Cell Signalling and Transmembrane Receptors in Biochemistry

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Questions and Answers

What is the primary function of transmembrane receptors in cellular signalling?

To activate gene expression in response to extracellular signals (correct)

To facilitate the breakdown of glycogen in the liver

To synthesise glucagon in response to adrenaline

To release histamine from mast cells

Which of the following is NOT an example of a second messenger?

Diacylglycerol

cAMP

Glucagon (correct)

Inositol triphosphate (IP3)

What is the primary function of G proteins in signal transduction pathways?

To transmit signals from transmembrane receptors to intracellular signalling pathways (correct)

To stimulate the release of histamine from mast cells

To activate receptor tyrosine kinases

To synthesise prostaglandins from arachidonic acid

Which of the following is an example of a cellular response to extracellular signals?

Change in gene expression in response to EGF Signup and view all the answers

What is the role of phospholipase C in signal transduction pathways?

To cleave phosphatidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol and inositol triphosphate (IP3) Signup and view all the answers

Which of the following is an example of an extracellular signal that binds to a transmembrane receptor?

Glutamate Signup and view all the answers

What is the role of Receptor Tyrosine Kinase (RTK) in signal transduction pathways?

To activate downstream signalling pathways by phosphorylating target proteins Signup and view all the answers

Which of the following is an example of a cellular response to extracellular signals that involves changes in gene expression?

Epithelial cell differentiation in response to EGF Signup and view all the answers

What is the primary characteristic of a paracrine signal?

The signal acts on other cells that are very close to the producing cell. Signup and view all the answers

Which type of receptor is typically associated with hydrophilic hormones such as adrenaline?

Cell surface receptor Signup and view all the answers

What is the primary difference between autocrine and paracrine signaling?

The distance the signal travels to reach its target cells. Signup and view all the answers

What is a characteristic of receptors in terms of their binding to signals?

They have high selectivity and high affinity. Signup and view all the answers

Which type of signaling involves the direct interaction between two cells?

Contact-dependent signaling Signup and view all the answers

What is a characteristic of signals in terms of their binding to different receptors?

They can bind to different types of receptors with high affinity. Signup and view all the answers

What is the primary mechanism of action for steroid hormones?

Binding to a receptor in the cytosol and triggering a response inside the cell Signup and view all the answers

What is the effect of acetylcholine binding to its receptor?

Depolarization of the plasma membrane due to the flow of ions Signup and view all the answers

What is the function of the DNA binding domain in steroid hormones?

To bind to hormone response elements in the sequence of target genes Signup and view all the answers

What is the role of G-protein-coupled receptors in signaling pathways?

To generate a secondary message inside the cell through cyclic AMP Signup and view all the answers

What is the effect of glucagon binding to its receptor?

Generation of a secondary message inside the cell through cyclic AMP Signup and view all the answers

What is the function of the hormone binding domain in steroid hormones?

To interact with other transcription factors Signup and view all the answers

What is the effect of EGF binding to its receptor?

Activation of an enzymatic kinase cascade through the MAP kinase pathway Signup and view all the answers

What is the role of phospholipase C in signaling pathways?

To generate inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG) Signup and view all the answers

What is the main function of the G-protein α subunit when GTP is bound to it?

It dissociates from the βγ subunit and activates the effector enzyme Signup and view all the answers

What is the result of the GTPase activity on the G-protein α subunit?

The G-protein α subunit becomes inactive and re-associates with the βγ subunit Signup and view all the answers

What is the function of the βγ subunit in the G-protein complex?

It plays a crucial role in the GTP/GDP exchange on the G-protein Signup and view all the answers

What is the result of the signal binding to the receptor in GPCR signalling?

The G-protein α subunit becomes active and activates the effector enzyme Signup and view all the answers

What is the function of the effector enzyme in GPCR signalling?

It produces the 2nd messenger that triggers the downstream response Signup and view all the answers

What is the purpose of the GTP/GDP exchange on the G-protein?

It allows the G-protein to switch between its active and inactive states Signup and view all the answers

What is the outcome of GTP hydrolysis in the G-protein complex?

G-protein complex re-association Signup and view all the answers

What is the role of cAMP in the activation of protein kinase A (PKA)?

cAMP binds to the regulatory subunit of PKA Signup and view all the answers

What is the effect of phosphorylase kinase a on glycogen?

It phosphorylates glycogen, converting it to glucose-1-phosphate Signup and view all the answers

What is the role of phosphodiesterase in the regulation of cAMP levels?

It hydrolyzes cAMP to AMP Signup and view all the answers

What is the outcome of CREB phosphorylation by PKA?

It activates transcription by binding to specific sequences Signup and view all the answers

What is the outcome of IP3 binding to the Ca2+ channel in the endoplasmic reticulum?

It activates the release of Ca2+ into the cytosol Signup and view all the answers

What is the role of DAG in the activation of protein kinase C?

