Question

What is the difference between a G protein and a receptor tyrosine kinase? Give an example of a hormone that uses each.

Short answer

G proteins are molecular switches activated by G protein-coupled receptors (e.g., adrenaline). RTKs activate signaling cascades by phosphorylation (e.g., insulin).

Step-by-step solution

- Understanding G Proteins

G proteins are a family of proteins that act as molecular switches inside cells. They are involved in transmitting signals from a variety of stimuli outside a cell to its interior. When a ligand binds to a G protein-coupled receptor, it activates the G protein, which then triggers various downstream effects in the cell.

- Example of Hormone Using G Protein

An example of a hormone that uses a G protein-coupled receptor is adrenaline (also known as epinephrine). Adrenaline binds to adrenergic receptors, which are G protein-coupled receptors.

- Understanding Receptor Tyrosine Kinases

Receptor tyrosine kinases (RTKs) are a class of cell surface receptors that also transmit signals into the cell. However, they work by adding a phosphate group to tyrosine residues on certain proteins within the cell. This initiates a series of signaling cascades that lead to cellular responses.

- Example of Hormone Using Receptor Tyrosine Kinase

An example of a hormone that uses a receptor tyrosine kinase is insulin. Insulin binds to the insulin receptor, which is a receptor tyrosine kinase, triggering pathways that regulate glucose uptake and metabolism.

Key concepts

G protein-coupled receptors

G protein-coupled receptors (GPCRs) are incredibly important proteins found on the surface of many cells. They help transmit signals from outside the cell to the inside.
When a molecule, or ligand, such as a hormone, binds to a GPCR, it activates a G protein inside the cell.
The activation of G proteins triggers various responses inside the cell. These responses can affect functions like cell growth, secretion of other hormones, or changes in cell metabolism.
For example, when adrenaline binds to its GPCR, known as adrenergic receptors, the body responds by increasing heart rate, releasing glucose from energy stores, and other 'fight or flight' responses.

Receptor Tyrosine Kinases

Receptor Tyrosine Kinases (RTKs) are another type of important cell surface receptor. Unlike GPCRs, RTKs transmit signals by adding a phosphate group to tyrosine residues on certain proteins inside the cell.
When a hormone or other molecule binds to an RTK, it activates the receptor's kinase activity, leading to phosphorylation.
This phosphorylation acts as a signal for various intracellular pathways, ultimately leading to specific cellular responses.
Insulin is a hormone that uses an RTK. When insulin binds to its receptor, it triggers pathways that help cells take in glucose and use it for energy.

Signal transduction

Signal transduction is the process of converting a signal from outside the cell to a functional change inside the cell.
Both GPCRs and RTKs are crucial players in signal transduction.
The process generally begins with the binding of a ligand, like a hormone, to a receptor on the cell surface.
This event triggers a cascade of molecular events inside the cell, leading to a specific response.
For example, in the case of adrenaline, the hormone binds to its GPCR, activating a G protein and leading to increased heart rate and energy mobilization.

Adrenaline

Adrenaline, also known as epinephrine, is a hormone that plays an essential role in the body's 'fight-or-flight' response.
Adrenaline is produced by the adrenal glands and quickly prepares the body for action.
It binds to adrenergic receptors, which are a type of GPCR.
Once the receptor is activated, it sets off a cascade of events within the cell, leading to increased heart rate, dilated airways, and elevated blood glucose levels, all of which help the body respond to a stressful situation.

Insulin

Insulin is a hormone that is crucial for regulating the body's blood sugar levels.
Produced by the pancreas, insulin helps cells absorb glucose from the blood, thereby lowering blood glucose levels.
Insulin binds to the insulin receptor, which is an RTK.
This binding activates the receptor's kinase activity, leading to the phosphorylation of specific proteins.
This signal transduction pathway helps cells take in glucose and use it as an energy source or store it for future use.

Phosphorylation

Phosphorylation is a key biochemical process where a phosphate group is added to a molecule, such as a protein.
In signal transduction, phosphorylation is often used to activate or deactivate proteins and enzymes, thereby determining the cell's response to external signals.
Receptor Tyrosine Kinases (RTKs) commonly use phosphorylation to transmit signals.
When a molecule like insulin binds to its receptor, it triggers the receptor's kinase activity, leading to phosphorylation of target proteins.
This process is essential for many cellular functions, including metabolism, growth, and communication between cells.