Problem 4 - Campbell Biology

Question

Consider this pathway: epinephrine \(\rightarrow\) G protein-coupled receptor \(\rightarrow\) G protein \(\rightarrow\) adenylyl cyclase \(\rightarrow\) CAMP. Identify the second messenger. (A) CAMP (C) GTP (B) G protein (D) adenylyl cyclase

Step-by-Step Solution

Answer

cAMP

Step 1: Understand the Pathway

The given pathway involves a sequence of molecules and reactions starting with epinephrine, which binds to a G protein-coupled receptor. This activates a G protein, which in turn activates adenylyl cyclase. The activation of adenylyl cyclase leads to the production of cAMP.

Step 2: Define Second Messenger

A second messenger is a molecule that relays signals received at receptors on the cell surface to target molecules inside the cell. It acts as an intermediary between the receptor and other signaling pathways within the cell.

Step 3: Identify the Second Messenger in the Pathway

In the pathway, epinephrine is the first messenger. The second messenger is produced after the activation of adenylyl cyclase. The molecule produced by adenylyl cyclase is cAMP (cyclic adenosine monophosphate). Therefore, cAMP is the second messenger.

Key Concepts

second messenger

A second messenger is a substance within a cell that helps relay signals from the cell's surface to its interior. When a signaling molecule (like a hormone or neurotransmitter) binds to a receptor on the cell membrane, it triggers the production of a second messenger. Second messengers are crucial for amplifying the signal and eliciting a physiological response.
Examples of second messengers include:

• cAMP (cyclic adenosine monophosphate)
• IP3 (inositol triphosphate)
• Ca2+ ions (calcium ions)

In our specific pathway, epinephrine is the first messenger that initiates the signal. The second messenger, cAMP, is then produced inside the cell to carry on the signal, resulting in a desired cellular response.

G protein-coupled receptor

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a key role in transmitting signals from the outside to the inside of a cell. They respond to various stimuli including hormones, neurotransmitters, and sensory signals.
Here’s how GPCRs function in a typical signaling pathway:

• A signaling molecule binds to a GPCR.
• This activates the receptor, causing it to change shape.
• The activated GPCR then interacts with a G protein, causing it to exchange GDP for GTP.
• The GTP-bound G protein activates downstream effectors, such as adenylyl cyclase.

In our pathway, epinephrine binds to a GPCR, triggering a series of events that eventually lead to the production of the second messenger cAMP.

adenylyl cyclase

Adenylyl cyclase is an enzyme located on the inner side of the cell membrane. It plays a crucial role in the signaling pathways initiated by GPCRs. When adenylyl cyclase is activated by a G protein, it catalyzes the conversion of ATP (adenosine triphosphate) to cyclic AMP (cAMP), a crucial second messenger.
Here’s a closer look at the process:

• A GPCR is activated by a ligand, such as epinephrine.
• The activated GPCR then activates a G protein by facilitating the exchange of GDP for GTP.
• This active G protein then binds to and activates adenylyl cyclase.
• Adenylyl cyclase converts ATP into cAMP.

In this pathway, adenylyl cyclase serves as a key link between the activation of the G protein and the production of the second messenger cAMP.

cAMP

Cyclic AMP (cAMP) is a well-known second messenger involved in many biological processes. It is derived from ATP and plays a vital role in transferring the signals from the cell surface to the interior.
Here’s what happens once cAMP is produced:

• cAMP activates protein kinase A (PKA).
• Activated PKA phosphorylates target proteins, leading to various cellular responses.
• The cellular responses can include changes in gene expression, ion channel activity, and metabolic pathways.

In the specific pathway we are discussing, cAMP is produced as a result of adenylyl cyclase activation. This cAMP then activates downstream effectors to bring about the physiological effects initiated by the primary signaling molecule, epinephrine.