Lipids are a diverse group of organic compounds that are essential for several biological functions, ranging from energy storage to cell signaling. They are loosely described as organic, water-insoluble compounds demonstrating high solubility in non-polar solvents.
The diversity of lipids is reflected in the variety of natural structures. Unlike other biological molecules that are comprised of relatively few components, lipids are complex. Their biosynthesis involves numerous biochemical transformations, generating vast quantities of lipid molecules.
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A specific set of nomenclature, chemical representation, and a classification system are therefore necessary to not only comprehensively characterize lipids, but also enable bioinformatic databases, tools and methodologies to study their role on a systems-biology level.
This was realized by The LIPID MAPS consortium, which comprises eight primary lipid categories, within which are several hierarchal subcategories. For the purposes of simplicity, lipids can be classified as:
Storage lipids
Fatty acids
Fatty acids are comprised of a polar head (a carboxyl group) and a nonpolar aliphatic tail. They span a length of between 4 and 36 carbons in length. The exhibition of both polar and non-polar properties is described as amphipathy. Within a cell, they are associated with other biological molecules.
Fatty acids can be broadly classified as saturated or unsaturated. The physical properties of fatty acids depend on length and degree of unsaturation of their aliphatic chains. In their fully saturated forms, the most stable conformation is the fully extended form, in which steric hindrance of neighboring atoms is minimized. This allows ordering into crystalline arrays with the aliphatic tails associating through van der waals forces.
In unsaturated fatty acids, double bonds cause kinks to appear in the chain; this prevents tight packing of fatty acids and alters the properties of the arrays they form. This affects membrane properties as fatty acids are important constituents of phospholipids, which comprise many membranes.
In the body, fatty acids are released from triacylglycerols during fasting to provide a source of energy. They circulate in the blood by binding to a protein carrier, serum albumin where they travel to the tissue for use in metabolism or biosynthetic pathways.
Triacylglycerols
Triacylglycerols are the primary storage form of long-chain fatty acids, which are broken down for energy and used in the structural formation of cells. Triacylglycerols are composed of glycerol (1,2,3-trihydroxypropane) and 3 fatty acids to form a triester.
Simple triacylglycerols contain identical fatty acids, however, most naturally occurring fatty acids are mixed. Triacylglycerols are stored in adipocytes in vertebrates or as soils in the seed of plants. Both adipocytes and seeds contain lipase enzymes to liberate fatty acids for export when they are required for fuel or biosynthetic purposes.
In some animals, triacylglycerols provide a means of insulation; this is particularly notable in arctic-dwelling mammals such as walruses, polar bears, and penguins. Polyunsaturated fatty acids are important as constituents of the phospholipids and form the membranes of the cells.
Tri-, Di- and Monoacylglycerols
Triacylglycerol, diacylglycerol, and monoacylglycerol contain three, two, or one fatty acid(s) respectively, which are esterified to trihydroxy-alcohol glycerol. While triacylglycerol functions predominantly as an energy storage molecule, diacylglycerol and monoacylglycerol species perform signaling roles as secondary messengers or ligands for signaling proteins such as protein kinases. These proteins are implicated in diverse pathways including cell proliferation, growth and protein transport.
Sterols
Sterols are comprised of tetracyclic rings, a feature common to human sex pheromones. Sterols can be conjugated to fatty acids, fatty acid esters, and sugars. Sterols have a fundamental effect in membrane properties, affecting fluidity, membrane transport and function of membrane proteins.
Sterols interact with phospholipids to stiffen and impermeabilize the membrane. They work specifically to alter the dynamics of a process known as phase transition. This describes the transition of a membrane from the solid phase (gel phase) to liquid at a defined temperature.
Specifically, sterols may eliminate this ability of membranes to transition. Alongside sphingolipids, sterols may form structures called lipid rafts which are implicated in signaling and membrane trafficking. Outside of the cell membrane, sterols, particularly cholesterols, are precursor of bile acids, vitamin D and steroidal hormones.
Structural lipids
Complex cell membrane lipids
Cellular membranes control the transport of materials, including signaling molecules and can change in form to enable budding, fission, and fusion. The cell membranes have a hydrophilic (water-loving) constituent and a hydrophobic (water repelling) constituent, making them amphiphilic.
Phospholipids
There are two classes of phospholipids. The first, glycerophospholipids, are comprised of glycerol fatty acid esters, phosphatidic acids, and alcohols. Three alcohols that form phosphatides are choline, ethanolamine, and serine.
Phospholipids differ from triglycerides in their ability to act at the cell membrane as well as functioning as emulsifiers in food products. This latter function exploits their ability to reduce the interfacial tension between oil and water. Consequently, they are useful for purposes of emulsification, solubilization, or dispersion.
The second are sphingolipids. Sphingolipids have a long-chain or sphingoid base, such as sphingosine, to which a fatty acid is linked by an amide bond. The simplest sphingolipid is ceramide. They have high phase transition temperatures, and as such, form lipid rafts along with cholesterols. They, therefore, play an important role in cell signaling processes.
Glycolipids
Glycolipids are acylglycerols, ceramides, and prenols that are attached to one or more monosaccharide residues. They are crucial during cell development as they affect cell-cell interactions, immune responses and cell proliferation.
Other lipids
Lipoproteins
Lipoproteins are complex proteins that are comprised of a hydrophobic core of triglycerides and cholesterol esters surrounded by a hydrophilic shell of phospholipids, apolipoproteins, and unesterified cholesterol.
Apolipoproteins both stabilize and target the complex to a tissue. They can be classified according to their density and in descending order, they are HDL (high-density lipoprotein), IDLs (intermediate-density lipoproteins) LDL (low-density lipoprotein), VLDL (very low-density lipoprotein).
Lipoproteins play a role in metabolism. They are used to store and transport excess dietary (exogenous) and liver-generated (endogenous) lipids and cholesterol. The type of particle in which they are packaged dictates their destination.
Polyketides
Polyketides are made by polymerization of acetyl and propionyl subunits using enzymes. Polyketides form a large number of secondary metabolites and natural products from animal, plant, bacterial, fungal sources. Antimicrobials or antibiotics like erythromycins, tetracyclines and anticancer agents like epothilones are polyketides.
Further Reading