Cell Biology and Signalling Flashcards
1
Q
what is a cell?
A
a semi-independent living unit within the body, sites the mechanisms for metabolism, growth and replication
2
Q
what is an organelle?
A
subunit within a cell with a defined structure and performing specific, integrated activities. different functions can operate under different conditions
3
Q
what is a tissue
A
organised assembly of cells which carry out coordinated activities within the body
4
Q
what is an organ?
A
assembly of tissues coordinated to perform specific functions within the body
5
Q
what is a system?
A
assembly of organs with specific activities sharing regulatory infuences
6
Q
what is a prokaryote?
A
single celled organism, chromosome circular and free, no membranous organelles
7
Q
what is a eukaryote?
A
chromosomes enclosed in a nucleus, linear DNA, membrane bound organelles, all complex organisms
8
Q
what is a virus?
A
an assemblage of nucleic acid (DNA or RNA) and proteins. invade cells, subvert their protein synthesis machinery to make more viruses
9
Q
genetic material (prokaryote vs eukaryote)
A
P: chromosomes - single circular location - nuclear region nucleolus - absent histones - absent extrachromosomal DNA - in plasmids ribosomes - 70S cell division - binary fission
E: chromosomes - paired linear location - membrane-bound nucleus nucleolus - present histones - present extrachromosomal DNA - in mitochondria ribosomes - 80S cytoplasmic / 70S mitchondrial cell division - mitosis or meiosis
10
Q
intracellular structure (prokaryote vs eukaryote)
A
P: mitotic spindle - absent sterols in plasma membrane - absent internal membranes - only for photosynthetic organisms endoplasmic reticulum - absent mitochondria - absent lysosomes - absent Golgi - absent peroxisomes - absent cytoskeleton - absent cell wall - present
E: mitotic spindle - present sterols in plasma membrane - present internal membranes - numerous membrane bound organelles endoplasmic reticulum - present mitochondria - present lysosomes - present Golgi - present peroxisomes - present cytoskeleton - present cell wall - absent (apart from some fungi)
11
Q
microscopes (SEM vs TEM)
A
SEM: cell surface shown electrons scattered off cell surface by heavy metal TEM: looks inside the cell
BOTH:
elaborate prep involved
can only use dead cells
12
Q
what limits max size of the cell?
A
diffusion at distance less than 50um is efficient, needs efficient SA:V (bigger cells less efficient)
13
Q
how do specialised cells overcome the max size limitation?
A
thin processes - directed transport of substances around cell via cytoskeleton
“giant” multinucleate cells - gene expression can occur in more than one place
gap junctions - channels between cells allow movt of substances between cells
14
Q
nucleus
A
largest organelle (3-10um)
only organelle clearly visible by light microscopy
contains genetic material:
- DNA organised as chromosomes; chromatin = complex of DNA/histone and non-histone proteins
- DNA winds round histones into nucleosomes
- unless cell is dividing chromatin is decondensed
nucleolus - where rDNA is transcribed and ribosome subunits assembled
nuclear envelope - surrounded by two layers of membrane
nuclear pores - allows transport in and out
15
Q
SER and RER
A
SER:
biosynthesis of membrane lipids and steroids, starts of N-linked glycosylation, detoxification of xenobiotics
RER:
coated with ribosomes (translation, proteins for secretion or insertion into cell membrane), proteins are folded (cya-cys bridges form), vesicles budded from RER and transported to the Golgi
16
Q
Golgi complex / body / apparatus
A
- 4-8 closely stacked membrane bound channels (cisterna)
- modifies proteins delivered from RER via vesicles (modifying N-linked carbohydrates, glycosylation of O-linked carbs and lipids)
- synthesise/package materials to be secreted
- direct new proteins in vesicles to their correct compartments
transport membrane lipids around cell - creates lysosomes
17
Q
secretory vesicles
A
- vesicles bud off from the Golgi
- vesicles fuse with the inner surface of the plasma membrane and release their contents (exocytosis)
18
Q
peroxisomes
A
Contain enzymes for breaking down toxic materials, also involved in phospholipid synthesis, oxidation of very long chain fatty acids
- enzymes which generate H202
- Zellweger syndrome
