Lipid

Although the term "lipid" is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, monoglycerides, and phospholipids), as well as other sterol-containing metabolites such as cholesterol.^[6] Although humans and other mammals use various biosynthetic pathways both to break down and to synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

History

In 1815, Henri Braconnot classified lipids (graisses) in two categories, suifs (solid greases or tallow) and huiles (fluid oils).^[7] In 1823, Michel Eugène Chevreul developed a more detailed classification, including oils, greases, tallow, waxes, resins, balsams and volatile oils (or essential oils).^[8][9][10]

The first synthetic triglyceride was reported by

In 1827, William Prout recognized fat ("oily" alimentary matters), along with protein ("albuminous") and carbohydrate ("saccharine"), as an important nutrient for humans and animals.^[13][14]

For a century, chemists regarded "fats" as only simple lipids made of fatty acids and glycerol (glycerides), but new forms were described later.

The word lipide, which stems etymologically from Greek λίπος, lipos 'fat', was introduced in 1923 by the French pharmacologist Gabriel Bertrand.^[19] Bertrand included in the concept not only the traditional fats (glycerides), but also the "lipoids", with a complex constitution.^[9] The word lipide was unanimously approved by the international commission of the Société de Chimie Biologique during the plenary session on July 3, 1923. The word lipide was later anglicized as lipid because of its pronunciation ('lɪpɪd). In French, the suffix -ide, from Ancient Greek -ίδης (meaning 'son of' or 'descendant of'), is always pronounced (ɪd).

In 1947, T. P. Hilditch defined "simple lipids" as greases and waxes (true waxes, sterols, alcohols).^{[20][page needed]}

Categories

Lipids have been classified into eight categories by the Lipid MAPS consortium^[3] as follows:

Fatty acyls

Fatty acyls, a generic term for describing fatty acids, their conjugates and derivatives, are a diverse group of molecules synthesized by chain-elongation of an

Glycerolipids are composed of mono-, di-, and tri-substituted glycerols,^[30] the best-known being the fatty acid triesters of glycerol, called triglycerides. The word "triacylglycerol" is sometimes used synonymously with "triglyceride". In these compounds, the three hydroxyl groups of glycerol are each esterified, typically by different fatty acids. Because they function as an energy store, these lipids comprise the bulk of storage fat in animal tissues. The hydrolysis of the ester bonds of triglycerides and the release of glycerol and fatty acids from adipose tissue are the initial steps in metabolizing fat.^{[31]: 630–1}

Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more

Glycerophospholipids, usually referred to as

Examples of glycerophospholipids found in biological membranes are phosphatidylcholine (also known as PC, GPCho or lecithin), phosphatidylethanolamine (PE or GPEtn) and phosphatidylserine (PS or GPSer). In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as phosphatidylinositols and phosphatidic acids are either precursors of or, themselves, membrane-derived second messengers.^[31]: 844 Typically, one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (plasmalogen) glycerophospholipids, as well as dialkylether variants in archaebacteria.^[38]

Sphingolipids

Sphingolipids are a complicated family of compounds

The major phosphosphingolipids of mammals are sphingomyelins (ceramide phosphocholines),^[40] whereas insects contain mainly ceramide phosphoethanolamines^[41] and fungi have phytoceramide phosphoinositols and mannose-containing headgroups.^[42] The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a glycosidic bond to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as cerebrosides and gangliosides.

Sterols

Sterols, such as

Plant thylakoid membranes have the largest lipid component of a non-bilayer forming monogalactosyl diglyceride (MGDG), and little phospholipids; despite this unique lipid composition, chloroplast thylakoid membranes have been shown to contain a dynamic lipid-bilayer matrix as revealed by magnetic resonance and electron microscope studies.[59]

A biological membrane is a form of

The formation of lipids into protocell membranes represents a key step in models of abiogenesis, the origin of life.^[64]

Energy storage

Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. They are a major source of energy in aerobic respiration. The complete oxidation of fatty acids releases about 38 kJ/g (9 kcal/g), compared with only 17 kJ/g (4 kcal/g) for the oxidative breakdown of carbohydrates and proteins. The adipocyte, or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone-sensitive enzyme lipase.^[65] Migratory birds that must fly long distances without eating use triglycerides to fuel their flights.^[2]: 619

Signaling

Evidence has emerged showing that

Triglyceride synthesis takes place in the endoplasmic reticulum by metabolic pathways in which acyl groups in fatty acyl-CoAs are transferred to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.^{[2]: 733–9}

• Solid lipid nanoparticle – Novel drug delivery system
• Simple lipid
• Emulsion test
• Lipid microdomain
• Membrane lipid – Lipid molecules on cell membrane
• Lipidomics – large scale study of an organism's lipid metabolism using high-end chemical analysis techniquesPages displaying wikidata descriptions as a fallback
  • Lipidome – Totality of lipids in cells
• Protein–lipid interaction
• Phenolic lipid – Class of organic compounds, a class of natural products composed of long aliphatic chains and phenolic rings that occur in plants, fungi and bacteria

References

Bibliography

• Bhagavan NV (2002). Medical Biochemistry. San Diego: Harcourt/Academic Press.
  ISBN 978-0-12-095440-7
  .
• Devlin TM (1997). Textbook of Biochemistry: With Clinical Correlations (4th ed.). Chichester: John Wiley & Sons.
  ISBN 978-0-471-17053-2
  .
• Stryer L, Berg JM, Tymoczko JL (2007). Biochemistry (6th ed.). San Francisco: W.H. Freeman.
  ISBN 978-0-7167-8724-2
  .
• van Holde KE, Mathews CK (1996). Biochemistry (2nd ed.). Menlo Park, California: Benjamin/Cummings Pub. Co.
  ISBN 978-0-8053-3931-4
  .

External links

Look up lipid in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Lipids.

Introductory

• List of lipid-related web sites
• Nature Lipidomics Gateway – Round-up and summaries of recent lipid research
• Lipid Library – General reference on lipid chemistry and biochemistry
• Cyberlipid.org – Resources and history for lipids.
• Molecular Computer Simulations – Modeling of Lipid Membranes
• Lipids, Membranes and Vesicle Trafficking – The Virtual Library of Biochemistry, Molecular Biology and Cell Biology

Nomenclature

• IUPAC nomenclature of lipids
• IUPAC glossary entry for the lipid class of molecules

Databases

• LIPID MAPS – Comprehensive lipid and lipid-associated gene/protein databases.
• LipidBank – Japanese database of lipids and related properties, spectral data and references.

General

• ApolloLipids – Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs
• National Lipid Association – Professional medical education organization for health care professionals who seek to prevent morbidity and mortality stemming from dyslipidemias and other cholesterol-related disorders.