Biochemistry - The Chemical Reactions of Living Cells Volume 2 - D. Metzler 1980

Biosynthesis; how new molecules are formed
Steroid compounds

A major Class of Steroids is characterized by a core Skeleton built from four rings, three of which are fused six-membered rings, while the fourth is a five-membered ring. Cholestanol (dihydrocholesterol) can be considered a typical representative of steroid alcohols, or sterols:

Most sterols, including cholestanol, have a C₈–C₁₀ side chain at the C-17 position. The Structure of this chain points to its polyprenyl origin. At the C-3 position, steroids typically contain an oxygen atom, which is part of the OH group in sterols or, frequently, part of a carbonyl group in other steroids. Most Steroid compounds bear two methyl groups attached to the ring skeleton, designated as C-18 and C-19. Note that these groups have an axial orientation. In standard structural projections, they are depicted as projecting forward toward the viewer. Similarly, the equatorially oriented 3-OH group of cholestanol and the side chain at C-17 also project forward toward the viewer.

The angular methyl groups C-18 and C-19, as well as the 3-OH group and the side chain, are all oriented on the same side relative to the plane of the steroid ring system; all of them have a ß-orientation. Substituents directed to the opposite side are designated as a-oriented. While the methyl groups (C-18 and C-19) almost invariably have a ß-orientation, the 3-OH group in certain sterols may adopt an a-orientation. In steroid structural drawings, bonds to a-oriented substituents are conventionally represented by dashed lines, whereas bonds to ß-oriented substituents are shown as solid lines.

In cholestanol, all ring junctions—between rings A and B, B and C, and C and D—are in the trans-configuration; this means that the hydrogen atoms or methyl groups attached to the bridgehead carbon atoms project in opposite directions from the plane of the ring. As a result, all three six-membered rings adopt a relatively strain-free chair conformation. However, the Introduction of a double bond significantly alters the molecular shape. For instance, Cholesterol features a double bond between C-5 and C-6 (∆⁵), which leads to a considerable distortion of the chair conformation in rings A and B. In some steroids, the A/B ring junction exhibits cis-isomerism. This dramatically changes the conformation of the steroid molecule, making it noticeably bent rather than flat like cholestanol. A prime example is ß-coprostanol, a compound formed from cholesterol through bacterial action and present in large quantities in feces.

In certain sterols, notably estrogens, ring A is fully aromatic, and the C-19 methyl group is absent.

¹⁾ The name squalene is derived from the shark genus Squalus, in whose Liver this compound accumulates due to the blockage of its oxidation into cholesterol. Squalene is also an important component of human Skin Lipids.