- adrenoleukodystrophy (ALD)
19
Q
lysosomes
A
- electron dense spheres in EM
- membrane-bound
- 50 different hydrolytic enzymes
- all require low pH
- involved in organelle turnover/replacement (autophagy)
20
Q
mitochondria
A
- 2 layers of membrane
- number per cell reflects metabolic activity
- contain DNA (encode some of their proteins - own genome)
- sugars oxidised (generate ATP krebs)
- inner membrane in folds (cristae inc SA)
- Krebs cycle enzymes/electron transport chain are located in diff parts of structure
21
Q
peptide bond between AA’s
A
formed by enzyme reaction strong carboxyl and amino group hydrolysis (h20 removed) to give CN link only happens under digestion and lysosome
22
Q
peptide bond features (like double bond)
A
C-N bond short no rotation -ve charge on O \+ve charge on N peptides can form H-bonds with other polar groups in polypeptide chain
23
Q
direction of polypeptides
A
first AA has NH3+ group
last AA has COO- group
24
Q
other covalent linkages (apart from peptide bonds)
A
disulphide S-S bridges between two cys
(intrachain and interchain)
glycosylation:
O-linked -OH of thr and ser
N-linked -NH2 of asn
modifying structure changes function:
phosphorylation (+/- phosphate group):
eg. cell signal transduction
eg. change in activity of enzyme
methylation (+/- methyl groups) via -NH2 groups of lys and arg:
eg. histones affect gene expression
25
how is the alpha helix formed?
formed by H-bonds in same polypeptide chain
particularly H-bonds between peptide bond carbonyl-O and H of N-H every 4th peptide
26
alpha helix features
3.6 AA residues
| R groups on outside
27
what bonds are in beta pleated sheets
linear peptide chains
| H-bonding between peptide chains
28
collagen triple helix features:
- where are H-bonds
- how many residues
- common repeating primary sequence
- H-bonds between chains
- 3 residues
Gly-X-Y-Gly-X-Y
X=mainly proline
Y=mainly hydroxy-proline
29
define tertiary structure
how the whole polypeptide is folded in 3D, will consist of a number of diff super secondary structures (domains)
30
define quaternary structure
how the whole functional protein is formed in 3D, may consist of a number of subunits (eg. haemoglobin)
31
forces that stabilise protein structure (2)
covalent:
- disulphide bridges
non-covalent:
- H bonds
- electrostatic interactions
- VdeW forces
- hydrophobic effect
32
electrostatic interactions and 2 examples
between charged side chains
Asp and Glu carboxyl groups are ionised
-COO-
Lys and Arg amino groups are ionised
-NH3+
33
define van de waal forces
sum of the attractive or repulsive forces between molecules
(excluding those due to covalent, hydrogen, electrostatic)
dependent on dipole affect caused by unequal distributions of electrons
34
hydrophobic regions are _____ to form hydrogen bonds
unable
35
proteins are sensitive to denaturation by
- pH
- temp
- ionic strength
36
Creutzfeldt-Jakob disease symptoms
| aggregation of misfolded proteins
neurological symptoms:
- difficulties with walking
- slurred speech
- numbness
- dizziness
psychological symptoms:
- severe depression
- withdrawal
- anxiety
37
3 key features of enzymes
- speed (rate enhancement by 10^6-10^14)
- selectivity (Some will only act on one type of substrate)
- specificity (eg. Will only add glucose onto 2 position of another glucose not 3,4 or 6 positions
38
classification of enzymes (6)*
- oxidoreductases
Lactate Dehydrogenase
(add O2 or remove 2H)
- transferases
Alanine aminotransferase
(catalyse transfer of functional groups from donors to acceptors)
- hydrolases
Trypsin
(catalyse cleavage of bonds by addition of water, hydrolysis)
- lyases
ATP-citrate lyase
(catalyse cleavage of C-C, C-O or C-N, form double bonds by removal of groups)
- isomerases
Phosphoglucose isomerase
(catalyse the transfer of functional groups within the same molecule, isomerisation reactions)
- ligases
DNA ligase
(use ATP to catalyse the formation of new covalent bonds)
classes are divided in subgroups according to their substrate and source
eg. alcohol dehydrogenase
IUB name, alcohol, substrate, reaction type followed by ace, IUB number, E.C.1.1.1.1
6 classes further divided into subgroups according to substrate or source, each enzyme is identified by its own 4 digit number
(eg. catalase is E.C.1.11.1.6)
39
what does the induced fit theory imply?
