Phospholipids are a class of lipids that 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. The structure of the
phospholipid molecule generally consists of hydrophobic tails and a
hydrophilic head. Biological membranes in eukaryotes also contain
another class of lipid, sterol, interspersed among the phospholipids and
together they provide membrane fluidity and mechanical strength.
Purified phospholipids are produced commercially and have found
applications in nanotechnology and materials science.
Amphipathic character
The 'head' is hydrophilic (attracted to water), while the hydrophobic
'tails' are repelled by water and are forced to aggregate. The
hydrophilic head contains the negatively charged phosphate group, and
glycerol. The hydrophobic tail usually consists of 2 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, sterols) in a bilayer such as a cell membrane.
Lipid bilayers occur when hydrophobic tails line up against one
another, forming a membrane of 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. Sterols
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 However, this model has now been superseded, as through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.
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
Types of phospholipid
Diacylglyceride structures
- 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 phosphoryllipid
Simulations
Computational simulations of phospholipids are often performed using molecular dynamics with force fields such as GROMOS, CHARMM, or AMBER.
Characterization
Phospholipids are optically highly birefringent, i.e. their refractive index is different along their axis as opposed to perpendicular to it. Measurement of birefringence can be achieved using cross polarisers in a microscope to obtain an image of e.g. vesicle walls or using techniques such as dual polarisation interferometry to quantify lipid order or disruption in supported bilayers.
Analysis
There are no simple methods available for analysis of Phospholipids
since the close range of polarity between different phospholipid species
makes detection difficult.
Oil chemists often use spectroscopy to determine total Phosphorus
content and then calculate content of Phospholipids based on molecular
weight of expected fatty acid species. Lipidomists use more absolute
methods of analysis of with nuclear magnetic resonance spectroscopy (NMR), particularly 31P-NMR, while HPLC-ELSD provides relative values.
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.
Sources
Common sources of industrially produced phospholipids are soya,
rapeseed, sunflower, chicken eggs, bovine milk, fish eggs etc. Each
source has a unique profile of individual phospholipid species and
consequently differing applications in food, nutrition, pharmaceuticals,
cosmetics and drug delivery.
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 by the enzyme Phospholipase C into inositol triphosphate (IP3) and diacylglycerol (DAG), which both carry out the functions of the Gq type of G protein in response to various stimuli and intervene in various processes from long term depression in neurons to leukocyte signal pathways started by chemokine receptors.
Phospholipids also intervene in prostaglandin signal pathways as the raw material used by lipase enzymes to produce the prostaglandin precursors. In plants they serve as the raw material to produce Jasmonic acid, a plant hormone similar in structure to prostaglandins that mediates defensive responses against pathogens.
Food technology
Phospholipids can act as an emulsifier, enabling oils to form a colloid with water. Phospholipids are one of the components of lecithin which is found in egg-yolks, as well as being extracted from soy beans, and is used as a food additive in many products, and can be purchased as a dietary supplement.
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
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