AP Biology

## Learning Outcomes

- Describe how a G-protein-coupled receptor converts an extracellular signal into an intracellular response.
- Explain the roles of the receptor, G protein, GDP, GTP, adenylyl cyclase, cAMP, and protein kinases.
- Predict how mutations, inhibitors, toxins, or added enzymes affect a GPCR signaling pathway.
- Distinguish GPCR signaling from ligand-gated ion channels and enzyme-linked receptors.
- Explain how second messengers and phosphorylation cascades amplify signals.
- Describe how GPCR pathways are turned off to prevent continuous cell responses.

## AP Biology Syllabus

In AP Biology Unit 4, signal transduction questions often ask how information outside a cell is received, relayed, amplified, and converted into a cellular response. G-protein-coupled receptors are a key example of membrane receptor signaling, especially for hydrophilic ligands that cannot cross the phospholipid bilayer.

- Reception: a ligand binds a specific GPCR on a target cell membrane.
- Transduction: the activated receptor causes a G protein to exchange GDP for GTP.
- Relay: activated G protein subunits regulate effector proteins such as adenylyl cyclase.
- Second messengers: cAMP and calcium ions can carry signals rapidly through the cytosol.
- Response: protein kinases phosphorylate target proteins, changing enzyme activity, gene expression, secretion, movement, or cell behavior.
- Regulation: GTP hydrolysis, phosphodiesterases, and phosphatases help stop the signal.
- Disruption: mutations, toxins, antibodies, or inhibitors can block or overactivate the pathway.
- Exam tasks: predict and justify effects using pathway order and cause-and-effect reasoning.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why do hydrophilic ligands usually need receptors on the plasma membrane?
2. What happens to a G protein when GDP is replaced by GTP?
3. How does adenylyl cyclase connect G-protein activation to protein kinase activation?
4. Predict the effect of adding cAMP-dependent phosphodiesterase to a cell using a cAMP signaling pathway.
5. Why can a mutation in a receptor protein prevent a cell from responding to a hormone even if the hormone is present?

## Introduction

Cells constantly receive information from their surroundings. A hormone in the bloodstream, a neurotransmitter at a synapse, or a local growth factor released by a nearby cell may all act as signals. For a cell to respond, it must have the correct receptor. If the receptor is missing or has the wrong shape, the ligand may be present, but the cell will not respond.

G-protein-coupled receptors, often shortened to GPCRs, are one of the major types of plasma membrane receptors tested in AP Biology. They are especially useful for signaling molecules that cannot pass directly through the phospholipid bilayer. A GPCR detects the signal outside the cell, changes shape, activates a G protein inside the cell, and starts a signal transduction pathway that can affect many target proteins.

> **Key Term: G-protein-coupled receptor**
> A membrane receptor that binds an extracellular ligand and activates an intracellular G protein to begin a signal transduction pathway.
>
> **Key Term: Ligand**
> A signaling molecule that binds specifically to a receptor.
>
> **Key Term: Target cell**
> A cell that has the correct receptor for a particular ligand and can respond to that ligand.

### The GPCR pathway from ligand binding to active G protein

A typical signal transduction pathway has three main stages: reception, transduction, and response. In GPCR signaling, reception begins when a ligand binds to the extracellular side of the receptor. The ligand does not need to enter the cell. Instead, ligand binding changes the shape of the receptor protein. This shape change is passed across the plasma membrane to the cytoplasmic side of the receptor.

GPCRs are membrane proteins, commonly described as having seven membrane-spanning regions. On the inside of the cell, the receptor interacts with a G protein. The G protein is called this because it binds guanine nucleotides: GDP and GTP.

In its inactive state, the G protein is bound to GDP. When the ligand-bound GPCR becomes active, it causes the G protein to release GDP and bind GTP. This switch from GDP to GTP activates the G protein. In many textbook models, the active G protein then separates into parts, such as an alpha subunit and a beta-gamma pair. One or more of these parts can move along the inner surface of the membrane and regulate another protein.

> **Key Term: G protein**
> An intracellular signaling protein that is inactive when bound to GDP and active when bound to GTP.

The next protein in the pathway is often called an effector protein. In GPCR pathways, the effector may be an enzyme that makes a second messenger or an ion channel that changes ion movement. AP Biology questions often focus on the enzyme adenylyl cyclase, which produces cyclic AMP.

A simple GPCR sequence is:

1. Ligand binds GPCR.
2. GPCR changes shape.
3. G protein exchanges GDP for GTP.
4. Active G protein regulates an effector protein.
5. Effector protein changes the amount of a second messenger.
6. Second messenger activates proteins that create the cellular response.

> **Test Tip:**
> On AP questions, keep the order clear. The ligand does not directly activate cAMP or protein kinase in the usual GPCR model. The pathway is ligand → receptor → G protein → effector enzyme → second messenger → protein kinase → response.