enzymes undergo conformational changes upon substage binding
these changes ca affect residues in AS as well as repositioning entire domains
it serves to bring specific functional group within enzyme in the proper position to catalyse reaction
40
catalysis of peptide bond hydrolysis by chymotrypsin
- polypeptide substrate binds to hydrophobic pocket
- H+ is transferred from Ser to His, substrate forms tetrahedral transition state with enzyme
- H+ transferred to C-terminal fragment, which is released by cleavage of the C-N bond. The N-terminal peptide is bound through acyl linkage to Ser
- water molecule binds to enzyme in place of departed polypeptide
- water molecule transfers its proton to His and its -OH to the remaining substrate fragment. tetrahedral transition state formed
- the second peptide fragment is released: acyl bond cleaved, proton transferred from His back to Ser, enzyme returns to initial state
41
enzyme with low substrate specificity
Papain
a cysteine protease from papaya
used as a meat tenderiser
42
other ways of plotting enzyme rate vs [S]
Lineweaver-Burk (double reciprocal) plot
1/v against 1/[S]
Intercepts: 1/Vmax and -1/Km
Eadie-Hofstee plot
v/[S] against v
intercepts: Vmax and Vmax/Km
Hanes plot
[S]/v against [S]
Intercepts: Km/Vmax and -Km
43
relationship between Km and E-S affinity
lower the value of Km the higher the affinity of a particular substrate for the enzyme that catalyses it
44
Types of E-S inhibition
Competitive:
Bind directly to the AS of an enzyme, competing with substrate
Increases Km but does not affect Vmax
Non-competitive:
Binds to enzyme away from AS, alters shape of enzyme so even if substrate can bind, the AS functions less effectively
Reduces Vmax but does not affect Km
Uncompetitive:
Only bind once the E-S complex has formed. The E-S-I complex does not produce any product
Reduces both Km and Vmax
45
Example of competitive inhibition
Succinate dehydrogenase:
- oxidation of succinate to fumarate
- inhibited reversibly by malonate (resembles substrate, can’t be oxidised)
Increases Km
Doesn’t affect Vmax
46
Example of non-competitive inhibition
Fluoride inhibition of enolase
- key enzyme of glycolysis
- forms PEP
F- is a non-competitive inhibitor
Fluoride ions replace oxygen s of carboxylate of PEP
Reduces Vmax
Doesn’t affect Km
47
Example of uncompetitive inhibition
Examples involved with certain types of cancer
A number of genes that code for TSAPs are inhibited uncompetitively by amino acids such as leucine and phenylalanine
Reduces both Km and Vmax
48
What is allosteric regulation
A form of regulation where the regulatory molecule (an activator or inhibitor) binds to an enzyme someplace other than the AS
All cases of non-competitive inhibition are forms of allosteric regulation
49
Features of allosteric enzymes
Multiple activate sites located on different protein subunits
50
Allosteric inhibitor
Allosteric activators
Cooperatively
Allosteric inhibitors:
Bind to enzyme away from AS, all AS’s on protein subunits are changed so they work less well
T-state
Allosteric activators:
Bind to locations on an enzyme other than AS causing inc in function of the active site
R-state
Cooperativity:
Substrate itself serves as an allosteric activator: binds to one AS, the activity of the other AS’s goes up
51
Feedback inhibition (cycle)
In metabolic pathways an end product of a chain of enzymatic reactions can act as an allosteric inhibitor using a feedback mechanism
- substrate binds to enzyme
- intermediate substrate
- this binds to another enzyme, producing another intermediate
- finally producing end product
- if want to inhibit the formation of this final product, the product can bind away from active site on the first enzyme
- this depresses the formation of the end product
52
What does an allosteric graph look like?
