Concept: Eicosanoids

Definition and Classification

Eicosanoids are lipids derived from C20 polyunsaturated fatty acids, such as arachidonic acid. The term is derived from the Greek word for 'twenty.' Eicosanoids are classified into three main groups:

• Prostaglandins

• Thromboxanes

• Leukotrienes

All eicosanoids are synthesized from arachidonic acid, which is released from membrane phospholipids.

General Structure

• Eicosanoids typically contain multiple conjugated double bonds.

• They are not stored in cells but are synthesized as needed.

Example:

Arachidonic acid is the precursor for all major eicosanoids.

Eicosanoid Functions

Biological Roles

• Eicosanoids act as local hormones, regulating physiological processes such as inflammation, pain, fever, and blood clotting.

• They have a paracrine mode of action, affecting cells near their site of synthesis.

• They play a role in immune responses and smooth muscle contraction.

Example:

Prostaglandins regulate pain and inflammation, while thromboxanes promote blood clotting.

Types of Eicosanoids

1. Prostaglandins

Prostaglandins (PG) contain a five-carbon ring (cyclopentane) and are involved in regulating pain, inflammation, and smooth muscle contraction.

• Derived from arachidonic acid via the cyclooxygenase pathway.

• Example: Prostaglandin E2 is involved in fever and pain responses.

2. Thromboxanes

Thromboxanes have a six-membered ring with an ether. They are produced by platelets and promote blood clotting by stimulating platelet aggregation and vasoconstriction.

• Example: Thromboxane A2 induces blood clot formation.

3. Leukotrienes

Leukotrienes contain conjugated double bonds and are synthesized by leukocytes. They are involved in immune responses, especially in inflammation and allergic reactions.

• Leukotrienes cause smooth muscle contraction, especially in the lungs, and play a role in asthma attacks.

• Example: Leukotriene D4 is a potent bronchoconstrictor.

Biochemical Pathways

Arachidonic Acid Metabolism

• Arachidonic acid is released from membrane phospholipids by phospholipase A2.

• It is then converted into prostaglandins and thromboxanes via the cyclooxygenase (COX) pathway, and into leukotrienes via the lipoxygenase pathway.

Key Equations

• Release of arachidonic acid:

• Prostaglandin synthesis:

• Thromboxane synthesis:

• Leukotriene synthesis:

Practice Questions and Key Facts

• All eicosanoids contain three conjugated double bonds.

• Arachidonic acid is the precursor for eicosanoids.

• Prostaglandins and leukotrienes both contain a ring structure.

• Thromboxanes, prostaglandins, and leukotrienes all contain a carboxyl group.

• Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen lower body temperature by blocking prostaglandin synthesis.

Sample Practice Question

Summary Table: Eicosanoid Types and Functions

Additional info: Eicosanoids are crucial mediators in many physiological and pathological processes, and their synthesis is a major target for anti-inflammatory drugs.