Lecture 26

Question 1

Signal transduction

Answer 1

• conversion of a signal or impulse from one form to another

Question 2

Cell communication

Answer 2

• cell communication can very on how “public” a message is made
• signalling molecule can take up many forms: proteins, peptides, nucleotides, amino acids, fatty acid derivatives, gases

Question 3

Endocrine signalling

Answer 3

• the most “public” signalling system
• endocrine cells produce signal molecules known as hormones to be delivered through the bloodstream
• signal can be broadcast to the entire body
• example: glucagon and insulin are hormones used to regulate blood sugar levels

Question 4

Paracrine signalling

Answer 4

• signalling cell produce signal molecules known as local mediators that diffuse locally through the extracellular fluid
• signal is limited and can only be delivered to nearby cells
• if the signalling cell responds to its own signal this is a form of paracrine signalling known as autocrine signalling
• example: cancer cells secrete local mediators which promote their own survival

Question 5

Neuronal signalling

Answer 5

• signals can be delivered very quickly over a long distance (>1m)
• instead of broadcasting signal widely, signal is sent to a specific target cell
• the signal is transmitted along a neuron in form of an action potential. The electrical signal is converted into a chemical signal in form of neurotransmitter at the nerve terminals
• the neurotransmitter binds to the receptor on the target cell which can be converted back into an electrical signal

Question 6

contact dependent

Answer 6

• most intimate and short range
• contact is made between two molecules embedded in the signalling cell and the receptor on the target cell

Question 7

example of contact dependent signaling

Answer 7

lateral inhibition in drosophila

Question 8

cell responses

Answer 8

• the ability for a cell to respond is dependent on wether or not it has an appropriate receptor
• even if two cells have the same receptor they may respond differently
• the extracellular signal is not the message: the information conveyed is also dependent on how the target cell receives and interprets the signal
• example: muscle cell (contraction) vs. salivary gland cell (secretion)

Question 9

Extracellular signals can fall into two general classes

Answer 9

1. molecules that do not cross the plasma membrane and bind to surface receptors
   - usually large and hydrophilic
2. molecules that cross the plasma membrane and enter the cytosol and bind to intracellular receptors
   - usually small and hydrophobic

Question 10

steroid hormone

Answer 10

• hydrohphobic molecule that can cross the plasma membrane and binds to a nuclear receptor
• example: cortisol

Question 11

what is a nuclear receptor

Answer 11

• a receptor that when bound to a ligand can enter the nucleus and initiate transcription
• can initially be found in the cytosol or nucleus

Question 12

Nitric oxide

Answer 12

• it is a gas that diffuses across the plasma membrane and binds to proteins like guanylyl cyclase forming cyclic GMP
• only works locally because it is quickly converted into nitrates and nitrites
• NO is produced in endothelial cells causing smooth muscle to relax and blood vessels to dilate

Question 13

cell surface receptors

Answer 13

• most extracellular signals bind to cell surface receptors
• this binding generates an intracellular signals using intracellular signalling molecules which activate effector proteins to cause a cellular response

Question 14

Functions of intracellular signalling pathways

Answer 14

1. relay signal onwards
2. transduce and amplify signal
3. integrate
4. distribute

Question 15

molecular switches

Answer 15

• are signalling proteins that toggles between active and inactive states in response to a signal

Question 16

two classes of molecular switches

Answer 16

• proteins activated or inactivated by phosphorylation
• GTP-binding proteins

Question 17

what do protein kinases do

Answer 17

• phosphorylate proteins

Question 18

what do protein phosphatases do

Answer 18

• dephosphorylates proteins

Question 19

What do serine, threonine, and tyrosine all have in common?

Answer 19

• they can serve as phosphorylation sites, where phosphate groups are added to their hydroxyl groups by protein kinase

Question 20

Explain the phosphorylation cascade

Answer 20

• the phosphorylation of the molecular switch causes it to phosphorylate another molecular switch allowing for transmission, amplification, distribution and regulation of signals
• example: MAP kinase

Question 21

GTP binding proteins posses

Answer 21

• GTP hydrolyzing (GTPases) activity

Question 21

GTP-binding proteins

Answer 21

• toggles between active and inactive state depending on wether we have GTP or GDP bound

Question 22

What are the two main types of GTP-binding proteins

Answer 22

1. large trimeric GTP-binding proteins
2. monomeric GTPases

Question 23

What are two regulatory proteins that aid small GTP-binding proteins

Answer 23

1. Guanine nucleotide exchange factors (GEFs): which activates proteins by exchanging GDP for GTP
2. GTPases activating proteins: which inactivate proteins by promoting GTP hydrolysis

Question 24

3 major types of cell surface receptors

Answer 24

• ion channel coupled receptor
• G-protein coupled receptor
• enzyme coupled receptor

Question 25

ion channel coupled receptor

Answer 25

• we’ve seen it when a chemical signal in form of neurotransmitter is transduced into an electrical signal by opening of ion channel and causing a change in membrane potential
• important in neutrons and electrically excited cells like muscle cells

Question 26

g-protein coupled receptors

Answer 26

• G protein coupled receptors activate membrane bound trimeric GTP binding proteins causing the activation or inactivation of enzymes or ion channels in the plasma membrane

Question 27

enzyme coupled receptor

Answer 27

• the receptor itself can act as an enzyme or associate with enzymes in the cell