Reaction velocity against substrate conc
Sigmoidal
Can be considered to be a result of combing two Michaelis-Menten enzymes
One with high Km value (T-state)
One with low Km value (R-state)
The sigmoidal response is retained but shifted by regulators
53
Example of allosteric enzyme
ATCase
Aspartate transcarbamylase
Catalyses the condensation of aspartate and carbamoyl phosphate to form N-carbamoylaspartate, in pyrimidine synthesis
(CTP end product)
Enzymes exists in T state (favoured by CTP binding) and R state (favoured by substrate binding)
In absense of substrate/regulators ATCase exists in equilibrium between 2 states, where T state is favoured
54
amino acid structure
central carbon with amino group, carboxylic group, and a R group attached
20 universal amino acids
55
essential amino acids
de novo
cannot be synthesised by humans
need to be obtained nutritionally
56
the 20 coded amino acids
| and their side chain charges
```
aspartic acid - negative (carboxylic)
glutamic acid - negative (carboxylic)
arginine - positive (amino)
lysine - positive (amino)
histidine - positive (complex ring amino)
asparagine - uncharged polar (amide)
glutamine - uncharged polar (amide)
serine - uncharged polar (hydroxyl)
threonine - uncharged polar (hydroxyl)
tyrosine - uncharged polar (benzene uncharged)
alanine - non polar (aliphatic)
glycine - non polar (none)
valine - non polar (aliphatic)
leucine - non polar (aliphatic)
isoleucine - non polar (aliphatic)
proline - non polar (ring structure including alpha amino)
phenylalanine - non polar (benzene ring)
methionine - non polar (sulphur)
tryptophan - non polar (ring structure)
cysteine - non polar (thiol)
```
57
not obvious 3 letter codes (3)
Gln - glutamine
Asn - asparagine
Ile - isoleucine
58
not obvious single letter amino acid codes
```
W - tryptophan
E - glutamate
D - aspartate
K - lysine
Q - glutamine
N - asparagine
```
59
other roles of amino acids
taste enhancement
- MSG (monosodium glutamate)
neurotransmitters
- glutamate
synthesis of neurotransmitters
- eg. tyrosine to dopamine
- eg. tryptophan to serotonin
synthesis of hormones
- eg. tyrosine to melanin
60
what does acidity depend on?
depends only on free hydrogen ions
not those still bound to anions
61
where do acids in the body come from?
- breakdown of proteins
- incomplete oxidation of fats or glucose
- loading and transport of carbon dioxide in the blood
62
how is acid-base balance regulated in the body?
- lungs
- kidneys
- chemical buffers
63
what is blood pH
7.4
64
what is pKa?
the pH at which the acid is half dissociated
the lower the pKa, the stronger the acid
65
what are buffers mixtures of?
weak acids and their conjugate bases
66
why is the pKa the best buffering point
if H+ is added they can be picked up by the conjugate base
if OH- is added the acid can donate a proton to form H2O
67
examples of:
metabolic acidosis
respiratory acidosis
conjugate base (bicarbonate) is low, probs diabetic
the acid (carbonic acid) is high, carbon dioxide partial pressure is proportional to carbonic acid conc so can monitor
68
why is glycine not a good physiological buffer?
it has pKa points at 2.3 and 9.6 which means it buffers best at non-useful pH's
the alpha carboxyl and alpha amino groups are involved in the peptide bond so wouldn't be able to dissociate anyway
69
best amino acid as a physiological buffer
histidine
pKa of 6
most amino acids do not buffer in the physiological range
70
what makes haemoglobin a good blood buffer
has a large number of histidine residues
in haemoglobin, the pKa of histidine is different from that of free histidine (which is 6), neighbouring groups affect the pH
oxyhaemoglobin pKa = 6.8
deoxyhaemoglobin pKa = 7.8
71
What are the 3 key residues in the catalytic triad
Ser
His
Asp
72
What is albinism caused by
Defective tyrosinase
| Affects the synthesis of melanin from Tyrosine
73
What is a cytoskeleton (give 3 components)
Protein filaments
Actin = thinnest (muscle)
Microtubules = thickest (pull daughter cells apart)
Intermediate filaments = mechanical strength of cell
74
What’s another name for lipids
Triacylglycerols
75
Two types of lipids in membranes and their structures
Phospholipids
- glycerol & 2 FA’s & phosphate containing group
- composed of a polar head group attached to glycerol backbone through a phosphate group (hydrophilic)
- FA’s linked time glycerol via ester bonds (hydrophobic)
Glycolipids
- glycerol & 2 FA’s & sugars
76
Define amphipathic
Polar head group and non-polar FA tail (polar intact with aqueous environment)
77
Common head groups found in phospholipids (4)
Learn their struct?
