Lipid

Along with proteins, nucleic acids and carbohydrates, lipids are one of the major classes of biologically important molecules or biomolecules. They are water-insoluble organic compounds that are highly soluble in nonpolar organic solvents.

Although the term lipid is often used informally as a synonym for fat, the latter is in fact a subgroup of lipids referred to as a triglyceride. Unlike other groups of molecules, lipids comprise a broad and diverse range of structures, which also include phospholipids (components of cell membranes), sterols (most notably cholesterol and the steroid hormones), and more complex lipid derivatives such as glycolipids (sugar-linked lipids).

Consistent with their diverse chemical and structural properties, lipids have a variety of functions in the body:

• Structuring cell membranes. The cell membrane constitutes a barrier for the cell and controls the flow of material in and out of the cell
• Energy storage. Triglycerides are an efficient form of energy storage that can be mobilized
• Signal transduction, or the transmission of information in cells. Lipid hormones like steroids and eicosanoids also mediate communication between cells.
• Lipid vitamins are required for metabolism, usually as coenzymes.

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.

The major classes of lipids and their properties

Many lipids consist of a polar head group (P) and a nonpolar tail (U for unpolar). The lipid shown is a phospholipid (which has two tails).

The term lipid really is a catch-all phrase for a wide variety of hydrocarbon-based molecules of biological origin. Lipids encompass a huge range of structures, which can be aliphatic or aromatic, acyclic or cyclic, straight or branched, saturated or unsaturated, flexible or rigid. This diversity makes it impossible to define lipids on the basis of a single core structural feature or biosynthetic origin.

However, one shared property of lipids 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.

Their amphipathic qualities allow lipids to form lipid bilayers (left) and micelles (right).

The basic classes of lipids are:

• Fatty acids
• Glycerides or glycerolipids: Monoglycerides, Diglycerides, Triglycerides (neutral fats), as well as phosphoglycerides or glycerophospholipids
• Nonglycerides: Sphingolipids, [Sterol]] lipids (includes cholesterol and steroid hormones), Prenol lipids (includes terpenoids), Waxes, Polyketides
• More complex lipid derivatives, such as sugar-linked lipids (glycolipids) and protein-linked lipids

An alternative classification system has been proposed (J. Lipid Res. 46:839), which instead divides lipids into the following groups: (1) fatty acyls, (2) glycerolipids, (3) glycerophospholipids, (4) sphingolipids, (5) sterol lipids, (6) prenol lipids, (7) saccharolipids and (8) polyketides.

Fatty acids: the building blocks of lipids

Three-dimensional representation of the saturated fatty acid myristic acid.

Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH₃(CH₂)_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.

A trans fatty acid (commonly shortened to trans fat) is an unsaturated fatty acid molecule that contains a trans double bond between carbon atoms, which makes the molecule less 'kinked' in comparison to fatty acids with cis double bonds. These bonds are characteristically produced during industrial hydrogenation of plant oils. Research suggests that increasing amounts of trans fats are, for causal reasons not well understood, correlate with circulatory diseases such as atherosclerosis and coronary heart disease, than the same amount of non-trans fats.

Fatty acids can be bound or attached to other molecules, like triglycerides or phospholipids. When they are not attached to other molecules, they are known as "free" fatty acids. The uncombined fatty acids or free fatty acids may come from the breakdown of a triglyceride into its components (fatty acids and glycerol).

Free fatty acids are an important source of fuel for many tissues since they can yield relatively large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. However, heart and skeletal muscle prefer fatty acids. On the other hand, the brain cannot use fatty acids as a source of fuel, relying instead on glucose, or on ketone bodies produced by the liver from fatty acid metabolism during starvation, or periods of low carbohydrate intake.

Glycerides: the energy storage lipids

The characteristic structure of a triglyceride, with a glycerol head (left) attached to three fatty acid tails. This example of an unsatured fat contains the fatty acids palmitic acid, oleic acid, and alpha-linolenic acid.

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.

