Peptide hormones: mechanisms & biosynthesis Flashcards
Protein/peptide hormones =
most numerous type of hormone
• Many peptide hormones belong to ….
families that share genetic and
peptide structure homologies
- Evolutionary emergence from a common ancestral gene
Most peptide/protein hormones require the
__ of a __ gene
Most peptide/protein hormones require the
transcription of a single gene
• e.g., human INS gene (insulin)
• Most peptide/protein hormones require the
transcription of a single gene
however exceptions include…
Glycoprotein hormones
Glycoprotein hormones examples (4)
• Follicle-stimulating hormone (FSH), Luteinizing
hormone (LH), Thyroid-stimulating hormone
(TSH) and human chorionic gonadotropin (hCG)
Glycoprotein hormones are Heterodimers …..
-Heterodimers consisting of alpha and beta
subunits (encoded by two different genes),
with carbohydrate side-chains
• Alpha subunit: same for all four of these
hormones; Beta subunit: unique for each
hormone (and confers biological specificity)
Genes: ____ regions (exons + introns) and ___ regions
• Genes: coding regions (exons + introns) and regulatory regions
promoter region
• Most important regulatory region: promoter region on 5’ end of gene… includes an
initiator element (w/ transcription start site, generally just downstream of TATA box)
and regulatory elements (specific binding sites for transcription factors: proteins that
may initiate, activate, or repress transcription)
transcription factors
transcription factors: proteins that
may initiate, activate, or repress transcription)
what is the most important regulatory region
Most important regulatory region: promoter region on 5’ end of gene
wha are enhancers/silencers
Enhancers/silencers (regulatory regions that are binding sites for activators or
repressors) are usually located further upstream or downstream of the promoter
Some hormones cause activators or repressors to bind to regulatory elements,
___ or ____ transcription of target genes as part of their cellular effects.
give an example
Some hormones cause activators or repressors to bind to regulatory elements,
increasing or decreasing transcription of target genes as part of their cellular effects.
e.g., estrogen receptor binds to specific DNA sequences: estrogen response elements
(EREs). Hormones can both affect and be affected by regulation of gene expression.
what is DNA
DNA: tightly packaged (e.g., histone proteins forming nucleosomes, which bundle together)
what is chromatin remodeling
• To allow dynamic access to condensed DNA, chromatin remodeling alters this architecture to
expose or hide regions for transcriptional regulation.
Chromatin modeling is important for controling dna expression
chromatin remodeling could involve
Histone modifying enzymes,
ATP-dependent chromatin
remodeling complexes
Histone modifying enzymes example
Histone modifying enzymes, e.g., histone acetyltransferases (HATs) add acetyl groups to histones, causing more open conforma=on (vs. histone deacetylases, HDACs)
ATP-dependent chromatin
remodeling complexes
• ATP-dependent chromatin remodeling complexes which can move, eject or restructure nucleosomes to expose binding sites for transcription activators or repressors at gene promoters or enhancers
ATP-dependent chromatin remodeling complexes can be
recruited to specific sites such as gene promoters, and
cooperate with histone modifying enzymes.
_____ located far from the
transcriptional start site can impact gene expression
Distal regulatory elements located far from the
transcriptional start site can impact gene expression
• They can come into closer proximity due to
changes in the 3-D structure of DNA & chromatin
In the promoter region, the
preinitiation complex forms:
Incl. RNA polymerase II (the enzyme that transcribes genes to mRNA), general transcription factors, and additional regulators (e.g., specific transcription factors, chromatin remodeling complexes, etc.)