It binds to protein kinase C, making it more active Signup and view all the answers

What is the outcome of signal amplification via kinase cascade?

A small signal is amplified to a large response Signup and view all the answers

What is the role of Gαq (Gq) subunit in the GPCR pathway?

It activates phospholipase C, leading to the production of IP3 and DAG Signup and view all the answers

What is the outcome of the binding of cAMP to the regulatory subunit of PKA?

The tetramer dissociates, and the catalytic subunits are activated Signup and view all the answers

Study Notes

Cell Biology and Signalling

The course covers the foundations of medical science, specifically cell biology and signalling.

Cellular Responses

Cellular responses to signals include:
- Change in metabolic activities (e.g. glucagon switches liver from synthesizing glycogen to breaking down glycogen)
- Secrete and release (e.g. binding of antigen to mast cell stimulates the secretion of histamine)
- Changes in gene expression (e.g. Epidermal Growth Factor (EGF) activating genes involved in cell growth)
- Sensory perception (e.g. light activation of rhodopsin)

Extracellular Signals

Factors that act as extracellular signals include:
- Amino acids (and derivatives) (e.g. glutamate, adrenaline, dopamine)
- Steroids (e.g. oestradiol, testosterone, cortisol, aldosterone)
- Prostaglandins (eicosanoids) derived from arachidonic acid
- Proteins and Peptides (e.g. insulin, glucagon, growth factor, EGF)
- Gases (e.g. NO, CO)

Ways for Cells to Signal to Each Other

Different ways for cells to signal to each other include:
- Endocrine (signal produced by cells in one part of the body and travels in blood to target cells somewhere else)
- Autocrine (signal acts on the same cell that produces it)
- Paracrine (signal produced by cell and acts on other cells that are very close)
- Contact dependent (signal is integral part of one cell and interacts directly with another cell)
- Neuronal (electrical signal transmitted down cell and message passed to another via synapse)

Receptor/Signal

A cell must express a receptor in order to respond to a signal
Receptors have high selectivity and high affinity
Signals can bind to different types of receptors
Signals are eventually turned off

Location of Receptors

Cell surface receptors (e.g. hormone is hydrophilic, binding of hormone triggers response inside cell)
Intracellular receptors (e.g. hormone is hydrophobic, hormone crosses the plasma membrane, hormone binds to receptor in the cytosol and triggers a response inside cell)

Types of Signalling

Binding of signal to receptor:
- Depolarization of membrane due to flow of ions (e.g. acetylcholine)
- Direct activation of transcription factor (e.g. steroid)
- Generation of secondary message inside cell (e.g. glucagon – cAMP)
- Direct activation of enzymatic kinase cascade (e.g. EGF – MAP kinase pathway)

Second Messengers

Cyclic AMP (cAMP)
Cyclic GMP (cGMP)
Inositol 1, 4, 5-trisphosphate (IP3)
Diacylglycerol (DAG)
Calcium ions (Ca2+)

G-Protein-Coupled Receptors

G-protein-coupled receptors (GPCRs) have 7 transmembrane-spanning domains
Activation of GPCRs leads to the activation of G-proteins
G-proteins are guanine nucleotide binding proteins

G-Protein Signalling

G-protein signalling involves:
- Signal binding to receptor
- G-protein (GDP bound) associates with receptor
- GTP/GDP exchange on G-protein (GTP bound)
- G-protein dissociates into α (GTP bound) and βγ subunits
- α subunit (with GTP bound) activates effector enzyme
- Effector enzyme produces 2nd messenger
- GTP hydrolysed to GDP, G-protein complex re-associates, signalling ends

cAMP Signalling

cAMP is a second messenger produced by the activation of adenylyl cyclase
cAMP activates protein kinase A (PKA)
PKA phosphorylates and activates target proteins

cAMP Dependent Protein Kinase A (PKA)

PKA is a tetrameric enzyme consisting of 2 regulatory (R) and 2 catalytic (C) subunits
cAMP binds to the regulatory subunit and tetramer dissociates
Catalytic monomers (C) are now active enzymes

cAMP Mediated Effects on Glycogen Breakdown

cAMP dependent protein kinase (PKA) phosphorylates and activates phosphorylase kinase
Phosphorylase kinase phosphorylates and activates phosphorylase
Phosphorylase breaks down glycogen to glucose-1-phosphate

Signal Amplification via Kinase Cascade

Signal amplification occurs through the activation of multiple enzymes in a kinase cascade
The numbers of molecules involved in each step of the cascade increase, amplifying the signal

cAMP and Gene Transcription

PKA phosphorylates CREB (cAMP response element binding protein)
CREB binds to specific sequences in target genes and stimulates transcription
Long-term adaptation to starvation involves changes in gene expression

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Description

Quiz on cell biology and signalling, covering cell-to-cell signalling, second messengers, and transmembrane receptors in biochemistry.