A key feature of GPCR signaling is specificity. Only cells with the matching receptor respond to a ligand. For example, a hormone may travel through the blood and contact many cell types, but only target cells with the correct receptor activate the pathway. This explains why one signal can affect some tissues but not others.

### cAMP, adenylyl cyclase, and protein kinase A

One classic GPCR pathway uses adenylyl cyclase and cyclic AMP. After the G protein is activated, it can stimulate adenylyl cyclase, an enzyme embedded in the plasma membrane. Adenylyl cyclase converts ATP into cyclic AMP, usually written as cAMP.

> **Key Term: Adenylyl cyclase**
> A membrane-associated enzyme that converts ATP into cyclic AMP during some GPCR signaling pathways.
>
> **Key Term: cyclic AMP (cAMP)**
> A small intracellular second messenger made from ATP that helps relay and amplify signals inside the cell.

cAMP is not the original ligand. It is made inside the cell after receptor activation. That is why it is called a second messenger. The ligand is the first messenger because it carries information to the outside of the cell. cAMP carries the message through the cytosol to intracellular targets.

> **Key Term: Second messenger**
> A small non-protein molecule or ion that relays a signal inside the cell after receptor activation.

One major target of cAMP is protein kinase A, often abbreviated PKA. Inactive PKA becomes active when cAMP binds to it. Active PKA phosphorylates target proteins by transferring phosphate groups, usually from ATP, onto specific amino acids in those proteins.

> **Key Term: Protein kinase**
> An enzyme that transfers phosphate groups to target molecules, often changing the activity of proteins.

Phosphorylation can activate or inhibit a protein, depending on the protein and the site phosphorylated. This means one GPCR pathway can produce many possible responses, including:

- changing the activity of metabolic enzymes
- opening or closing ion channels
- altering secretion of molecules
- changing gene expression through transcription factors
- affecting cell growth, division, or survival

Epinephrine and glucagon are useful examples. Both are hormones involved in energy mobilization. In some target cells, these hormones activate GPCR pathways that increase cAMP. The rise in cAMP activates PKA, which phosphorylates proteins that help make stored energy available.

> **Key Term: Signal amplification**
> An increase in signal strength when one activated molecule activates many molecules downstream in a pathway.

Signal amplification is a major reason GPCR pathways are effective. One ligand-bound receptor may activate several G proteins. Each active G protein may stimulate an effector enzyme. Each enzyme may produce many cAMP molecules. Each cAMP-activated kinase may phosphorylate many target proteins. The result is that a small amount of hormone can lead to a large cellular response.

> **Test Tip:**
> If an AP question asks why a low concentration of ligand can cause a strong response, discuss amplification. Do not simply say “the cell works harder.” Explain that activated proteins or second messengers can activate many downstream molecules.

### Turning GPCR signals off

Cells must be able to stop signaling. If a pathway stayed active after the signal disappeared, the response could continue too long and damage homeostasis. GPCR pathways include several shutoff mechanisms.

First, the ligand may detach from the receptor or be broken down outside the cell. Without ligand, the receptor returns to its inactive shape.

Second, the G protein has a built-in timer. The active G protein is bound to GTP, but it can hydrolyze GTP to GDP. Once GTP is converted to GDP, the G protein becomes inactive again. The inactive G protein can rejoin its original subunits and wait for another signal.

Third, second messengers can be removed. In cAMP pathways, phosphodiesterase breaks down cAMP into AMP. This reduces cAMP concentration, so cAMP-dependent protein kinases are no longer activated.

> **Key Term: Phosphodiesterase**
> An enzyme that breaks down cyclic AMP, helping terminate cAMP-mediated signaling.

Fourth, protein phosphatases reverse phosphorylation. If protein kinases add phosphate groups, phosphatases remove them. This helps return target proteins to their previous states.

> **Key Term: Protein phosphatase**
> An enzyme that removes phosphate groups from target molecules, often reversing the effect of protein kinases.

This balance between kinases and phosphatases is a common AP Biology theme. Kinases are often associated with signal propagation because they phosphorylate proteins. Phosphatases often help reset the pathway by dephosphorylating proteins. However, phosphorylation does not always mean “on,” and dephosphorylation does not always mean “off.” The effect depends on the protein.

> **Exam Warning:**
> Do not memorize “phosphorylation always activates.” AP questions may ask about a specific pathway where phosphorylation inhibits a protein. Safer wording: phosphorylation changes protein activity.

A common exam scenario involves adding cAMP-dependent phosphodiesterase to a cell. Because phosphodiesterase breaks down cAMP, less cAMP is available to activate protein kinase A. Therefore, the step in which inactive protein kinase becomes active would be reduced or blocked. Downstream phosphorylation and the final cellular response would decrease.

