What are steroids?
Many compounds of widely differing functions are classified as steroids because they have the same general structure: a fused-ring system consisting of three six-membered rings (the A, B, and C rings) and one five-membered ring (the D ring).
There are many important steroids, including sex hormones. The steroid that is of most interest in our discussion of membranes is cholesterol (Figure 8.9). The only hydrophilic group in the cholesterol structure is the single hydroxyl group.
As a result, the molecule is highly hydrophobic. Cholesterol is widespread in biological membranes, especially in animals, but it does not occur in prokaryotic cell membranes. The presence of cholesterol in membranes can modify the role of membrane-bound proteins. Cholesterol has a number of important biological functions, including its role as a precursor of other steroids and of vitamin D3. We will see a five-carbon structural motif (the isoprene unit) that is common to steroids and to fat-soluble vitamins, which is an indication of their biosynthetic relationship. However, cholesterol is best known for its harmful effects on health when it is present in excess in the blood. It plays a role in the development of atherosclerosis, a condition in which lipid deposits block the blood vessels and lead to heart disease.