Adipose tissue is an anatomical term for loose connective tissue composed of adipocytes. Its main role is to store energy in the form of fat, although it also cushions and insulates the body.

Triglycerides play an important role in metabolism as energy sources. They contain more than twice as much energy (9 kcal/g) as carbohydrates and proteins.

Phospholipids: membrane components

The structure of sphingomyelin, a phospholipid derived from sphingosine.

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. 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.

Sterol lipids: structure and signaling

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Cholesterol

Cholesterol.

Cholesterol is a sterol (a combination steroid and alcohol) and a lipid found in the cell membranes of all body tissues, and transported in the blood plasma of all animals. Lesser amounts of cholesterol are also found in plant membranes. The name originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix -ol for an alcohol, as researchers first identified cholesterol (C₂₇H₄₅OH) in solid form in gallstones in 1784.

Most cholesterol is not dietary in origin; it is synthesized internally. Cholesterol is present in higher concentrations in tissues which either produce more or have more densely-packed membranes, for example, the liver, spinal cord and brain, and also in atheromata. Cholesterol plays a central role in many biochemical processes, but is best known for the association of cardiovascular disease with various lipoprotein cholesterol transport patterns and high levels of cholesterol in the blood.

When doctors talk to their patients about the health concerns of cholesterol, they are often referring to "bad cholesterol", or low-density lipoprotein (LDL). "Good cholesterol" is high-density lipoprotein (HDL); this denotes the way cholesterol is bound in lipoproteins, the natural carrier molecules of the body.

Cholesterol is required to build and maintain cell membranes; it makes the membrane's fluidity - degree of viscosity - stable over wider temperature intervals (the hydroxyl group on cholesterol interacts with the phosphate head of the membrane, and the bulky steroid and the hydrocarbon chain is embedded in the membrane). Some research indicates that cholesterol may act as an antioxidant. Cholesterol also aids in the manufacture of bile (which helps digest fats), and is also important for the metabolism of fat soluble vitamins, including vitamins A, D, E and K. It is the major precursor for the synthesis of vitamin D, of the various steroid hormones, including cortisol and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Further recent researchTemplate:Citationneeded shows that cholesterol has an important role for the brain synapses as well as in the immune system, including protecting against cancer.

Steroids

A steroid is a lipid characterized by a carbon skeleton with four fused rings. All steroids are derived from the acetyl CoA biosynthetic pathway. Different steroids vary in the functional groups attached to these rings. Hundreds of distinct steroids have been identified in plants, animals, and fungi. Their most important role in most living systems is as hormones. Steroid hormones produce their physiological effects by binding to steroid hormone receptor proteins. The binding of steroid hormones to their receptors causes changes in gene transcription and cell function.

Steroid skeleton. Carbons 18 and above can be absent.

In human physiology and medicine, the most important steroids are cholesterol, the steroid hormones, and their precursors and metabolites. In the bloodstream steroids are bound to carrier proteins.

Cholesterol is an important steroid alcohol, being a common component of animal cell membranes. However, a high level of it can cause various conditions and diseases, such as atherosclerosis. Most other steroids are synthesized from cholesterol. Also, various hormones, including vertebrate sex hormones, are steroids created from cholesterol.

Some of the common categories of steroids include:

• Anabolic steroids are a class of steroids that interact with androgen receptors to increase muscle and bone synthesis. There are natural and synthetic anabolic steroids. These are the "steroids" used by athletes to increase performance.
• Corticosteroids include glucocorticoid and mineralocorticoids:
  • Glucocorticoids regulate many aspects of metabolism and immune function, and often prescribed by doctors to reduce inflammatory conditions like asthma and arthritis.
  • Mineralocorticoids are corticosteroids that help maintain blood volume and control renal excretion of electrolytes.
• Sex steroids are a subset of sex hormones that produce sex differences or support reproduction. They include androgens, estrogens, and progestagens.
• Phytosterols - steroids naturally occurring in plants.

External links

• 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.

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