Question

Explain why polar homnones 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 without second messengers as they are unable to pass through the hydrophobic lipid bilayer of the cell membrane. Instead, they bind to cell membrane receptors, such as G-protein coupled receptors, initiating a signal transduction process involving second messengers, like cyclic AMP (cAMP). The process includes hormone binding, activation of G proteins, activation of adenylate cyclase, formation of cAMP, activation of protein kinase A (PKA), phosphorylation of target proteins, and finally signal termination through cAMP hydrolysis. This pathway helps amplify and transmit the hormone signal within the target cell, resulting in a specific cellular response.

Step-by-step solution

1. Define polar hormones and the lipid bilayer barrier

Polar hormones, also known as hydrophilic hormones, are hormones that are water-soluble and cannot pass through the lipid bilayer of a cell membrane. Since cell membranes are composed of a lipid bilayer, which is primarily hydrophobic, polar hormones are unable to enter the cell directly.

2. Explain the role of second messengers in signal transduction

As polar hormones cannot pass through the cell membrane, they need an alternative way to relay their signals to the target cells. This is where second messengers come into play. Second messengers are intracellular signaling molecules that are responsible for transmitting the hormone signal into the cell. This process is known as signal transduction. The polar hormone binds to a specific receptor on the cell membrane, activating the receptor and triggering a series of biochemical events within the cell.

3. Introduce cyclic AMP (cAMP) as a second messenger

Cyclic AMP (cAMP) is a common second messenger used in hormone action. It is derived from adenosine triphosphate (ATP) and helps in amplifying the hormone signal within the target cell, initiating a specific cellular response. Now, let's discuss its role in hormone action in a step-by-step manner.

4. Step 1: Hormone binding to the cell membrane receptor

The first step is the binding of the polar hormone to its specific receptor on the cell membrane. This receptor is commonly a G-protein coupled receptor (GPCR), which spans the lipid bilayer.

5. Step 2: Activation of G proteins

Upon hormone binding, the GPCR undergoes a conformational change, activating the G protein that is associated with the receptor. The G protein consists of three subunits: alpha, beta, and gamma. The activated G protein exchanges its bound guanosine diphosphate (GDP) for a guanosine triphosphate (GTP), causing the alpha subunit to dissociate from the beta-gamma subunits.

6. Step 3: Activation of adenylate cyclase

The activated G protein alpha subunit binds to the membrane-bound enzyme adenylate cyclase, activating it. Adenylate cyclase is responsible for the conversion of ATP to cAMP.

7. Step 4: Formation of cyclic AMP (cAMP)

Activated adenylate cyclase converts ATP to cAMP by catalyzing the cyclization of the ATP molecule. This results in an increase in the intracellular concentration of cAMP.

8. Step 5: Activation of protein kinase A (PKA)

The increase in cAMP activates an enzyme called protein kinase A (PKA) by binding to its regulatory subunits, releasing the active catalytic subunits of PKA.

9. Step 6: Phosphorylation of target proteins

Activated PKA catalytic subunits can now phosphorylate specific target proteins within the cell. This phosphorylation event alters the activity of the target proteins, leading to a specific cellular response.

10. Step 7: Termination of the signal

Finally, the signal is terminated by the hydrolysis of cAMP to AMP and inactivation of PKA. This ensures that the cell returns to its basal state until the next hormone signal is received. In conclusion, polar hormones cannot regulate their target cells without second messengers because they cannot pass through the lipid bilayer of the cell membrane. Cyclic AMP is an important second messenger in hormone action, helping to amplify and transmit the hormone signal within the target cell, ultimately leading to a cellular response.

Key concepts

Polar Hormones

Polar hormones are fascinating chemical messengers vital for maintaining balance in biological systems. These hormones, also known as hydrophilic hormones, boast a water-soluble nature. This main characteristic sets them apart. Unlike non-polar or lipid-soluble hormones, polar hormones cannot freely cross the lipid bilayer of cell membranes.
This barrier is due to the hydrophobic nature of lipid bilayers. The cell membrane consists of tightly packed lipid molecules. They create a protective barrier that is difficult for polar molecules to penetrate. This separation keeps the cell environment stable by controlling what can enter and exit the cell.

• Example of polar hormones: protein hormones like insulin and growth hormone.
• Important for fast-acting responses in the body due to their inability to directly enter cells.

Since polar hormones can't pass through the membrane directly, they require alternate methods to influence cell activity. This need paves the way for the use of second messengers, which we'll discuss next.

Second Messengers

Second messengers are central to the process of signal transduction by polar hormones. They act as vital intermediaries that relay hormone signals from the outside to the inside of a cell. The primary hormone, often referred to as the "first messenger," binds to a receptor on the cell's surface. This binding initiates a cascade of chemical events within the cell.
These events rely heavily on second messengers to transmit the signal deeper into the cell's interior. These messengers are typically small molecules that are rapidly produced or released in response to hormone-receptor interactions. One of the most prevalent second messengers in this category is cyclic AMP (cAMP).

• Second messengers help amplify the signal inside the cell, ensuring a swift and precise response.
• They are involved in various cellular processes, supporting diverse bodily functions.

The use of second messengers allows cells to fine-tune their responses to external signals. This specialization ensures that cells respond appropriately to various stimuli, enhancing overall cellular function.

Signal Transduction

Signal transduction is the series of steps cells go through to convert a hormone signal from outside the cell into a meaningful response inside it. This process is crucial because it allows cells to respond to external changes and manage internal processes efficiently. Cyclic AMP (cAMP) is a key player as a second messenger involved in many signal transduction pathways.
When a polar hormone binds to its receptor on the cell surface, it activates an enzyme called adenylate cyclase via a G-protein coupled receptor. This enzyme converts ATP, a common energy-carrying molecule, to cAMP.

• cAMP is responsible for activating protein kinase A (PKA), a crucial enzyme in phosphorylating target proteins.
• The phosphorylation of these proteins catalyzes changes within the cell, facilitating specific cellular responses.

Once the response is initiated, mechanisms exist to stop or "turn off" this signal, allowing the cell to reset until the next signal is received. This dynamic process exemplifies how cells maintain balance and respond to ongoing challenges.