Another common scenario involves adenylyl cyclase. If adenylyl cyclase is defective, inhibited, or missing, ATP will not be efficiently converted into cAMP. Even if the ligand binds and the G protein activates normally, the cell may fail to activate cAMP-dependent protein kinase. The pathway stops at the effector enzyme step.

### Disruptions, examples, and AP-style predictions

GPCR pathways can be disrupted at many points. AP Biology often tests this by giving an experimental treatment or mutation and asking you to predict the effect on the cellular response.

A mutation in the GPCR gene may change the receptor’s shape. If the ligand-binding site changes, the ligand may no longer bind. If the cytoplasmic side changes, the receptor may bind the ligand but fail to activate the G protein. Either way, the cell may not respond normally.

A mutation in the G protein may also change the response. If the G protein cannot exchange GDP for GTP, it may remain inactive. If it cannot hydrolyze GTP back to GDP, it may remain active too long. These two mutations would have opposite effects: one blocks signaling, while the other can cause overactive signaling.

A mutation in adenylyl cyclase can prevent cAMP production. If no cAMP is produced, cAMP-dependent protein kinase activation will be reduced. This is a common AP-style cause-and-effect chain:

- Defective adenylyl cyclase means less cAMP.
- Less cAMP means less activation of protein kinase A.
- Less active protein kinase means less phosphorylation of target proteins.
- The final cellular response is reduced or absent.

Environmental molecules and toxins can also interfere with signaling. Cholera toxin is a classic example connected to GPCR signaling. It affects G protein signaling in intestinal cells, causing abnormal activation of pathways that raise cAMP. High cAMP changes ion movement across the intestinal epithelium, water follows by osmosis, and severe diarrhea can result. The danger is dehydration.

> **Key Term: Cholera toxin**
> A bacterial toxin that disrupts G protein signaling in intestinal cells, leading to excessive cAMP-related effects and severe water loss.

Antibodies can also block receptors. In some disorders, antibodies bind to cell surface receptors and prevent normal ligands from interacting with them. If the receptor is blocked, the target cell may not respond to the ligand even when the ligand is present in normal amounts. The best AP explanation is receptor specificity: the ligand must bind the correct receptor to begin transduction.

GPCRs should also be distinguished from other membrane receptor types. Ligand-gated ion channels open or close an ion channel when a ligand binds. Enzyme-linked receptors, such as receptor tyrosine kinases, have or activate enzyme activity directly on the cytoplasmic side. GPCRs do not usually act as enzymes themselves; they activate G proteins, which then regulate effector proteins.

> **Exam Warning:**
> Do not mix receptor types. If a question describes GDP, GTP, G proteins, adenylyl cyclase, or cAMP, it is pointing toward GPCR signaling, not a ligand-gated ion channel.

When answering free-response questions, use pathway logic. For example:

> **Question:**
> A drug prevents GTP from binding to the G protein. Predict the effect on cAMP levels and justify your answer.

A strong answer would say that cAMP levels would decrease or fail to rise after ligand binding. The G protein needs GTP to become active. If GTP cannot bind, the G protein will not activate adenylyl cyclase, so ATP will not be converted into cAMP at the normal rate.

> **Question:**
> A molecule blocks phosphodiesterase activity. Predict the effect on protein kinase A activity.

A strong answer would say that protein kinase A activity may remain elevated for longer because cAMP is not broken down efficiently. More cAMP remains available to activate PKA.

Notice how both answers name the affected molecule, place it in the pathway, and connect it to the final effect. This is the style AP Biology rewards.

## Key Point Checklist

This article has covered the following key knowledge points:

- GPCRs are plasma membrane receptors used by many hydrophilic ligands that cannot cross the phospholipid bilayer.
- A target cell responds to a ligand only if it has the correct receptor.
- Ligand binding changes the shape of the GPCR and activates an intracellular G protein.
- Inactive G proteins are bound to GDP; active G proteins are bound to GTP.
- Active G proteins regulate effector proteins such as adenylyl cyclase.
- Adenylyl cyclase converts ATP into cyclic AMP.
- cAMP is a second messenger that can activate protein kinase A.
- Protein kinases phosphorylate target proteins, changing cell activity.
- Signal amplification allows a small external signal to produce a large cellular response.
- Phosphodiesterase breaks down cAMP and helps stop the pathway.
- Protein phosphatases remove phosphate groups and help reset signaling proteins.
- Mutations, toxins, antibodies, or inhibitors can block or overactivate GPCR signaling pathways.

## Key Terms and Concepts

- G-protein-coupled receptor
- Ligand
- Target cell
- G protein
- Adenylyl cyclase
- cyclic AMP (cAMP)
- Second messenger
- Protein kinase
- Signal amplification
- Phosphodiesterase
- Protein phosphatase
- Cholera toxin


Introduction to signal transduction - G-protein-coupled receptors