27.6 Steroids

Structure and Characteristics of Steroids

Steroids are a class of lipids built on a distinctive four-ring skeleton. Unlike the fats and phospholipids you've seen in earlier sections, steroids don't contain fatty acid chains. Instead, their biological diversity comes from small functional group changes on a rigid ring framework. That structural rigidity is central to how they work.

The Gonane Skeleton

All steroids derive from gonane, a tetracyclic (four-fused-ring) hydrocarbon:

• Rings A, B, and C are cyclohexane rings (six-membered)
• Ring D is a cyclopentane ring (five-membered)
• Carbon atoms are numbered 1 through 17 in a standardized system, which makes it possible to precisely describe where functional groups sit on the skeleton

Ring Fusion and Stereochemistry

The way the rings connect to each other determines the overall 3D shape of the molecule:

• A/B ring junction: typically trans-fused in most natural steroids (substituents on opposite faces)
• B/C and C/D ring junctions: trans-fused as well in the most common natural steroids

Trans fusion across all junctions produces a relatively flat, rigid molecule. Rings B and C are locked into chair conformations with very little flexibility. Ring A can adopt a half-chair in some steroids, and ring D (being five-membered) can pucker into an envelope conformation. Overall, though, the fused ring system is far more rigid than open-chain lipids.

Common Functional Group Positions

Small changes to the gonane skeleton produce dramatically different biological activities:

• Angular methyl groups at C-10 and C-13 (these project above the ring plane)
• Alkyl side chain at C-17 (present in cholesterol; shortened or removed in many hormones)
• Hydroxyl ($-OH$), ketone ($C=O$), or alkene ($C=C$) groups at various positions, especially C-3 and C-17

The takeaway: steroids share the same core skeleton, and it's the specific pattern of functional groups that gives each steroid its unique activity.

Classes and Functions of Steroid Hormones

Steroid hormones fall into two broad categories based on where they're produced and what they regulate.

Sex Hormones

These control the development and maintenance of reproductive characteristics:

• Androgens (e.g., testosterone): promote male secondary sexual characteristics. Synthesized primarily in the testes.
• Estrogens (e.g., estradiol): promote female secondary sexual characteristics and regulate the menstrual cycle. Synthesized primarily in the ovaries.
• Progestogens (e.g., progesterone): prepare and maintain the uterine lining for pregnancy, and help regulate the menstrual cycle. Synthesized in the ovaries and, during pregnancy, the placenta.

Adrenocortical Hormones

These are produced in the adrenal cortex (the outer layer of the adrenal glands, which sit on top of the kidneys):

• Glucocorticoids (e.g., cortisol): regulate glucose metabolism, suppress inflammatory and immune responses, and mediate the body's stress response.
• Mineralocorticoids (e.g., aldosterone): control salt and water balance by acting on the kidneys, which in turn affects blood pressure.

Specific Natural and Synthetic Steroids

Testosterone

• Structure: gonane skeleton with a ketone at C-3 and a hydroxyl group at C-17
• Functions: drives development of male secondary sexual characteristics (facial hair, deepened voice), promotes muscle growth, and maintains bone density

Estradiol

• Structure: gonane skeleton with hydroxyl groups at both C-3 and C-17, and an aromatic A ring (the A ring is fully unsaturated, making it a benzene ring)
• Functions: drives development of female secondary sexual characteristics, regulates the menstrual cycle, and helps maintain bone density

The aromatic A ring is a key structural difference from androgens. It's produced by the enzyme aromatase (more on that below), and it's what makes estradiol an estrogen rather than an androgen.

Oral Contraceptives (Synthetic Steroids)

Combination oral contraceptive pills contain two synthetic hormones that mimic natural estrogen and progesterone:

• Ethinylestradiol (synthetic estrogen): gonane skeleton with an ethynyl group ($-C \equiv CH$) at C-17 and a hydroxyl group at C-3. The ethynyl group makes it orally active because it resists breakdown in the liver.
• Norethindrone (synthetic progestin): gonane skeleton with an ethynyl group at C-17 and a ketone at C-3.

These synthetic steroids prevent pregnancy through three mechanisms:

1. Suppress ovulation by providing steady hormone levels that prevent the hormonal surge needed to release an egg
2. Thicken cervical mucus, making it harder for sperm to reach the egg
3. Alter the endometrium (uterine lining), reducing the likelihood of implantation

Steroid Biosynthesis and Regulation

Biosynthesis from Cholesterol

Cholesterol is the precursor for all steroid hormones. The conversion involves a series of enzymatic steps:

1. Cholesterol's side chain at C-17 is cleaved to produce pregnenolone, the first steroid intermediate.
2. Pregnenolone is then modified through oxidation, reduction, and isomerization reactions to yield the various hormone classes (progestogens, glucocorticoids, mineralocorticoids, androgens).
3. Aromatase converts androgens into estrogens by aromatizing the A ring, removing the C-19 methyl group and introducing the three double bonds that create the aromatic system.

Aromatase is clinically significant: aromatase inhibitors are used in breast cancer treatment to reduce estrogen production in patients with estrogen-receptor-positive tumors.

How Steroids Act on Cells

Because steroids are hydrophobic, they can pass directly through cell membranes. Once inside the cell:

1. The steroid binds to a specific nuclear receptor (a protein inside the nucleus).
2. The receptor-steroid complex acts as a transcription factor, binding to DNA and turning specific genes on or off.
3. This changes the pattern of protein synthesis in the target cell, which produces the hormone's physiological effects.

This mechanism means steroid hormones tend to act more slowly than water-soluble signaling molecules (which use surface receptors), but their effects are longer-lasting because they alter gene expression.

Anabolic Steroids

Anabolic steroids are synthetic derivatives of testosterone designed to enhance the muscle-building (anabolic) effects while minimizing the masculinizing (androgenic) effects. They promote protein synthesis and muscle growth, which is why they're sometimes misused in athletics. Prolonged use carries serious health risks, including liver damage, cardiovascular problems, and hormonal disruption.