- choline (involved in signalling)
- serine
- ethanolamine
- inositol (involved in signalling)
78
What is ceramide composed of?
Sphingosine and fatty acyl chain together
79
Variation in composition of cellular membranes
Plasma membrane
Outer mitochondrial membrane
Inner mitochondrial membrane
Nuclear membrane
Only plasma membrane contains carbohydrates
Higher amount of protein in the inner mitochondria membrane and nuclear membrane %
All are different in lipid composition %
Plasma membrane has far more cholesterol %
80
Composition of the 2 halves of the bilayer
External side:
- glycolipid
- PC and SPH
Internal side:
- PS and PE
81
Regulation of fluidity of the bilayer
Increase fluidity:
- inc in short chain fatty acids reduce VdeW interactions between so inc fluidity
- kinks in unsaturated fatty acids reduce VdeW with other lipids so inc fluidity
Decrease fluidity:
- high cholesterol content restricts random movt of polar heads
82
Composition of lipid rafts
Specialised membrane
Less fluid
Inc level of cholesterol and sphingomyelin
83
Membrane protein classes:
Integral (intrinsic) proteins
Anchored proteins
Peripheral (extrinsic) proteins
Integral
- embedded in lipid bilayer
Anchored
- have covalent bonds with fatty acids from phospholipids
- example is RAS a signalling G-protein
Peripheral
- attach to membrane surface by ionic interactions with integral proteins or phospholipid heads
- example is Spectrin (structural protein on erythrocytes that interact with other proteins such as ankyrin)
84
Cation vs anion
Cations carry one or more positive charges.
Anions carry one or more negative charges.
85
pKa values for amino acids
Alpha carboxyl group:
~2.5
Alpha amino group:
~9.6
86
Equivalence point
When an equivalent number of MOLES of base has been added to the weak acid
87
The Bohr effect
An increase in pH (and decrease in CO2 conc) increases Hb’s affinity for O2
88
BPG effect
Whole blood must contain something that lowers its affinity for O2 compared to pure Hb
89
Globin chains in adult Hb and fetal Hb
HbA:
Alpha 2 beta 2
HbF:
Alpha 2 gamma 2
(Practically none left after 3months)
90
Ferrous vs ferric iron
```
Ferrous = Fe2+
Ferric = Fe3+ (rust/cannot react with O2)
```
91
Deoxygenated structure vs oxygenated Hb
When no oxygen:
Iron centre is shifted 0.4armstrong from the plane of the Haem
When oxygen binds, iron centre moves into plane of the Haem (due to partial change in electron distribution, Fe2+ temp goes Fe3+)
Allosteric transition from a low O2 affinity state (T) to a high O2 affinity state (R)
Distal histidine stabilises other side of oxygen
92
Molecular equations in respiring tissues and in the lungs
In respiring tissues:
CO2 enters erythrocyte
CO2 + H2O -> HCO3- + H+ + Cl-
Bicarbonate dissolves in blood plasma whilst Cl- enters erythrocyte
H+ produced decreases pH so less affinity for O2
In lungs = opposite equation and CO2 leaves erythrocyte and is exhaled
93
Molecular cause of sickle cell anaemia
Genetic disease resulting from a mutation that converts Glu (acidic & hydrophilic) in the beta chains to Val (non-polar & hydrophobic)
Oxygenated molecules are soluble but upon deoxygenation, conformation of HbS differs from HbA and it aggregates into insoluble fibers -> sickle shaped cell
94
Protein signalling at ER (example)
Newly synthesised protein at the ribosome
Signal sequence on growing polypeptide chain is recognised by SRP (signal recognising protein)
SRP binds to receptor in the ER membrane
SRP displaced (& recycled), newly synthesised polypeptide guided through translocation channel on membrane and into ER lumen
95
When does the protein remain in the ER membrane and not fully through?
