Problem 6 - Campbell Biology

Question

Which observation suggested to Sutherland the involvement of a second messenger in epinephrine's effect on liver cells? (A) Enzymatic activity was proportional to the amount of calcium added to a cell-free extract. (B) Receptor studies indicated that epinephrine was a ligand. (C) Glycogen breakdown was observed only when epinephrine was administered to intact cells. (D) Glycogen breakdown was observed only when epinephrine and glycogen phosphorylase were mixed.

Step-by-Step Solution

Answer

Option (C) Glycogen breakdown was observed only when epinephrine was administered to intact cells.

Step 1: Understanding the Problem

Determine which observation suggested to Sutherland the involvement of a second messenger in the action of epinephrine on liver cells.

Step 2: Analyzing Each Option

Go through each listed option to understand its relevance to Sutherland's discovery. (A) Enzymatic activity being proportional to calcium:Not directly related to second messenger in the context of epinephrine.(B) Epinephrine as a ligand: This indicates receptor binding but not necessarily a secondary messenger.(C) Glycogen breakdown with intact cells: This implies the requirement of a cellular component, which can hint at a second messenger.(D) Glycogen breakdown with epinephrine and glycogen phosphorylase: Direct mixing does not indicate the necessity of a second messenger.

Step 3: Selecting the Correct Observation

From analysis, Option (C) is correct. Sutherland observed that glycogen breakdown occurred only when epinephrine was administered to intact cells, indicating that something within the cell (a second messenger) was required for the response.

Key Concepts

epinephrine

Epinephrine, also known as adrenaline, is a hormone released from the adrenal glands in response to stress or excitement.
Its primary function is to prepare the body for 'fight-or-flight' reactions. This preparation includes increasing heart rate, expanding air passages in the lungs, and, crucially in the context of our exercise, promoting the breakdown of glycogen to glucose in the liver.
Epinephrine acts by binding to specific receptors on the surface of liver cells. These receptors are proteins that recognize and respond to the hormone, setting off a chain reaction within the cell. This binding does not directly cause changes within the cell. Instead, it triggers the production of a second messenger inside the cell, which then leads to the desired cellular response.
This second messenger system is crucial for amplifying the signal of the epinephrine, ensuring that even small amounts of the hormone can produce a significant cellular response.

glycogen breakdown

Glycogen breakdown is an important process that provides quick energy to cells.
In the liver, glycogen is stored as a large, branched molecule composed of glucose units. When needed for energy, glycogen is broken down into glucose-1-phosphate, which can then be converted to glucose.
The enzyme responsible for this breakdown is called glycogen phosphorylase. However, this enzyme's activity is regulated and does not act unless signaled to do so by a molecule like the second messenger cAMP (cyclic Adenosine Monophosphate).
When epinephrine binds to its receptor on the liver cell, it triggers the production of cAMP. This small molecule acts within the cell to activate glycogen phosphorylase. Only when this second messenger is present does glycogen breakdown occur, illustrating why Sutherland's observation about the requirement of intact cells was so critical.

cell signaling

Cell signaling involves the communication between cells to control biological processes and coordinate activities.
This occurs through various signaling pathways, of which the epinephrine signaling pathway leading to glycogen breakdown is an example.
The process involves several steps:

• Epinephrine, the first messenger, binds to its specific receptor on the cell membrane.
• This binding activates an associated G-protein on the inside of the membrane.
• The G-protein then activates adenylate cyclase, an enzyme that converts ATP to cAMP, generating the second messenger.
• cAMP activates protein kinase A (PKA), which in turn activates glycogen phosphorylase through a series of phosphorylation events.
• Active glycogen phosphorylase then catalyzes the breakdown of glycogen into glucose-1-phosphate.

This detailed cascade ensures that the signal from the small amount of epinephrine is amplified, allowing for a robust and rapid cellular response. Understanding these steps can elucidate how external signals like hormones can result in significant changes in cellular activity.