Steroids

Figure 123.

Terpenes may be cyclic or linear with more than 23,000 terpenes known in nature. The simple terpenes like myrcene have a molecular formula of C10H15 with multiples of five carbons added to make C15, C20, C25, etcetera, as numbers of carbon atoms per terpene. Terpenes do not have to have double bonds and can be substituted with ketones and hydroxyl groups. Menthol, limonene, and camphor are examples of terpenes. Certain molecules in plants, such as vitamin A and beta-carotene, are considered terpenes. Figure 124 shows terpenes that are fragrant in nature:

STEROIDS

Figure 124.

Steroids are lipids by virtue of being lipid soluble and hydrophobic. These are essentially specific types of terpenes that have a common polycyclic framework. Figure 125 shows what a numbered steroid ring looks like with the rings labelled A, B, C, and D:

Figure 125.

In biological systems, the most important and common steroid in the human body will be cholesterol. This is relatively flat structure from the side view in three dimensions. Figure 125 shows the numbered steroid, which is cholesterol in this case. Note it has just one hydroxyl group, five methyl groups, and one double bond. There are many different steroids that come from cholesterol, such as cortisol, estradiol, progesterone, testosterone, and norethindrone. The functional groups will react as all the functional groups do in organic molecules.

The cholesterol/steroid backbone seems complex but it is just a series of ring closure reactions that take a long-chain fatty acid and close the molecule to make rings. It starts with a squalene molecule, which is a terpene. This undergoes enzymatic oxidation to make an epoxide intermediate. Once folded and enzymatically acted upon, it gives rise to lanosterol, which is a steroid/cholesterol precursor molecule.