Question

Explain why polar hormones cannot regulate their target cells without using second messengers. Also explain in a step-by-step manner how cyclic AMP is used as a second messenger in hormone action.

Short answer

Polar hormones cannot regulate their target cells directly due to their polar nature and inability to pass through the lipid bilayer of cell membranes. They require second messengers like cyclic AMP (cAMP) to transmit their signal. The process involves the following steps: 1) hormone binding to a cell surface receptor, 2) activation of adenylyl cyclase, 3) conversion of ATP to cAMP, 4) activation of protein kinases, 5) phosphorylation of target proteins, 6) cellular response, and 7) termination of cAMP signaling.

Step-by-step solution

Introduction to Polar Hormones and Second Messengers

Polar hormones are water-soluble hormones that are unable to pass through the lipid bilayer of target cell membranes due to their polar nature. As a result, they cannot directly interact with internal components of the target cell and require an intermediary molecule, known as a second messenger, to transmit their signal.

Second Messengers

Second messengers are molecules that relay signals from receptors on the cell surface (activated by the polar hormone) to target molecules in the cytosol and/or nucleus, ultimately leading to a response. There are different types of second messengers, and one of them is cyclic AMP (cAMP).

Formation of Cyclic AMP (cAMP)

Let's explore the steps involved in the formation and use of cAMP as a second messenger in hormone action: 1. A polar hormone (such as adrenaline) binds to a specific receptor (G-protein-coupled receptor) on the surface of the target cell. This interaction activates the receiver. 2. The activated receptor stimulates a nearby membrane-bound enzyme called adenylyl cyclase by activating a G-protein. 3. Adenylyl cyclase catalyzes the conversion of adenosine triphosphate (ATP) into cyclic AMP (cAMP).

Role of Cyclic AMP (cAMP) as a Second Messenger

4. cAMP acts as a second messenger by binding to and activating specific proteins called protein kinases (primarily Protein Kinase A or PKA). 5. Activated PKA will then phosphorylate (add a phosphate group) to various target proteins, which in turn modify the cellular response. 6. The response can include changes in gene expression, protein synthesis, enzyme activity, ion channel openings, or other cellular processes, depending on the target cell and the hormone's specific function.

Termination of cAMP Signaling

7. Over time, the cAMP signaling pathway is deactivated by the action of another enzyme, called phosphodiesterase, which converts cAMP back into AMP (adenosine monophosphate). This ensures that the response is properly regulated, and the cell returns to its baseline state. In summary, polar hormones, like adrenaline, cannot interact directly with internal components of the target cell due to their inability to pass through the lipid bilayer. They rely on second messengers like cAMP to transmit their signal. The process involves hormone binding to a receptor, activation of adenylyl cyclase, conversion of ATP to cAMP, activation of protein kinases, phosphorylation of target proteins, cellular response, and termination of cAMP signaling.

Key concepts

polar hormones

Polar hormones are a type of hormone that has difficulty crossing the cell membrane because they are water-soluble. They include hormones like adrenaline and others that interact with receptors on the cell surface. These hormones can’t pass directly through the cell's lipid bilayer because they are not lipid-soluble. Instead, they must communicate from the outside of the cell. This is where the concept of second messengers comes in.
Since polar hormones can't get into the cell on their own, they rely on signaling pathways that utilize these second messengers to carry out the changes and responses needed inside the cell.

• The cell membrane is composed of fatty components, making it a barrier for small, charged molecules like polar hormones.
• Polar hormones must bind to specific receptors on the cell's surface to initiate a reaction inside the cell.

This makes the process efficient and ensures that the specific signal is passed accurately without the hormone entering the cell. This signal must be modified internally to bring about specific effects, which is where cyclic AMP plays a pivotal role.

cyclic AMP

Cyclic AMP, abbreviated as cAMP, serves as an essential second messenger within the cell. When a polar hormone like adrenaline binds to a receptor on the cell's surface, this sets off a chain reaction. The receptor involved is often linked to a G-protein, which subsequently activates an enzyme known as adenylyl cyclase.

Adenylyl cyclase plays a crucial role by facilitating the transformation of ATP (Adenosine Triphosphate) into cyclic AMP. Once formed, cAMP activates protein kinases, particularly Protein Kinase A (PKA). This activation enables PKA to phosphorylate target proteins, leading to a variety of cellular responses.

• cAMP is fundamental for translating extracellular signals into meaningful cellular actions.
• This process allows cells to respond dynamically to external signals and maintain proper function.

This signaling pathway is efficient but temporary. The presence of camphor and the involvement of specific enzymes like phosphodiesterase ensure that once the signal is no longer needed, the cAMP is degraded to ensure the system resets for future signaling events.

G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are a large family of receptors that reside on the surface of cells, key to initiating signaling pathways for various physiological processes. They feature prominently in the mechanism by which polar hormones act because they serve as the gateway for external signals. When a hormone like adrenaline attaches to a GPCR, this alters the shape of the receptor and activates a G-protein located inside the cell.

This activated G-protein then interacts with and stimulates adenylyl cyclase to produce cyclic AMP from ATP. GPCRs have an essential role in translating the external hormonal signal into a format that the cell can respond to internally.

• They are versatile receptors responsible for many functions, including senses like smell and taste.
• GPCRs are integral to many cellular processes due to their involvement with multiple second messengers and pathways.

Their ability to utilize different second messengers, including cAMP, allows them to regulate a variety of cellular activities. Understanding GPCRs and their function helps explain how signals from outside get converted into complex biological responses inside the cell.