Lipid microdomain
Encyclopedia
Lipid microdomains are formed when lipid
s undergo lateral phase
separations yielding stable coexisting lamellar domain
s. These phase separations can be induced by changes in temperature
, pressure
, ionic strength
or by the addition of divalent
cations or protein
s. The question of whether such lipid microdomains observed in model lipid systems also exist in biomembranes had motivated considerable research efforts. Lipid domains are not readily isolated and examined as unique species, in contrast to the examples of lateral heterogeneity.
One can disrupt the membrane and demonstrate a heterogeneous range of composition
in the population
of the resulting vesicles
or fragment
s. Electron microscopy can also be used to demonstrate lateral inhomogeneities in biomembranes.
Often, lateral heterogeneity has been inferred from biophysical techniques where the obsereved signal indicates multiple populations rather than the expected homogenous population. An example of this is the measurement of the diffusion coefficient of a fluorescent lipid analogue in soybean protoplast
s. Membrane microheterogeneity is sometimes inferred from the behavior of enzyme
s, where the enzymatic activity does not appear to be correlated with the average
lipid physical state exhibited by the bulk of the membrane. Often, the methods suggest regions with different lipid fluidity
, as would be expected of coexisting gel
and liquid crystal
line phases within the biomembrane. This is also the conclusion of a series of studies where differential effects of perturbation caused by cis and trans fatty acid
s are interpreted in terms of preferential partitioning of the two liquid crystalline and gel-like domains.
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s undergo lateral phase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
separations yielding stable coexisting lamellar domain
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
s. These phase separations can be induced by changes in temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, ionic strength
Ionic strength
The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...
or by the addition of divalent
Divalent
In chemistry, a divalent ion or molecule has a valence of two and thus can form two bonds with other ions or molecules. An older term for divalent is bivalent....
cations or protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. The question of whether such lipid microdomains observed in model lipid systems also exist in biomembranes had motivated considerable research efforts. Lipid domains are not readily isolated and examined as unique species, in contrast to the examples of lateral heterogeneity.
One can disrupt the membrane and demonstrate a heterogeneous range of composition
Composition
Composition may refer to:* Composition , in which one assumes that a whole has a property solely because its various parts have that property* Compounding is also known as composition in linguistic literature* in computer science...
in the population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
of the resulting vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
or fragment
Fragment
Fragment may refer to:* A small part/portion broken off something; debris* Fragment , all the data necessary to generate a pixel in the frame buffer* Sentence fragment, a sentence not containing a subject or a predicate...
s. Electron microscopy can also be used to demonstrate lateral inhomogeneities in biomembranes.
Often, lateral heterogeneity has been inferred from biophysical techniques where the obsereved signal indicates multiple populations rather than the expected homogenous population. An example of this is the measurement of the diffusion coefficient of a fluorescent lipid analogue in soybean protoplast
Protoplast
Protoplast, from the ancient Greek πρῶτον + verb πλάθω or πλάττω , initially referred to the first organized body of a species.Protoplast has several biological definitions:...
s. Membrane microheterogeneity is sometimes inferred from the behavior of enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s, where the enzymatic activity does not appear to be correlated with the average
Average
In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....
lipid physical state exhibited by the bulk of the membrane. Often, the methods suggest regions with different lipid fluidity
Fluidity
Fluidity may refer toIn science*reciprocal of viscosity*Cognitive fluidity*Membrane fluidity*Sexual fluidityOthers*Fluidity *Dark Fluidity – a literature magazine*Empire Fane ship...
, as would be expected of coexisting gel
Gel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
and liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
line phases within the biomembrane. This is also the conclusion of a series of studies where differential effects of perturbation caused by cis and trans fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s are interpreted in terms of preferential partitioning of the two liquid crystalline and gel-like domains.
See also
- BiochemistryBiochemistryBiochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
- Essential fatty acidEssential fatty acidEssential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them...
- Lipid signalingLipid signalingLipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses...
- Saturation (chemistry)Saturation (chemistry)In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
External links
- http://www.biophysj.org/cgi/content/abstract/94/8/3104 - Lipid microdomain formation.
- http://www.jimmunol.org/cgi/reprint/168/6/2737.pdf - Lipid microdomain clustering.
- http://medcat.wustl.edu/catflat/BME/B720468.html - Lipid microdomain signaling.