Cholesterol synthesis condensation mechanism

Figure 26.9. Condensation Mechanism in Cholesterol Synthesis. The mechanism for joining dimethylallyl pyrophosphate and isopentenyl pyrophosphate to form geranyl pyrophosphate. The same mechanism is used to add an additional isopentenyl pyrophosphate to form famesyl pyrophosphate.

Carbonyl condensations are among the most widely used reactions in the biological world for the assembly of new carbon-carbon bonds in such important biomolecules as fatty acids, cholesterol, and steroid hormones. One source of carbon atoms for the synthesis of these biomolecules is acetji-CoA, a thioester of acetic acid and the thiol group of coenzyme A. The function of the coenzyme A group of acetyl-CoA is to anchor the acetyl group on the surface of the enzyme systems that catalyze the reactions we examine in this section. In the discussions that follow, we will not be concerned with the mechanism by which each enzyme-catalyzed reaction occurs. Rather, our concern is with recognizing the type of reaction that takes place in each step. 

The surface rheological proprieties can be controlled by using monolayers of different lipid compositions. We used saturated and unsaturated lipids and their mixtures with different amounts of cholesterol. At the experimental temperature and film pressure the saturated lipids were in the liquid condensed or solid phase whereas the unsaturated lipids in the liquid expanded phase [3]. The mechanical properties of the monolayers can be tuned with addition of different amounts of cholesterol. The results are used for the proof of bilayer or multilayer synthesis and the conditions of their occurrence. 

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