Classification and Action of Hormones

• Serotonin: This molecule is unique because it acts as both a hormone and a neurotransmitter, influencing overall disposition and happiness.
• Melatonin: Produced by the pineal gland, melatonin is responsible for regulating the body's normal sleep cycles.

Exam Tip: Remember that some chemicals, like serotonin, do "double duty" by acting as both hormones and neurotransmitters.

Peptide Hormones

Peptide hormones are essentially short chains of amino acids, typically ranging from ten to fifteen in length. Because they are made of amino acids, they are often "charged" molecules, which means they typically cannot pass directly through the hydrophobic (fatty) center of a cell membrane.

Prohormones and Activation

Many peptide hormones are initially released in an inactive state known as a prohormone (or "pre-hormone"). To become biologically active, the prohormone must undergo enzymatic cleavage—essentially, an enzyme must "cut" a portion of the protein chain to reveal the active hormone.

Glycoproteins

A specialized subgroup of peptide hormones includes glycoproteins, which are proteins with sugar molecules attached to them. Key examples include:

• Thyroid-stimulating hormone (TSH)
• Luteinizing hormone (LH)
• Follicle-stimulating hormone (FSH)

Lipid Derivatives

The third group shifts away from protein-based structures to molecules derived from fats (lipids). Because these are non-polar and hydrophobic, they can often diffuse directly through the phospholipid bilayer of cell membranes.

Eicosanoids

A major subgroup of lipid-derived hormones is the eicosanoids, which are synthesized from arachidonic acid. These are typically released in response to bodily damage or inflammation.

• Prostaglandins: Involved in various cellular activities and inflammatory responses.
• Leukotrienes: These are notably involved in the clotting process and other cellular responses to injury.

Steroid Hormones

While eicosanoids are a major lipid group, steroid hormones are perhaps the most well-known. These are derived from cholesterol. Unlike peptide hormones, which are often "free" in the blood and act quickly, steroid hormones often travel bound to proteins in the blood. This allows them to act as a "savings account" or reserve, where they circulate in the blood ready to be released when needed. Examples include estrogens, progesterone, and androgens.

Summary of Hormone Action Mechanisms

The chemical classification of a hormone dictates its mechanism of action at the target cell.

The Second Messenger System

For peptide hormones that cannot enter the cell, a multi-step communication process occurs:

1. First Messenger: The hormone itself binds to a receptor on the cell surface.
2. G Protein: This acts as a link (often an enzyme) that is activated by the receptor.
3. Second Messenger: The activated G protein catalyzes the production of an intermediary molecule, such as cyclic AMP (cAMP), ATP, or the release of calcium.
4. Amplification: This system allows a single hormone molecule to trigger the release of thousands of second messengers, greatly amplifying the signal.

Key Concept: The cell can regulate its sensitivity to hormones through up-regulation (increasing the number of receptors when hormone levels are low) or down-regulation (decreasing the number of receptors after prolonged hormone exposure).