Hydrophilic hormones and enzyme cascades Flashcards
What are hydrophilic hormones
Proteins (e.g. insulin), peptides, or amino acid derivatives (e.g.
adrenaline).
* Can dissolve in plasma without needing transport by binding
proteins, although there are exceptions (about half of
catecholamines are loosely bound to plasma albumin).
* Hormones act by altering intracellular proteins, because
hydrophilic hormones can’t cross plasma membrane, they act by
binding to receptors on outer plasma membrane surface of target
cell.
* Binding of hormone to receptor causes an ↑ in concentration of
‘second messenger’ in cell, which alters activity of other proteins.
What are the 4 types of ways hormones can act
Hormones can be classified according to distance over which they
act:
Endocrine
Act on cells form site of release. Secreted into blood.
Only target cells express receptor (e.g. insulin and
adrenaline).
Paracrine
Act on nearby cells only. Diffuse in interstitial fluid and
are rapidly inactivated by local enzymes (e.g.
histamine).
Juxtacrine
Hormone is either bound to membrane (requires
physical contact between cells) or hormone is secreted
into extracellular matrix.
Autocrine Act on cell that released hormone (e.g. T cells and IL2).
What are the features of hormone receptors
o Binding site faces outside of cell.
o Cytosolic domain is affected by binding to extracellular
binding site.
o Contains a membrane-spanning portion to connect
binding site to cytosolic domain that changes
conformation when hormone binds to extracellular part
of protein.
What are the 4 receptor types
Receptor Example Information
Ligand-gated
ion channel ACh receptor
Signal is transduced to cell via
change in membrane potential
when ion channel is opened.
Receptor
enzymes
Insulin
receptor
Enzymatic activity of receptor is
activated by hormone-binding.
Enzyme recruiting
receptor
Cytokine
receptor
Hormone binding induces
recruitment and activation of
protein kinases.
G-protein
coupled
receptor
Adrenaline
receptor
Hormone binding activates GTP
binding proteins.
Signal transduction steps
[1] Hormone is released. Hormone may enter bloodstream in
order to reach targets all over body, or may only be released into
tissue fluid, to reach nearby cells.
* [2] Hormone binds to receptor on plasma membrane of cell.
Hormone binding induces a conformational change in receptor’s
cytosolic region that alters its function.
[3] Concentration of a second
messenger ↑ through enzymatic
action.
* [4] Effectors are stimulated or
inhibited by second messenger.
These may be pumps, enzymes
or gene transcription factors,
for example.
* [5] Signalling pathway is shut down. Effectors return to original
state, and messengers are removed or become ineffective.
Describe adrenaline (beta adrenergic receptor)
[1] In absence of adrenaline, G-protein is bound to GDP, effector
protein, PKA and AC is inactive.
* [2] When adrenaline binds to receptor, it changes 3/4 and 5/6
cytosolic loops.
* [3] These interact with G-protein to remove GDP and pick up GTP.
* [4] G𝛼 subunit with GTP bound to it can bind to AC which is
activated and converts ATP to cAMP.
[5] PKA which is a heterotetramer with 2 regulator subunits and 2
catalytic subunits. Regulatory subunits stop PKA from
phosphorylating.
* [6] cAMP binds to regulatory subunits and releases catalytic
subunits which phosphorylate regulatory proteins.
* Enzyme catalysed steps (AC and cAMP) allows a small amount of
adrenaline to have a large effect.
Describe activation of heterodimeric G protein
Switch II is part of 𝛼 subunit. Can form an 𝛼 helix. Extra phosphate
on GTP stabilises switch II which then activates adenylyl cyclase.
When GTP is hydrolysed, switch II becomes disordered.
* Overactive adenylyl
cyclase in enterocytes
of intestine is
responsible for water
diarrhoea.
Describe Protein Kinase A
Protein kinase A, an
R2C2 heterotetramer,
is a serine/threonine
kinase.
* It recognises consensus sequence (target): Arg,Arg,X,Ser/Thr,Z.
This means it only phosphorylates serine/threonine if they sit
within this consensus sequence.
