Lipid
Lipids are a class of
hydrocarbon-containing
organic compounds essential for the structure and function of living
cells. Lipids are characterized by being
water-
insoluble and soluble in nonpolar organic solvents. Although the term
lipid is often used as a synonym for
fat, the latter is in fact a subgroup of lipids called
triglycerides.
 |
Figure 1: Structure of a Lipid. Many lipids consist of a polar head group (P) and a nonpolar tail (U for unpolar). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on to represent lipids with one, two, or three chains. |
Along with
proteins,
nucleic acids and
carbohydrates, lipids are one of the major classes of biologically important molecules or
biomolecules. However, unlike these other groups of molecules, lipids comprise a broader and more diverse range of structures. The term lipid really is a catch-all phrase for a wide variety of
hydrocarbon-based molecules of biological origin that are predominantly nonpolar or
hydrophobic ("water-fearing"), meaning that they do not interact well with polar solvents like water, and are instead soluble in less polar organic solvents. Most lipids also have some
polar or
hydrophilic ("water-loving") character. This makes them
amphipathic or
amphiphilic molecules (having both hydrophobic and hydrophilic portions). In the case of
cholesterol, the polar group is a mere -OH (
hydroxyl or alcohol). In the case of
phospholipids, the polar groups are considerably larger and more polar. Lipids encompass a huge range of structures. They can be
aliphatic or
aromatic. They can be acyclic or cyclic, straight or branched, saturated or unsaturated. Lipids can be flexible or rigid. This diversity makes it impossible to define lipids on the basis of a single core structural feature or biosynthetic origin, as can be done with the other major groups of biomolecules. The basic classes of lipids are:
*
Fatty acids
** Saturated
** Unsaturated
*
Glycerides or glycerolipids
***
Monoglycerides
***
Diglycerides
***
Triglycerides (neutral
fats)
** Phosphoglycerides or
glycerophospholipids
* Nonglycerides
**
Sphingolipids
**
Sterol lipids (includes
cholesterol and steroid
hormones)
**
Prenol lipids (includes
terpenoids)
**
Waxes
**
Polyketides
* More complex lipid derivatives
** Sugar-linked lipids:
glycolipids
** Protein-linked lipids
Note: There are different ways to classify lipids. A comprehensive new classification system has been proposed recently (J. Lipid Res. 46:839), which instead divides lipids into: (1) fatty acyls, (2) glycerolipids, (3) glycerophospholipids, (4) sphingolipids, (5) sterol lipids, (6) prenol lipids, (7) saccharolipids and (8) polyketides.
* Cell membrane structure
** Constitutes a barrier for the cell
** Controls the flow of material in and out of the cell
* Energy storage (for instance, fats stored in adipose tissue)
* Lipid
hormones like
steroids and
eicosanoids - mediate communication between cells
*
Signal transduction - function in the transmission of information in cells
* Lipid
vitamins - required for metabolism, usually as coenzymes
Fatty acids and glycerides
Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH
3(CH
2)
nCOOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no
double bonds, it is called saturated. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in
cis or
trans geometric isomers. In most naturally occurring fatty acids, the double bonds are in the cis configuration.
Glycerides are lipids possessing a
glycerol core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by
ester linkages. Glycerides with three acyl groups (
triglycerides or neutral fats) are the main storage form of fat in animals and plants.
taba mo
An important type of glyceride-based molecule found in
biological membranes, such as the cell's
plasma membrane and the intracellular membranes of
organelles, are the phosphoglycerides or
glycerophospholipids. These are
phospholipids that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely,
ether linkages and to one "head" group by a
phosphate ester linkage. The head groups of the phospholipids found in
biological membranes are phosphatidylcholine (
lecithin), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. These phospholipids are subject to a variety of reactions in the cell: for instance, polar head groups or fatty acid tails can be released from specific phospholipids through enzyme-catalyze hydrolysis to generate
second messengers involved in
signal transduction. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one to three phosphate molecules, and this constitutes another mechanism of
cell signaling. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like
sphingolipids and
sterols (such as
cholesterol in animal cell membranes) are also found in biological membranes.
A biological membrane is a form of lipid bilayer, as is a
liposome. Formation of lipid bilayers is a spontaneous process when the glycerophospholipids described above are placed in water. In an aqueous milieu, the polar heads of lipids tend to oriente toward the polar, aqueous environment, while the hydrophobic tails tend to minimize their contact with water. The nonpolar tails of lipids (
U) tend to cluster together, forming a
lipid bilayer (1) or a
micelle (2). The polar heads (
P) face the aqueous environment. Micelles form when single-tailed
amphiphilic lipids are placed in a polar milieu, while lipid bilayers form when two-tailed phospholipids are placed in a polar environment (Fig. 2). Micelles are "monolayer" spheres and can only reach a certain size, whereas bilayers can be considerably larger. They can also form tubules. Bilayers that fold back upon themselves form a hollow sphere, enclosing a separate aqueous compartment, which is the basis of biological membranes.
Micelles and bilayers separate out from the polar milieu by a process known as the "hydrophobic effect." When dissolving a nonpolar substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved nonpolar substance, since the polar molecules cannot form
hydrogen bonds to the nonpolar molecule. Therefore, in an aqueous environment, the polar water molecules form an ordered "
clathrate" cage around the dissolved nonpolar molecule. However, when the nonpolar molecules separate out from the polar liquid, the
entropy (state of disorder) of the polar molecules in the liquid increases. This is essentially a form of phase separation, similar to the spontaneous separation of oil and water into two separate phases when one puts them together.
 |
Figure 2: Self-organization of lipids. A lipid bilayer is shown on the left and a micelle on the right. |
The self-organisation depends on the concentration of the lipid present in solution. Below the
critical micelle concentration, the lipids form a single layer on the liquid surface and are dispersed in solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at the second critical micelle concentration (CMC-II) into elongated pipes, and at the lamellar point (LM or CMC-III) into stacked lamellae of pipes. The CMC depends on the chemical composition, mainly on the ratio of the head area and the tail length.
Lipids play diverse and important roles in
nutrition and
health. Many lipids are absolutely essential for life. However, there is also considerable awareness that abnormal levels of certain lipids, particularly cholesterol (in
hypercholesterolemia) and, more recently,
trans fatty acids, are risk factors for
heart disease and other diseases.
*
Chapter 12 in "Biochemistry" by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer (2002) W. H. Freeman and Co.
*
Biochemistry*
Fat*
Saturation (chemistry)*
ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs.*
Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology*
The Lipid library - provides information on the chemistry, analysis and biochemistry of lipids*
LIPID MAPS: LIPID Metabolites and Pathways Strategy*
IUPAC glossary entry for the lipid class of molecules what is IUPAC?*
"A comprehensive classification system for lipids"*
CCMDWeb, a lipid health educational resource on global risk reduction for dyslipidemia and cardiovascular disease as well as online CME programs.
