Specific Roles

Membrane lipids can have specific roles in addition to their normal function in membrane structure. Clearly the role of amphipathic lipids in forming the bilayer that is the foundation of normal membrane structure is one of the most important functions for any molecule in Nature. However, there are more specialized functions that we should not forget. In the following sections, we will deal firstly with the role of phospholipids as components of lung surfactant (where they, literally, help the breath of life) and then in cell signalling.

Lipid signalling is one of the most rapidly growing areas of lipid biochemistry. We have already come across examples of this in the section on eicosanoids (Section 2.4). In addition, there are many examples of intact lipids or their derivatives (Table 7.8) which are important in controlling such aspects of cellular activity as hormone action, cell differentiation and apoptosis.

Of course, whenever a biochemical plays an important role in biology then things can go wrong. Examples of diseases where lipids are involved are also included in the following sections. In addition, some previous sections of the book also cover aspects of disease (Sections 4.2.3, 4.2.4, 4.3, 4.4 and 5.4).

Table 7.8 Examples of lipids or their Precursor

20C Polyunsaturated fatty acid a-Linolenic acid

Platelet activating factor (PAF)

Phosphatidylcholine

Phosphatidic acid

Phosphatidyl inositol-4,5-bisphosphate

Sphingomyelin

Ceramide derivatives that have roles in signalling Signalling molecule

Eicosanoid (animals) Jasmonic acid (plants) PAF

Phosphatidic acid Diacylglycerol

Diacylglycerol; inositol-1,4,5-frisphosphate

Ceramide

Sphingosine; sphingosine-1-phosphate

Example of role

Inflammation, pain Stress responses Inflammation

Phagocytic respiratory burst

Protein kinase C regulation

Protein kinase C regulation; calmodulin kinase control

Apoptosis

Protein phosphorylation; release of calcium

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