When the protein has an additional stop sequence it stops the process so protein is embedded within membrane
96
Newly translated protein targeted to the mitochondria
Protein completed and released by ribosome into cytoplasm
Chaperone (HSP70) takes protein to mitochondria
Signal sequence on protein, binds to receptor on mitochondrial membrane, protein guided through translocator contact site and into mitochondrial matrix
Signal sequence cleaved off protein
Protein is folded into mature protein
97
Targeting proteins to nucleus
Newly translated protein is fully folded and released into cytoplasm
Nuclear proteins contain NLS (nuclear localisation signal) which has lots of basic AA’s
NLS binds to importin and this complex is transported through nuclear pore
(Exportin performs reverse function)
98
Targeting proteins to lysosome
Lysosomal protein tagged with sugar in Golgi
(Mannose-6-phosphate)
Sugar receptor in golgi directs proteins into transport vesicles
Vesicle becomes the lysosome
99
Composition of cytoskeleton
| Smallest to largest
Actin microfilaments:
- monomer is globular protein G-actin
- dynamic
- 2 tightly wound chains (polymerised)
- eg. In microvilli = mechanical support
Intermediate filaments:
- different IF proteins in diff cell types
- epithelia = keratin
- axons = neurofilamin
- universal (nucleus) = lamin A, B, C
- structure process: monomer-dimers-tetramers-link up end to end
Microtubules (polymers of tubulin):
- tubulin monomer is heterodimer: alpha&beta
- 13 parallel protofilaments arranged in hollow tube
- dynamic scaffold (chromatid separation)
- movt of cargo within cells
100
Lamellapodia
Extensions of cells containing actin network
(Generated by rapid actin growth in cell membrane)
Contraction involving myosin allows cell movt
101
Spindle formation
Spindle formation initiated from centrosome
(type of microtubule organising centre)
Centrosomes contain centrioles
(a form of stable microtubules)
Centrosomes form 2 poles of cells
Kinetochore MT attach to centromere of chromatid
Aster MT attach centromere to cell membrane
102
Movement of cargo within cells
2 motor proteins asso with microtubules
ATP hydrolysed to move cargo along microtubule
Kinesin moves to + end (cell periphery)
Dynein moves to - end (near nucleus)
103
Cilia
Microtubules central support
MTOC called Basal body close to membrane
MT’s move components up and down cilia
104
Primary vs motile cilium
In cross section
Primary have no central doublet (9+0)
Motile have a central doublet
(9+2)
Contains additional dyenin component that provides the ATP synthesis for movement of cilia
105
Selective permeability
Block almost all hydrophilic molecules
(Charged polar molecules needed specialist proteins)
Small, uncharged or hydrophobic can freely cross
106
Passive transport
Rate of diffusion depends on partition coefficient of solute
Solutes that are more hydrophobic have higher partition coefficient and equilibrate more quickly
107
Facilitate diffusion of glucose
GLUT1
- most cells
- high affinity
GLUT2
- liver, pancreatic cells
- low affinity
- never fully saturated so will work at all glucose concs
GLUT3
- neurones
- low Km
GLUT4
- muscle, adipocytes
- regulated by insulin
- (muscle glucose to glycogen) (adipocyte glucose to fatty acids)
108
Insulin stimulated uptake of glucose
I stimulates uptake of glucose in muscle and adipose
I inc the amount of GLUT4 in plasma membrane (via vesicles)
Inc uptake of glucose into cell
(When decrease of glucose, transporters move into intra-cellular storage pool - endosome)
109
Gated ion channels
Open or close in response to stimulus
Either:
Ligand-gated:
(Acetylcholine on Na+/K+ channel on post synaptic membranes)
Voltage-gated:
(Na+ and K+ channels in axons)
110
Na+/K+ pump
| Primary active transport
Na+/K+ pump consists of tetramer
Na+ enters open cytoplasmic access
Phosphorylation from ATP at cytoplasmic site causes conformational change (closes cytoplasmic access)
Conformational change means pump binds K+ and releases Na+ outside cell
Hydrolysis of phosphate group closes external access, opens cytoplasmic, releases K+
111
Secondary active transport
Pre-establishes gradient is used to drive transport of solute across membrane against gradient
ATP hydrolysis used to establish primary gradient
Example:
Na+-glucose cotransporter
Symport
(Na+/K+ used before to set up gradient)
