Please mail me with any comments, suggestions or reviews.
If you are an instructor and wish to order a desk copy of Biochemistry 2nd edition by Mathews and van Holde simply click the image of the text above.
In contrast to what is seen for nucleic acids, proteins and polysaccharides lipid are not polymeric in structure. Lipid molecules do have a tendency, however, to associate together, the form of this aggregation being determined by lipid structure and molecular properties.
Lipids are amphipathic in character, i.e. they are composed, in general, of a hydrophobic tail (typically a hydrocarbon chain), and a hydrophilic head group. An example of such a structure is the fatty acid shown below, palmitic acid.
The hydrophobic portion of a fatty acif may be saturated as we see in palmitic acid, or may be unsaturated as is the case for oleic acid below. Note that the cis arrangement of the double bond causes oleic acid to have a bend in its shape.
This fatty acid chain, with carbon in its most reduced form, yields the maximum amount of energy on oxidation. Fats take advantage of this property for the purposes of energy storage. Fats, or triacylglycerols, as shown below, are comprised of a glycerol molecule esterified with three fatty acids.
Cholesterol, a steroid, can also be considered a lipid even though it superficially beras little structural resemplance to palmitic acid above. In cholesterol the hydrophobic tail is provided by the steroid backbone, balck in the image below, with the alcohol group, in blue, providing the hydophilic head group. Cholesterol is an important lipid component of membranes.
Lipids are often found associated, either complexed or covalently bound, with proteins. Such lipoproteins serve a number of functions:
Such an apoprotein, i.e. the polpeptide component of a lipoprotein can be viewed by clicking here. As can be seen the structure is ideally suited for lipid transport, in this case stearic acid, with the hydrophobic residues forming a barrel structure within which the fatty acid is covalently bound.
allows for an interactive display of the apoprotein.
allows for an interactive display of the now twisted fatty acid.
