexadecyl-sn-glycero-3-phosphate diammonium salt
I-7968 - 1,2-Isopropylidene-sn-glycerol-3-phosphat calcium salt
O-0302 - 1-O-Octadecyl-2-O-methyl-rac-glycero-3-phosphate disodium salt


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