* Regulatory subunits have sequence: Arg,Arg,Gly,Ala,Ile. This
means catalytic subunit binds to regulatory subunits but won’t
phosphorylate it.
* Binding of 4 x cAMP to 2 R subunits causes them to dissociate
from catalytic subunits, activating them.
* Protein kinase A phosphorylates several enzymes, such as
hormone-sensitive lipase (+), acetyl CoA carboxylase (-), glycogen
synthase (-), and transcription factor CREB (+).
* Protein kinase A is thus able to immediately alter metabolic
pathways, and have longer term effects via gene transcription.
What are the diverse effects
In different cell types, different hormones can stimulate cAMP
pathway, and activated PKA will have different target proteins.
* Liver (adrenaline, noradrenaline, glucagon): ↑ glycogenolysis and
gluconeogenesis.
* Adipose tissue (adrenaline, ACTH): ↑ lipolysis.
* Ovarian follicles (FSH, LH): ↑ synthesis of oestrogen and
progesterone.
What are the points of amplification
Adrenaline: receptor complex is able to catalyse GDP:GTP
exchange on multiple G-proteins. Each activated G𝛼 subunit can
only bind to 1 adenylyl cyclase.
* Each active adenylyl cyclase can catalyse formation of many
molecules of cAMP. It takes 4 molecules of cAMP to activate 2 x
PKA subunits.
* Each active PKA subunit can phosphorylate many proteins.
What are receptor enzymes
These receptors have an extracellular ligand-binding domain, and
an enzyme active site on intracellular section, connected by a
single transmembrane segment.
* Many of these enzymes are tyrosine kinases (RTKs) (e.g. insulin
receptor).
* There are also some with serine/threonine kinase activity.
* Another group have guanylyl cyclase activity (convert GTP to
cGMP).
* In these receptors, ligand binding either activates enzyme activity,
or brings it in proximity to its target.
Describe tyrosine kinase receptors
- Receptor tyrosine kinases functions as dimers, with an
extracellular hormone-binding domain, and an intracellular
protein tyrosine kinase domain. - Upon binding of hormone (many are growth factors), RTK
monomers cross-phosphorylate each other. - Phosphorylation of RTK makes it a site of attachment for proteins
with SH2 domains, or PTB domains (localising proteins at
membrane). - For insulin receptor, cross-phosphorylation causes kinase to
become fully active.
Describe the MAPK pathway
[1] Binding of EGF to each EGFR monomer induces a structural
change that allows monomers to dimerize. Proximity of cytosolic
domains allows cross-phosphorylation. Initially PTK was active, but
not in close proximity to function.
* [2] Tyrosine-phosphates act as docking sites for Grb-2, which is
attached to Sos.
* [3] Sos catalyses exchange of GDP for GTP on membrane-bound
Ras, activating it.
* [4] GTP:Ras binds and activates Raf, a membrane-bound protein
kinase.
* [5] A series of protein kinases are phosphorylated and activated,
resulting in phosphorylation of several transcription factors,
altering their activity.
* MAPK: mitogen activated protein kinase
What is SOS and RAS
Sos is a guanine nucleotide exchange factor (GEF). It catalyses
exchange of GDP for GTP on Ras protein. But only when it has
been recruited to membrane via Grb-2.
* Ras is a small G-protein. Unlike heterotrimeric G-proteins, Ras is
monomeric.
* Ras also has a slower GTPase activity (0.02 per min vs ~ 3 per min)
than heterotrimeric G-proteins.
* GTPase activity can be ↑ ~ 105
-fold by GAPs (GTPase activating
proteins).
* Ras:GTP binds to, and activates, Raf.
Describe link between ESR signalling and cancer
EGFR is overexpressed in some epithelial cancers.
* In this case, a small amount of receptor can dimerize in absence of
ligand.
* Since tyrosine kinase activity is already present, this is enough to
initiate signal transduction, thereby sending an inappropriate
‘grow and divide’ signal to cell.
* A therapeutic antibody (cetuximab) targets extracellular domain
of receptor, sterically blocking ability of receptor to dimerise. This
has been successfully used in colorectal cancers