what influences the rate of gene transcription
Presence of regulators & interactions between regulators and
DNA regulatory elements influence the rate of gene transcription
Once transcription is initiated, \_\_\_\_ continues the elongation process until a sequence is reached and transcribed that signals \_\_\_\_ and \_\_\_\_\_ of the RNA, leading to \_\_\_\_\_
Once transcription is initiated, RNA polymerase II continues the elongation process until a sequence is reached and transcribed that signals cleavage and polyadenylation of the RNA, leading to transcription termination
Pre-mRNA/mRNA processing:
(during transcription): 5’ end of RNA is capped with modified guanine nucleotide (7-methylguanosine) • Key for nuclear export, translation and stability • 3’ end is polyadenylated (adenosine residues added) • Key for nuclear export, translation and stability • Introns (intervening sequences), spliced out, and exons (expressed) incl. 5’ and 3’ UTRs (untranslated regions) ligated • Mature mRNA is exported via nuclear pore
Protein and mRNA are degraded at ___ rates
(often fast for signalling molecules like hormones)
_____ and
___ of 3’ poly-A tail
affect ____ half-life
Protein and mRNA are degraded at variable rates
(often fast for signalling molecules like hormones)
5’ mRNA cap and
length of 3’ poly-A tail
affect mRNA half-life
Calcitonin gene-related peptide
37-a.a. peptide primarily
secreted in nervous system.
Vasodilator, implicated in
sensing pain
Calcitonin
32-a.a. peptide hormone
secreted by the thyroid gland. Reduces
blood calcium, opposing the effects of
parathyroid hormone.
Alternative splicing regulates___
Alternative splicing regulates the synthesis of
(typically tissue-specific) hormone or receptor variants
Hormone \_\_\_ (as well as specific blood binding proteins) are comprised of proteins, therefore \_\_\_ by genes…
Subject to the same types of regulatory control of their gene’s
transcription/translation as peptide hormones
• Some hormones affect the synthesis of their own receptors
(feedback regulation)
•
Hormones might regulate their target genes at ___,
___ and____ levels
Hormones might regulate their target genes at transcriptional,
posttranscriptional and posttranslational levels
initiation usually involves….
initiation usually involves interactions of certain key proteins (the iniHaHon factors) with the 5’ mRNA cap. These proteins bind the small (40S) ribosomal subunit and hold the mRNA in place
Control point
Control point: forma4on
of ini4a4on complex
during translation
transcription
• The mature mRNA is used as a template for the assembly of
amino acids via tRNAs
• Translation of mRNA to protein starts at a specific site on
mRNA = start codon (usually AUG, encodes methionine)
• Translation will continue until a stop codon is reached
Initiation stage
: ribosome assembles around the mRNA, and
the first tRNA is attached at the start codon
Elongation stage:
peptide bond between methionine and
subsequent encoded amino acid, and the ribosome then
translocates three nucleotides along the mRNA
Termination stage
when a stop codon is reached, the
ribosome releases the polypeptide
Translation
= very energetically expensive, and is driven by
GTP and ATP hydrolysis
____ hormones are stored in &
secreted from ___ – therefore,
must be synthesized within the ____
Protein/peptide hormones are stored in &
secreted from secretory vesicles – therefore,
must be synthesized within the rough ER
First amino acids that are translated from the
mRNA template form a ___
First amino acids that are translated from the
mRNA template form a signal sequence
signal sequence causes ….
• This causes the nascent peptide chain to enter
a secretory pathway at the beginning of its
formation. The signal sequence bind to a signal recognition particle (SRP), which docks with an SRP receptor complex. A translocation channel transports the nascent peptide chain into the ER lumen; translation continues)
Most peptide hormones: ….
Most peptide hormones: synthesized as large precursors, which then undergo several
modifications during and/or after translation
• e.g., preprohormone (w/ signal sequence) → prohormone → mature hormone
After the transfer of ribosomes to the ____, the growing polypeptide chain enters the ____ as translation continues, and the ____ is rapidly cleaved enzymatically
• After the transfer of ribosomes to the ER membrane, the growing polypeptide chain enters the ER lumen as translation continues, and the signal sequence is rapidly cleaved enzymatically (in some cases, the polypeptide generated is the mature hormone; in other cases, a prohormone needing further processing)
Additional modifications that may occur during translation in the ER lumen..
