2.20 Steroids and Prostaglandins

This section details two important classes of lipids: steroids, which act as hormones and structural components, and prostaglandins, which function as local signaling molecules involved in processes like inflammation.

Steroids

Steroids are a class of lipids characterized by a specific four-ring carbon structure. Unlike acylglycerols, they do not contain fatty acids but are classified as lipids because they are hydrophobic and insoluble in water.

General Properties:

• High molecular weight.
• Can be crystalline.
• Synthesized from isoprene units.

Structure:

• The core structure is a steroid nucleus composed of 17 carbon atoms arranged in four fused rings.
  • Three rings have six carbon atoms each (cyclohexane rings).
  • One ring has five carbon atoms (cyclopentane ring).
• Different steroids are distinguished by the length and structure of the side chains attached to these rings.

Examples and Functions:

• Cholesterol: A crucial structural component of animal cell membranes (providing fluidity) and the precursor molecule for all other steroids. It is generally absent from prokaryotic membranes.
• Bile Acids/Salts: Emulsify fats in the digestive system, aiding in their breakdown and absorption.
• Sex Hormones:
  • Testosterone: Male sex hormone.
  • Progesterone and Estrogen: Female sex hormones.
• Vitamin D: Regulates calcium metabolism in the body.

Prostaglandins

Prostaglandins are lipid compounds that act as local hormones, meaning they are produced in and act on the tissues where they are needed.

Source: Derived from the 20-carbon unsaturated fatty acid called arachidonic acid.

Location: Found in virtually every mammalian tissue.

Functions: Their roles are diverse and often specific to the tissue:

• Blood Pressure Regulation: Some prostaglandins reduce blood pressure, while others raise it.
• Immune Response: They help induce fever and inflammation and intensify the sensation of pain.
• Blood Clotting: They regulate the aggregation of platelets, which is an initial step in forming blood clots.

Science, Technology and Society Connections

Inflammation and Aspirin:

• Prostaglandins are key mediators of inflammation, a protective response characterized by redness, heat, pain, and swelling.
• They cause local blood vessels to dilate, increasing blood flow, and become more permeable, allowing white blood cells to move into the affected tissue.
• The drug aspirin reduces fever and pain by inhibiting the synthesis of prostaglandins (specifically by inhibiting the enzyme cyclooxygenase).

Biochemical Tests for Lipids (STEAM Activity 2.3)

Since steroids are lipids, their presence can be detected using general lipid tests.

Possible Questions/Answers

Q: What is the fundamental structure of all steroids? A: A steroid nucleus consisting of 17 carbon atoms arranged in four attached rings (three 6-carbon rings and one 5-carbon ring).

Q: Name two functions of cholesterol. A: 1. It is a structural component of animal cell membranes. 2. It is the precursor for other important steroids like sex hormones and Vitamin D.

Q: What are prostaglandins, and what are they derived from? A: Prostaglandins are local hormones that act on nearby tissues and are derived from the fatty acid arachidonic acid.

Q: How does aspirin work to relieve pain and fever? A: Aspirin inhibits the enzyme responsible for synthesizing prostaglandins. Since prostaglandins mediate pain, fever, and inflammation, blocking their production reduces these symptoms.