Lipids are defined as organic molecules that share the property of being water insoluble but are highly soluble in organic solvents. They are structurally diverse and have many different functions, including:
- components of cell membranes (Chapter 2)
- fuel molecules (Chapter 11)
- signalling molecules.
Some of the most important biological lipids include fatty acids, triglycerides and cholesterol.
Fatty acids
Structure
Fatty acids are composed of a long hydrocarbon chain with a terminal carboxylic acid. They normally contain between 14 and 24 carbon atoms and have the general formula CH3(CH2)nCOOH (Figure 5.1a). They exist in a saturated (no double bonds in hydrocarbon chain), monounsaturated (one double bond) and polyunsaturated (two or more double bonds) form. Most of the double bonds in naturally occurring unsaturated fatty acids exist in a cis configuration (same side of the double bond) (Figure 5.1b). A trans configuration (opposite sides of the double bond) can also occur (Figure 5.1c). The addition of the double bond causes a bend or kink in the hydrocarbon chain, which impacts on the packing of these molecules and causes a reduction in the melting point of the molecule. For example, stearate, an 18C saturated fatty acid has a melting point of 69°C, while oleate, a monosaturated 18C fatty acid, has a melting point of 13°C. This is important when considering the role of fatty acids in cell membrane structure (Chapter 2). The chain length also affects the melting point; the longer the chain length, the higher the melting point.
Nomenclature
The naming of fatty acids is slightly confusing, as while systematic names exist, naturally occurring fatty acids are generally called by their common names. A numbering system is also used, whereby the left-hand number depicts the number of carbon atoms and the right hand number reflects the number of double bonds in the hydrocarbon chain. Table 5.1 lists some of the naturally occurring common fatty acids. Normally the name of the fatty acid ends in ‘oic acid’, but most fatty acids are ionised at physiological pH (COO− instead of COOH) and end in ‘ate’. For example, palmitic acid (CH3(CH2)14COOH) will occur physiologically as palmitate (CH3(CH2)14COO−).
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