• Additional modifications that may occur during translation in the ER lumen
(contributing to the stability and folding of the final peptide hormone product):
• Covalent addition of oligosaccharide side chains (glycosylation)
• Folding and assembly of proteins, and quality control
(mediated by molecular chaperones in the ER)
• Disulfide bond formation
Peptide/protein hormones – post-translational protein processing
Once ___ is terminated and proper folding and assembly have occurred, the
___ or ___ transferred to ____ (membranous regions)
Once translation is terminated and proper folding and assembly have occurred, the
hormone or prohormone is transferred to Golgi apparatus (membranous regions)
Peptide/protein hormones – post-translational protein processing
• In the ____ further processing may incl…..
• In the Golgi apparatus, further processing may incl.: proteolytic cleavage, glycosylation/modifications of carbohydrates, acylation, acetylation, phosphorylation
Peptide/protein hormones – post-translational protein processing
• Incorporated into ____ (membrane vesicles),
often as inactive ____:
cleavage by a ____ needed to
produce active hormone
• Incorporated into secretory granules (membrane vesicles), often as inactive prohormone: cleavage by a prohormone convertase (PC) needed to produce active hormone
insulin post-translational processing
Preproinsulin
Preproinsulin: 110-a.a.,
biologically inactive.
In ER.
insulin post-translational processing
Proinsulin:
Proinsulin: 86-a.a., biologically inactive. • In ER: folding, disulphide bonds • In Golgi apparatus: further modifications (glycosylation)
insulin post-translational processing
Insulin
Insulin: 51-a.a.,
biologically active as monomer.
In secretory granules.
glucagon/GLP-1 & GLP-2 post-translational processing
____ is processed in a
tissue-specific manner,
depending in part on the local
dominant ____
Proglucagon is processed in a tissue-specific manner, depending in part on the local dominant prohormone convertase (PC)
glucagon/GLP-1 & GLP-2 post-translational processing
• In pancreatic α cells ,___
is synthesizes
• In pancreatic α cells, glucagon is synthesized (“opposes” insulin: raises blood glucose)
glucagon/GLP-1 & GLP-2 post-translational processing
• ___ cells synthesize
glucagon-like peptides (GLP)
• GLP-1 is what?
• Enteroendocrine cells synthesize glucagon-like peptides (GLP) • GLP-1: a peptide hormone which amplifies glucosestimulated insulin secretion (among other effects)
Post-translational modifications in the ER and Golgi apparatus
regulates….
Post-translational modifications in the ER and Golgi apparatus
regulates the synthesis of (typically tissue-specific) hormone variants
Cytoskeletal protein-mediated migration of vesicles
Peptide hormones: released from secretory granules by exocytosis
• Cytoskeletal protein-mediated migration of vesicles toward cell surface,
vesicle (granule) fusion with the plasma membrane,
expulsion of contents (e.g., hormones) into the extracellular space
Sorting
: Specific proteins meant to be secreted are
concentrated into secretory granules at the far side of
the Golgi apparatus. Sorting receptors that ”gather”
specific vesicle cargo may help mediate this.
Budding
: Proteins that coat membrane-bound
transport vesicles gather and cause vesicle to break
free. These coat proteins mediate transfer of vesicles
within the cell.
Trafficking
vesicles moved via motor proteins along
cytoskeleton (e.g. microtubules); requires energy.
Docking/fusion
mediated by interactions with docking
proteins at the destination site
Exocytosis is regulated.
