Question

Explain the mechanism of action of steroid hormones and thyroxine.

Short answer

Both steroid hormones and thyroxine are lipid-soluble molecules that can pass through the plasma membrane of target cells and bind to their specific intracellular receptor proteins. Steroid hormones, like cortisol and testosterone, are derived from cholesterol, whereas thyroxine (T4) is an amino acid-derived hormone. The hormone-receptor complexes of both types translocate to the nucleus and bind with hormone response elements (HREs) or thyroid hormone response elements (TREs), regulating the transcription of target genes. Consequently, new proteins are produced or existing proteins are modulated, controlling various cellular functions. Although they have similar mechanisms of action, they differ in chemical structures and the specific cellular receptors they interact with.

Step-by-step solution

Understand the basics of steroid hormones

Steroid hormones are derived from cholesterol and are lipid-soluble molecules. Some examples of steroid hormones include cortisol, testosterone, and estradiol. They can diffuse through the plasma membrane of target cells due to their lipid solubility.

Mechanism of action for steroid hormones

As steroid hormones are lipid-soluble, they can pass through the plasma membrane and bind to their specific intracellular receptor proteins found in the cytoplasm or nucleus of the target cells. This binding forms a hormone-receptor complex.

Steroid hormone-receptor complex and gene regulation

The hormone-receptor complex then translocates to the nucleus where it binds with specific DNA sequences called hormone response elements (HREs). This binding process activates or represses the transcription of certain target genes, depending on the hormone involved. Eventually, this leads to the production of new proteins or modulation of existing proteins, which ultimately control specific cellular functions.

Understand the basics of thyroxine

Thyroxine, also known as T4, is a thyroid hormone responsible for regulating metabolism, growth and development. It is an amino acid-derived hormone and slightly more polar than steroid hormones, but still able to cross the plasma membrane of target cells due to its lipophilic nature.

Mechanism of action for thyroxine

Thyroxine enters the target cell and is converted into its active form, triiodothyronine (T3). T3 then binds to its specific intracellular receptor proteins, which are part of the nuclear receptor superfamily, similar to steroid hormone receptors.

Thyroxine-receptor complex and gene regulation

Similar to steroid hormone-receptor complexes, the T3-receptor complex binds to specific DNA sequences known as thyroid hormone response elements (TREs). This binding process either activates or represses the transcription of target genes. As a result, the production of new proteins or modulation of existing proteins takes place, which controls various cellular functions. In conclusion, both steroid hormones and thyroxine share a similar mechanism of action where they bind to intracellular receptor proteins to form hormone-receptor complexes, which then regulate gene transcription by binding to specific DNA sequences. However, they differ in their chemical structures and the specific cellular receptors they interact with.