112
Cholera treatment
Cholera toxin stimulates inc in cAMP level that activates CFTR and secretion of chloride ions out of cell
Na+ and water follow via osmosis
Oral rehydration therapy includes high glucose conc which drives Na+ (and therefore Cl- and water) uptake into the cells via SGLUT
113
Secretion of insulin by beta-cells
Glucose transported into beta cells by facilitated diffusion by GLUT2
Glucose is metabolised increasing ATP level
ATP/ADP ratio closes ATP-sensitive K+ channels, leading to cell membrane depolarisation
Voltage-gated Ca2+ channels open, intracellular Ca2+ increases
Inc in Ca2+ triggers executors is of insulin in vesicles
114
Different ways for cells to signal to each other
Endocrine:
Signal produced by cells in one part of body travels in blood to target cells elsewhere
Autocrine:
Signal acts on same cell that produced it
Paracrine:
Signal produced by cell and act on other cells that are close
Contact dependent:
Signal is integral part of one cell and interacts directly with another cell
Neuronal:
Electrical signal transmitted down cell and message to another via synapse
115
Location of receptor
| In terms of hormones
Cell surface receptor
- hormone is hydrophilic (adrenaline) so doesn’t enter
Intracellular receptor
- hormone is hydrophobic
- binds to receptor in cytosol
116
2nd messengers generated by enzymes
G protein coupled receptors
(GPCR)
(Transmembrane alpha helix)
- activation of adenylyl cyclase (forms cAMP)
- activation of phospholipase C
(forms IP3, DAG)
117
G-proteins
Heterotrimeric complex
(Alpha, beta, gamma)
Dissociates when GTP binds
Free active G-alpha activates effector enzymes (leads to production of secondary messenger)
Complex re-associates when GTP hydrolysed to GDP
118
Signal amplification via kinase cascade
Glucagon -> glucagon receptor
ATP -> cAMP
inactive PKA -> active PKA
inactive phosphorylase kinase -> active phosphorylase kinase
Inactive phosphorylase -> active phosphorylase
Glycogen -> glucose-1-phosphate
119
cAMP dependent protein kinase A (PKA)
Tetrameric enzyme, 2 regulatory (R) and 2 catalytic (C) subunits
cAMP binds to R subunit and tetramer dissociates
C monomers now active enzymes (PKA)
120
IP3 / DAG
| Secondary messengers
Some GPCR contain G(alpha)q subunit
Dissociated Gq activates phospholipase C
Phospholipase C cleaves inositol phospholipids in membrane (-> DAG & IP3)
```
IP3 = activates Ca2+ channel in ER (inc conc in cytosol)
DAG = together with Ca2+ activates protein kinase C
```
121
Types of signalling
| After binding of signal to receptor
Depolarisation of membrane due to flow of ions
(Acetylcholine)
Direction activation of transcription factor
(Steroid)
(Steroid into cell, binds to receptor, complex binds to specific DNA)
Generation of secondary message inside cell
(Glucagon - cAMP)
Direct activation of enzymatic kinase cascade
(EGF - MAP kinase pathway)
122
Hyperpolarisation vs depolarisation
Hyperpolarisation:
Membrane potential becomes more negative
Depolarisation:
Membrane potential becomes less negative (more positive)
(Opening of a channel that lets positive ions flow into the cell cause depolarisation)
123
Direct activation of transcription factors
Binding of steroid hormones to receptor (in cytoplasm) induces conformational change that allows DNA binding and activation of transcription of target genes
Steroids are ligand-dependent transcription factors
124
GCPR signalling to effector enzymes
| Glycogen breakdown
Signal binds to receptor
GTP/GDP exchange on G protein (GTP bound)
GTPalpha activates effector enzyme (cyclase)
Cyclase produces cAMP (2nd messenger)
cAMP binds to R subunit
(on R2C2, PKA enzyme)
C monomers now active enzymes
(Refer to kinase cascade for the rest of breakdown)
125
cAMP on gene transcription
cAMP activates PKA
PKA phosphorylates CREB
(cAMP response element binding protein)
CREB binds to specific sequences in target genes & stimulates transcription
126
EGF (no secondary messenger)
Binding of EGF triggers autophosphorylation of tyrosine residues -> receptor tyrosine kinase (RTK)
Adaptor proteins contain phosphotyrosine binding domains (SH2, PTB)
Adaptor proteins Grb2 and Sos bind to receptor
This activates the exchange:
GDP-Ras -> GTP-Ras (Ras is a G protein)
GTP-Ras triggers kinase cascade:
(Ras-MAPkinase pathway)
MAPKKK activates MAPKK activates MAPK activates transcription factor