Often triggered by changes in calcium concentrations in the cytoplasm
that affects secretory granule fusion
• Not continuous secretion – instead, acutely, in response to a stimulus
• Allows for rapid release of lots of hormone, vs. if hormone needed to be
synthesized de novo in response to the secretory signal
• The stimulatory factors may include metabolites, other hormones or
neuropeptides (which are released in response to detection of a changed variable
that affects homeostasis, for instance), or direct nerve innervation
feedback loops
Input
Input: change in
extracellular
environment
or innervation
Only peripheral
endocrine glands
involved
(no CNS)
Tropic hormone
a hormone
that cause the release of
another hormone
pulsatile release
Hormone secretion = pulsatile release, may be important for physiological function
(rapid increase and subsequent decrease in hormone levels)
• Pulses may have variable frequency (from 4-30 min to 45-180 min) and
amplitude (e.g., up to 1000-fold changes for pituitary hormones)
- insulin release
secretion is pulsatile, and may be rhythmic
rhythmic changes
• Hormones may also show rhythmic changes – e.g., bursts every hour, every 24 h
(“circadian”) or less, or even longer
• Could be determined by environmental stimuli (e.g., light-dark cycle)
or an internal biological clock
secretion is pulsatile, and may be rhythmic
Adrenocorticotropic hormone (ACTH)
: Adrenocorticotropic hormone (ACTH) has a notable circadian rhythm,
characterized by high levels during early morning hours
ACTH
Adrenocorticotropic hormone (ACTH)
secretion is pulsatile, and may be rhythmic
gonadotropins
follicle-stimulating hormone (FSH) and luteinizing
hormone (LH) have peaks every ~30 days (during menstrual cycle)
Most peptide hormones: are …
soluble in aqueous
solutions (hydrophilic); therefore do not require
blood binding proteins for transport in the blood
stream
This makes them vulnerable to rapid degradation,
resulting in a relatively short plasma half-life and
duration of action
Hormones in plasma: either ___ or ___ e.g. blood binding proteins
Hormones in plasma: either free form
(= biologically active form) or bound to other
molecules, e.g. blood binding proteins
Blood binding proteins affect the controlled release
and stability of the pool of hormones
• Provides reservoir of hormone that exchanges
with free hormone fraction (making hormone
pool more stable, less dependent on
synthesis/release)
- More uniform/distant distribution
- Slowing hormone metabolism/breakdown
Not limited to hydrophobic hormones.
e.g., Growth Hormone and Insulin-like growth factor
1 are proteins with specific blood binding proteins.
Growth Hormone (GH)-Insulin-like growth factor 1 (IGF-1) Axis:
• Regulates linear growth (growth in
height) in children/adolescents
- GH
- IGF-1
• Growth Hormone (GH)-Insulin-like
growth factor 1 (IGF-1) Axis:
GH
191-a.a. polypeptide hormone synthesized by anterior pituitary; ~ half in circulation is bound to specific GH-binding proteins (reduce oscillations, prolong half-life)
Feedback regulation:
The bulk of acute adjustments
to hormone levels = changes to
hormone synthesis & secretion
key points of control regulation of synthesis/release (6)
Presence of regulators & interactions between regulators and
DNA regulatory elements influence the rate of gene transcription
Other control points: transport of RNA from the nucleus to the
cytoplasm, and the regulation of mRNA stability
Alternative splicing regulates the synthesis of
(typically tissue-specific) hormone or receptor variants
Control point: formation
of initiation complex
during translation
Post-translational modifications in the ER and Golgi apparatus
regulates the synthesis of (typically tissue-specific) hormone variants
Control of secretory granule release is an
important point of control for peptide hormones
Protein/peptide hormones are ___ after
being internalized by target cells (example) , or ___ by nontarget cells, or cleared/filtered by kidney. They
may also be ___by extracellular ___ . (example)
Protein/peptide hormones are degraded after
being internalized by target cells (e.g., receptormediated clearance, with hormone degraded in
intracellular lysosomes), or degraded by nontarget cells, or cleared/filtered by kidney. They
may also be degraded by extracellular proteases
(e.g., in the plasma).
