Chemical structure of unsaturated fatty acids
Unsaturated fatty acids contain one or more double/triple carbon-carbon bonds in the carbon chain. On this basis they can be divided into three classes:
- monounsaturated fatty acids (MUFAs), if only one double bond is present;
- polyunsaturated fatty acids (PUFAs), if at least two double bonds are present;
- acetylenic fatty acids, if one or more triple bonds are present.
Cis and trans double bonds
Carbon-carbon double bonds are the most common multiple bonds present in unsaturated fatty acids. They have a planar conformation; therefore, they can be considered as plans from whose opposite sides carbon skeleton attaches and continues. If the entry and the exit of the chain occurs on the same side of the plan, double bond is defined in cis configuration. Conversely, when the chain enters and exits on opposite sides of the plan, double bond is defined in trans configuration.
In nature, cis configuration is the most common one for unsaturated fatty acids.
Generally, unsaturated fatty acids into membranes are confined to the sn-2 position of phospholipids, and have 18 to 20 carbon atoms (in sn-1 position there are saturated 16-18 C fatty acids).
In triacylglycerols, MUFAs (or long chain saturated fatty acids) tend to be in sn-1 and sn-3 positions, whereas PUFAs in sn-2 position.
Note: sn = stereospecific numbering.
Classification of unsaturated fatty acids
Monounsaturated fatty acids
Although more than 100 MUFAs have been identified in nature, most of these are very rare.
Oleic acid, a long chain monounsaturated fatty acid, is widely distributed and abundant in nature (few fats are known to contain less than 10%). The highest content is found in extra-virgin olive oil, about 70 g/100 g edible portion.
In human diet, unsaturated fatty acids are almost exclusively oleic acid. However, if fish and/or rapeseed oil consumption play a large role in one’s diet, there are present moderate amounts of 20 and 22 C monounsaturated fatty acids.
Polyunsaturated fatty acids
Depending on the relative positions of the double bonds, PUFAs can be divided in three categories:
- conjugated, if double-bonds alternate with single bonds (–C=C–C=C–);
- unconjugated, if the double bonds are separated by carbon atoms bonded with single bonds, usually in a methylene-interrupted arrangement (–C=C–C–C=C–);
- the third group, in which double bonds are not entirely in a methylene-interrupted arrangement (present in certain microorganisms, marine lipids and some seed oils).
Also if conjugated dienes exist, they don’t occur extensively. Examples are sorbic acid (2,4-hexadienoic acid) and trans-2-cis-4-decadienoic acid, a known flavor component present in Williams pears, (Bartlett pears in the U.S. and Canada).
Among conjugated PUFAs, the most common ones are trienes, but they are present in small amount in animal fats and are abundant in only a few seed oils.
The omega (ω) or n designation system
The most important PUFAs in terms of extent of occurrence are the unconjugated and methylene-interrupted ones.
They are subdivided into families, depending on the position of the double bond closest to the methyl side, numbered 1, of the fatty acid molecule and are categorized in 12 omega families (letter n may be used in place of letter ω).
This classification system applies also to monounsaturated fatty acids; for example, oleic acid is an omega-9 fatty acid.
The omega designation system groups unsaturated fatty acids into families depending on the capability to be converted/interconverted to each other (the interconversion between PUFA occurs only within the same family, not among families).
This does not mean that all the fatty acids of a ω-family have the same physiological functions. In fact it appear clearly that each fatty acid has its own role in nutrition; examples are:
- within omega-6 family, dihomo-gamma-linolenic acid and arachidonic acid, that give rise prostaglandins (eicosanoids) with different physiological effects;
- between omega-6 and omega-3 families, arachidonic acid and eicosapentaenoic acid (EPA), that differ only by an additional double bond in EPA, have a metabolic antagonistic relationship.
Note: omega-3 and omega-6 PUFAs are essential fatty acids.
It is thought that polyunsaturated fatty acids exert their physiologic effects through a variety of mechanisms, ranging from modulating membrane structure to acting as precursors for signal molecules, e.g. eicosanoids.
Polyunsaturated fatty acids and human diet
In human diet, the consumed PUFAs are almost exclusively linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid). In the typical Western diet they account for more than 95% of PUFA taken up, unless one is eating much fish, in which case there are good amounts of EPA and docosahexaenoic acid (DHA), both omega-3 polyunsaturated fatty acids.
For healthy people, with no documented cardiovascular diseases, American Heart Association recommends eating one serving of oily fish at least twice a week, plus oils rich in alpha-linolenic acid. In the case of patients with documented cardiovascular diseases, the recommendations are 1 g/day of EPA plus DHA, and for those with hypertriglyceridemia, 2 to 4 g/day of EPA plus DHA are recommended.
Acetylenic fatty acids
They have a linear carbon chain characterized by the presence of one or more triple carbon-carbon bonds.
They are rarely found in naturally occurring lipids (fats and oils), being present only in some mosses and in few seed oils.
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