Lipids- Definition, Function, and Categories
The term lipid was coined by Bloor in 1943.
Lipids are Macro biomolecules, and are diverse groups of compounds including fats, oils, hormones, etc. that are insoluble in water due to their hydrophobic nature.
The lipids are soluble in organic solvents including fatty acids, waxes, sterols, fat-soluble vitamins (Vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, and phospholipids.
Characteristic features of lipid
The lipids show following characters:
- These are hydrophobic or amphiphilic small molecules
- The amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar or unilamellar liposomes, or membranes in an aqueous environment.
- Biological lipids originate entirely or in part from two distinct types of biochemical subunits or “building blocks”: Ketoacyl and isoprene groups.
Functions of lipids:
Important functions of lipis are:
- Component of biological membranes: Glycophospholipids are the main structural component of biological membranes, other lipids like sphingomyelin, and sterols are also found in biological membranes.
- Storing energy: Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. in Aerobic respiration, they are the major source of energy as they release the energy of twice more dioxygen than carbohydrates such as glycogen do, per mass. for example, migratory birds fly long without eating by using triglycerides to fuel their flight.
- Signaling: Lipid signaling is a vital part of cell signaling.
- Structural component: Acts as a structural component of cell membranes.
- Lipids are also used in the cosmetics and food industries,
- It has also application in nanotechnology
Categories of lipids
Following categories of lipids:
- Fatty acids
- Sterol lipids
- Prenol lipids
The fatty acids are a diverse group of molecules synthesized by chain-elongation of an Acetyl-CoA primer with malonyl-CoA or methymalonyl-CoA groups. The process of synthesis is called fatty acid synthesis.
They are made up of a hydrocarbon chain that terminates with a carboxylic acid group.
Structure of fatty acid: The fatty acid structure is one of the most fundamental categories of biological lipids, it is commonly used as a building block of structurally complex lipids.
- The carbon chains in fatty acids are typically four and 24 carbons long may be saturated or unsaturated,
- The carbon chain may be attached to functional groups containing oxygen, halogens, nitrogen, and sulfur.
- Cis –double bonds cause the fatty acid chain to bend
- Three double bonds in 18-carbon linolenic acid, the most abundant fatty acyl – chains of the plant thylakoid membrane, render these membranes highly fluid despite environmental low-temperature it makes linolenic acid give dominating sharp peak in high-resolution 13-C NMR spectra of the chloroplast. This plays important role in the structure and function of the cell membrane.
- Most naturally occurring fatty acids are of the Cis configuration.
- Trans form exists in some natural and partially hydrogenated fats and oils.
Examples of some important fatty acids:
- Eicosanoid: These are derived from arachidonic acid and eicosapentaenoic acid
- Docosahexaenoic acid: Important in a biological system (sight).
- Fatty esters: Includes important biochemical intermediates such as wax esters, fatty acid thioester coenzyme A derivative, etc.
- Fatty amides: include N-acyl ethanolamines, such as cannabinoid neurotransmitter anandamide.
The Glycerolipids are made up of mono, di, and tri-substituted glycerols.
- In these compounds, the three hydroxyl groups of glycerol are each esterified by different fatty acids.
- They function as an energy store, these lipids comprise the bulk storage fat in animal tissues.
- Additional subclasses of glycerolipids are represented by glycosylglcerols, which are characterized by the presence of one or more sugar residues attached to glycerol via a glycosidic linkage.
- Examples of Glycerolipids: digalactosyldiacylglycerols found in plant membranes, and Seminolipids in mammalian sperm cells.
These are a type of lipid usually referred to as phospholipids. The glycerophospholipid signifies a derivative of glycerophosphoric acid consisting of at least one O-alkyl, or O-acyl or O-alk1’enyl residue attached to glycerol (functional group).
- Glycerophospholipids are ubiquitous in nature. (Amphipathic nature)
- These are key components of the lipid bilayer of cells.
- These lipids serve as a structural component of biological membranes.
- They also play role in metabolism and cell signaling.
- The glycophospholipids can be also used as an emulsifying agent.
- A high amount of glycerophospholipids is found in Neural tissue.
- Structure: 3glycerol carbon atoms vertically with the phosphate attached to carbon atom number 3. Ex. Plasmalogen, phosphatidates.
- Example of glycerophospholipids: Phosphatidylcholine (PC, GPCho, lecithin), Phosphatidylethanolamine (PE or GPEtn), and Phosphatidylserine (PS or GPSer)
The Sphingolipids are complex lipids consisting of sphingoid base backbone, aliphatic amino alcohols including sphingosine.
- These lipids play important role in signal transduction and cell recognition.
- Disorder of sphingolipid metabolism shows a particular impact on neural tissue.
- The major phosphosphingolipids of mammals are sphingomyelins.
- Insects contain mainly ceramide – consists a hydrogen atom, fungi contain phytoceramide phosphoinositols, and mannose-contains headgroups.
- Sphingomyelins are the major Phosphosphingolipids of mammals.
- The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked by glycosidic bonds to the shingoides and gangliosides.
The saccharolipids are compounds that contain fatty acids linked directly to the sugar backbone, form compatible structures with membrane bilayers. In this type of lipid, a monosaccharide substitute for the glycerol backbone is present in glycolipids and glycerophospholipids.
- Acylated glucosamine is the most familiar sacchrolipids, are precursors of the lipid A, a component of the lipopolysaccharides in gram-negative bacteria.
- Acyl-trehaloses, like the Mycobacterial cord factor, are also examples of sacchrolipids.
- Saccharolipids are distinct from glycolipids as the latter is defined by IUPAC to have the sugar bound by a glycosidic linkage to a fatty acyl.
The polyketides are synthesized by polymerization of acetyl and propionyl subunits by classic enzymes as well as iterative and multimodular enzymes, that share mechanistic features with the fatty acid synthases.
- The polyketides comprise many secondary metabolites and natural products from animal, plant, bacterial, fungal, and marine sources.
- These have great structural diversity.
- Many polyketides are cyclic molecules whose backbones are often further modified by glycosylation, methylation, hydroxylation, oxidation, or other processes.
The function of polyketides: The polyketide or its derivatives such as erythromycins, tetracyclines, avermectins, and antitumor epothilones are commonly used as anti-microbial, anti-parasitic, and anti-cancer agents.
The sterol lipids are an important component of membrane lipids. The sterols are subgroups of steroids.
- Sterols are steroids in which one of the hydrogen atoms is substituted with a hydroxyl group, a position of 3 in the carbon chain. The chemical formula of sterol is C17H28O.
- The sterols are of two types: The phytosterol- Plant sterols; the Zoosterols- The sterols of animals.
Functions of sterols:
- The sterols and their derivatives such as cholesterol are an important component of membrane lipids.
- Bile acids and their conjugates are oxidized derivatives of cholesterol in mammals. These are synthesized in the liver.
- The plant equivalent is the phytosterols, such as stigmasterol, brassicasterol, and Bita sterol.
- Ergosterol is the predominant sterol in the fungal cell membranes.
Prenols are lipids synthesized from five – Carbon unit precursors isopentenyl diphosphate and dimethylallyl diphosphate, produced mainly via MVA (mevalonic acid) pathway.
- Carotenoids are important simple isoprenoids that function as antioxidants and as precursors of Vitamin A.
Reference: Some informations are taken from https://en.wikipedia.org/wiki/Lipid
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