Many peptide hormones: circulate in ____
un-bound to blood binding proteins (some
exceptions: ____). Therefore, relatively rapid
degradation/short duration of action
• Many peptide hormones: circulate in free form,
un-bound to blood binding proteins (some
exceptions: GH, IGF-1). Therefore, relatively rapid
degradation/short duration of action
___ of hormone clearance from circulation
varies, but half life of un-bound peptide
hormones is generally in the ___
• vs. ___ for catecholamines (e.g.,
norepinephrine, epinephrine), and ___ for steroid & thyroid hormones
Rates of hormone clearance from circulation
varies, but half life of un-bound peptide
hormones is generally in the order of minutes
• vs. seconds for catecholamines (e.g.,
norepinephrine, epinephrine), and
hours for steroid & thyroid hormones
____ (e.g., levels of
specific binding proteins or proteases) also
affects ____ levels
Control of hormone degradation (e.g., levels of
specific binding proteins or proteases) also
affects circulating hormone levels
GLP-1 and GLP-1 based treatments for type 2 diabeties
GLP-1: a peptide hormone
which amplifies glucosestimulated insulin secretion,
among other effects)
Example: drugs that inhibit the primary protease
that degrades GLP-1, leading to higher GLP-1 levels
Key points of control of hormone levels in circulation:
important for healthy physiological function ie…..
Key points of control of hormone levels in circulation:
important for healthy physiological function (i.e., they
allow hormone levels to respond to environmental &
internal changes, and thus carry out their function of
communicating & coordinating responses in other
cells in the body)
Key points of control of hormone levels in circulation:
important for healthy physiological function (i.e., they
allow hormone levels to respond to environmental &
internal changes, and thus carry out their function of
communicating & coordinating responses in other
cells in the body) BUT…..
these may also be control points that underlie
dysregulation of hormone levels, leading to
pathological effects.
Main types of cell surface receptors
for peptide hormones
• G-protein-coupled receptors
(or “7-transmembrane receptors”)
• Receptor-enzymes, including: • Growth factor receptors (have tyrosine kinase domain) • Guanylyl cyclase receptors (have guanylyl cyclase domain)
Type I cytokine receptors
(associated with an accessory protein
with a tyrosine kinase domain)
• Interactions between hormones and their receptors depends on:
- Number of receptors
- Affinity of the hormone for the receptor
- Concentra^on of circula^ng hormone
Regulation at the level of the receptors is also an important point of
endocrine function control,…
Increasing or decreasing receptor synthesis
• Internalization vs. cell membrane localization of cell surface receptors
• Desensitization of receptors (“uncoupled” from signal transduction pathway, due
to such mechanisms as phosphorylation of the receptor)
Cells with more receptors will have a \_\_\_\_ , at a given concentration of hormone. By the same principle, higher concentration of hormone will lead to \_\_\_. In both cases, this is only true up to a \_\_\_\_, where all available receptors are bound to the ligand)
Cells with more receptors will have a greater response (more [HR]), at a given concentration of hormone. By the same principle, higher concentration of hormone will lead to more [HR].
[HR] = concentration of hormone-receptor complex
(hormone bound to receptor)
In both cases, this is only true up to a
saturation point, where all available
receptors are bound to the ligand)
KD
KD is the equilibrium dissociation constant that
conventionally defines the affinity of the hormone-receptor binding.
KD is the concentration of free hormone [H]
at which half of available receptors are occupied.
The lower the KD, the higher the affinity of the receptor for the hormone.
Cells with \_\_\_ receptors will have a greater response, at a given concentration of hormone (up to a \_\_\_\_\_, where all available receptors are bound to the ligand)
Cells with higher-affinity receptors will have a greater response, at a given concentration of hormone (up to a saturation point, where all available receptors are bound to the ligand)
Iodocyanopindolol (ICYP)
Iodocyanopindolol (ICYP) is a β-adrenoceptor antagonist. Its [125I]-radiolabeled
derivative has been used to map the distribution of β-adrenoceptors in the body
