Bio Chemistry

BIOCHEMISTRY 3(2+1)

Lesson 8 : Lipids- Introduction, Classification and Functions

Fatty acids

1. The simplest with structural formula of R-COOH where R = hydrocarbon chain.
2. They differ from each other by the length of the tail, degree of unsaturation, and position of double bonds.
3. If there is no double bond, the fatty acid is saturated.
4. If there is at least one double bond, the fatty acid is unsaturated.
5. Monounsaturated fatty acids contain 1 double bond; polyunsaturated fatty acids have >2 double bonds.
6. Nomenclature =Dn represents where double bond occurs as you count from the carboxyl end



e.g:

-enoate -dienoate -trienoate -tetraenoate

one double bond 2 “ 3 “ 4 “

7. Can also use a colon separating 2 numbers, where the first number represents the number of carbon atoms and the second number indicates the location of the double bonds.

e.g: linoleate 18:2?9,12 or cis,cis -?9,12octadecadienoate

8. Physical properties differ between saturated and unsaturated fatty acids.

Saturated = solid at Room Temp; often animal source; e.g. lard
Unsaturated = liquid at Room Temp; plant source; e.g. vegetable oil

9. The length of the hydrocarbon tails influences the melting point.
10. As the length of tails increases, melting points increases due to number of van der Waals interactions.
11. Also affecting the melting point is the degree of unsaturation.
12. As the degree of unsaturation increases, fatty acids become more fluid--> melting point decreases.
13. Fatty acids are also an important source of energy. 9 kcal/g vs. 4 kcal/g for carbohydrates and proteins.

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 acetyl-CoA primer with malonyl-CoA or methylmalonyl CoA groups in a process called fatty acid synthesis. They are made of a hydrocarbon chain that terminates with a carboxylic acid group; this arrangement confers the molecule with a polar, hydrophilic end, and a nonpolar, hydrophobic end that is insoluble in water. The fatty acid structure is one of the most fundamental categories of biological lipids, and is commonly used as a building block of more structurally complex lipids. The carbon chain, typically between four to 24 carbons long, may be saturated or unsaturated, and may be attached to functional groups containing oxygen, halogens, nitrogen and sulfur. Where a double bond exists, there is the possibility of either a cis or trans geometric isomerism, which significantly affects the molecule's molecular configuration. Cis-double bounds cause the fatty acid chain to bend, an effect that is more pronounced the more double bonds there are in a chain. This in turn plays an important role in the structure and function of cell membranes. Most naturally occurring fatty acids are of the cis configuration, although the trans form does exist in some natural and partially hydrogenated fats and oils.

Examples of biologically important fatty acids are the eicosanoids, derived primarily from arachdonic acid and eicosapentaenoic acid, which include prostaglandins, leukotrienes and thromboxanes. Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as wax esters, fatty acid thioester coenzyme A derivatives, fatty acid thioester ACP derivatives and fatty acid carnitines. The fatty amides include N-acyl ethanolamines, such as cannabinoid neurotransmitter anandamide.