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Fatty acid


In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from triglycerides or phospholipids. Fatty acids are important sources of fuel because, when metabolized, they yield large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. Long-chain fatty acids cannot cross the blood–brain barrier (BBB) and so cannot be used as fuel by the cells of the central nervous system; however, free short-chain fatty acids and medium-chain fatty acids can cross the BBB, in addition to glucose and ketone bodies.

Fatty acids that have carbon–carbon double bonds are known as unsaturated. Fatty acids without double bonds are known as saturated. They differ in length as well.

Fatty acid chains differ by length, often categorized as short to very long.

Unsaturated fatty acids have one or more double bonds between carbon atoms. (Pairs of carbon atoms connected by double bonds can be saturated by adding hydrogen atoms to them, converting the double bonds to single bonds. Therefore, the double bonds are called unsaturated.)

The two carbon atoms in the chain that are bound next to either side of the double bond can occur in a cis or trans configuration.


Examples of Unsaturated Fatty Acids
Common name Chemical structure Δx C:D nx
Myristoleic acid CH3(CH2)3CH=CH(CH2)7COOH cis9 14:1 n−5
Palmitoleic acid CH3(CH2)5CH=CH(CH2)7COOH cis9 16:1 n−7
Sapienic acid CH3(CH2)8CH=CH(CH2)4COOH cis6 16:1 n−10
Oleic acid CH3(CH2)7CH=CH(CH2)7COOH cis9 18:1 n−9
Elaidic acid CH3(CH2)7CH=CH(CH2)7COOH trans9 18:1 n−9
Vaccenic acid CH3(CH2)5CH=CH(CH2)9COOH trans11 18:1 n−7
Linoleic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH cis,cis912 18:2 n−6
Linoelaidic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH trans,trans912 18:2 n−6
α-Linolenic acid CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH cis,cis,cis91215 18:3 n−3
Arachidonic acid CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOHNIST cis,cis,cis,cis5Δ81114 20:4 n−6
Eicosapentaenoic acid CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH cis,cis,cis,cis,cis58111417 20:5 n−3
Erucic acid CH3(CH2)7CH=CH(CH2)11COOH cis13 22:1 n−9
Docosahexaenoic acid CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)2COOH cis,cis,cis,cis,cis,cis4710131619 22:6 n−3
Examples of Saturated Fatty Acids
Common name Chemical structure C:D
Caprylic acid CH3(CH2)6COOH 8:0
Capric acid CH3(CH2)8COOH 10:0
Lauric acid CH3(CH2)10COOH 12:0
Myristic acid CH3(CH2)12COOH 14:0
Palmitic acid CH3(CH2)14COOH 16:0
Stearic acid CH3(CH2)16COOH 18:0
Arachidic acid CH3(CH2)18COOH 20:0
Behenic acid CH3(CH2)20COOH 22:0
Lignoceric acid CH3(CH2)22COOH 24:0
Cerotic acid CH3(CH2)24COOH 26:0
System Example Explanation
Trivial nomenclature Palmitoleic acid Trivial names (or common names) are non-systematic historical names, which are the most frequent naming system used in literature. Most common fatty acids have trivial names in addition to their systematic names (see below). These names frequently do not follow any pattern, but they are concise and often unambiguous.
Systematic nomenclature (9Z)-octadecenoic acid Systematic names (or IUPAC names) derive from the standard IUPAC Rules for the Nomenclature of Organic Chemistry, published in 1979, along with a recommendation published specifically for lipids in 1977. Counting begins from the carboxylic acid end. Double bonds are labelled with cis-/trans- notation or E-/Z- notation, where appropriate. This notation is generally more verbose than common nomenclature, but has the advantage of being more technically clear and descriptive.
Δx nomenclature cis,cis912 octadecadienoic acid In Δx (or delta-x) nomenclature, each double bond is indicated by Δx, where the double bond is located on the xth carbon–carbon bond, counting from the carboxylic acid end. Each double bond is preceded by a cis- or trans- prefix, indicating the configuration of the molecule around the bond. For example, linoleic acid is designated "cis9, cis12 octadecadienoic acid". This nomenclature has the advantage of being less verbose than systematic nomenclature, but is no more technically clear or descriptive.
nx nomenclature n−3 nx (n minus x; also ω−x or omega-x) nomenclature both provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on the xth carbon–carbon bond, counting from the terminal methyl carbon (designated as n or ω) toward the carbonyl carbon. For example, α-Linolenic acid is classified as a n−3 or omega-3 fatty acid, and so it is likely to share a biosynthetic pathway with other compounds of this type. The ω−x, omega-x, or "omega" notation is common in popular nutritional literature, but IUPAC has deprecated it in favor of nx notation in technical documents. The most commonly researched fatty acid biosynthetic pathways are n−3 and n−6.
Lipid numbers 18:3
18:3ω6
18:3, cis,cis,cis91215
Lipid numbers take the form C:D, where C is the number of carbon atoms in the fatty acid and D is the number of double bonds in the fatty acid (if more than one, the double bonds are assumed to be interrupted by CH
2
units
, i.e., at intervals of 3 carbon atoms along the chain). This notation can be ambiguous, as some different fatty acids can have the same numbers. Consequently, when ambiguity exists this notation is usually paired with either a Δx or nx term.
Saturated Monounsaturated Polyunsaturated Cholesterol Vitamin E
g/100g g/100g g/100g mg/100g mg/100g
Animal fats
Lard 40.8 43.8 9.6 93 0.60
Duck fat 33.2 49.3 12.9 100 2.70
Butter 54.0 19.8 2.6 230 2.00
Vegetable fats
Coconut oil 85.2 6.6 1.7 0 .66
Cocoa butter 60.0 32.9 3.0 0 1.8
Palm kernel oil 81.5 11.4 1.6 0 3.80
Palm oil 45.3 41.6 8.3 0 33.12
Cottonseed oil 25.5 21.3 48.1 0 42.77
Wheat germ oil 18.8 15.9 60.7 0 136.65
Soybean oil 14.5 23.2 56.5 0 16.29
Olive oil 14.0 69.7 11.2 0 5.10
Corn oil 12.7 24.7 57.8 0 17.24
Sunflower oil 11.9 20.2 63.0 0 49.00
Safflower oil 10.2 12.6 72.1 0 40.68
Hemp oil 10 15 75 0 12.34
Canola/Rapeseed oil 5.3 64.3 24.8 0 22.21

cis 
A cis configuration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the cis configuration, the less flexibility it has. When a chain has many cis bonds, it becomes quite curved in its most accessible conformations. For example, oleic acid, with one double bond, has a "kink" in it, whereas linoleic acid, with two double bonds, has a more pronounced bend. α-Linolenic acid, with three double bonds, favors a hooked shape. The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer, or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore can affect the melting temperature of the membrane or of the fat.
trans 
A trans configuration, by contrast, means that the adjacent two hydrogen atoms lie on opposite sides of the chain. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.
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Wikipedia

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