Peptide Hormone Mechanisms & Biosynthesis Flashcards
peptide/protein hormones are the….
most numerous type of hormone
many peptide hormones belong to____
families that share genetic and peptide structure homologies - essential for aspect of conformation and biological activity
where are many of the peptide hormones produced from?
hypothalamus, anterior pituitary, pancreas, nontraditional endocrines cells (ie GI tract)
where does hormone synthesis of peptide hormones take place?
the nucleus and cytoplasm of secretory cells
peptide hormone gene transcription
form precursors RNA molecule from DNA in the nucleus -> mediated by RNA polymerase II
what happens with peptide hormone precursor RNA transcript?
post transcriptional modifications in the nucleus -> excision of intron sequences
peptide hormone translation…
of mature mRNA into the encoded peptide chain
what happens to the peptide chains?
cotranslational and posttranslational modifcations
what does the start of the mRNA encode?
a signal sequence
what does the signal sequence do?
indicate that peptide hormone must be packaged for secretion, therefore it must be translated in the ER
what is produced first: active hormone, prohormone, prehormone, preprohormone
preprohormone first - has the signal peptide causing the protein to be translated in the ER
what happens after the signal peptide causes translation?
the signal sequence is then cleaved enzymatically to produce prohormone
describe the prohormone
not biologically active, may need to be processed/cleaved to produce biologically active hormone
most peptide hormones only require ________
transcription of a single gene -> ie human INS gene (insulin)
what are the exceptions? (where transcription of 2 genes is required)
- Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), Thyroid-stimulating hormone (TSH) and human chorionic gonadotropin (hCG)
- Heterodimers with alpha and beta subunits (encoded by two different genes) and carb side chains
describe heterodimers
have alpha and beta subunits encoded by two different genes on separate chromosomes
alpha subunit
encoded by same gene for all four hormones (FSH, LH, TSH, hCG)
beta subunit
unique for each hormone, gives biological specificity
-> designates the specific hormone
how are the subunits linked?
covalently
can also have ____ encode multiple hormones, or a hormone requiring _____
one gene, two different genes
-> E.g. same cell produces LH and FSH, which one is currently produced is determined by increased expression of LH gene vs. FSH gene.
genes
Consist of coding regions (exons + introns) and regulatory regions
coding regions
– exons containing nucleotide sequences that are conserved in mature mRNA + intron (intervening) sequences excised during posttranscriptional modification of RNA transcript in nucleus
- encode protein in question
promoter region
most important regulatory region, 5’ end of gene immediately upstream of first transcribed nucleotide; needed for accurate initiation and efficiency of transcription
whats included in the promoter region?
initiator element, regulatory elements, enhances/silencers
initiator element
- has transcription start site 25-30 nt downstream of TATA box
- required for correct initiation of transcription
regulatory elements
specific binding sites for various transcription factors
transcription factors
proteins that initiate, activate, or repress transcription
enhancers/silencers
regulatory regions - binding sites for activators or repressors, are usually further upstream or downstream of promoter
peptide hormone gene expression
- DNA: tightly packaged (e.g., histone proteins forming nucleosomes, which bundle together)
- To allow dynamic access to condensed DNA, chromatin remodeling alters this architecture
to expose or hide regions for transcriptional regulation.
what could gene expression of peptide hormones involve
- histone modifying enzymes (eg HATS, add acetyl group to histones, causing more open conformation - vs histone deacetylates, HDACs)
- ATP dependent chromatin remodeling complexes can move, eject or restructure nucleosomes to expose binding sites for transcription activators or repressors at gene promoters or enhancers
in the promoter region, the pre initiation complex forms….
- incl RNA polymerase II (enzyme that transcribes genes to mRNA), general TFs and other regulators (specific TFs, chromatin, remodeling complexes, etc)
once transcription is initated….
RNa polymerase II continues to elongation until a sequence is transcribed that signals cleavage and polyadenylation of RNA : 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) plus 5’ and 3’ UTRs (untranslated regions) ligated
- Mature mRNA is exported via nuclear pore
calcitonin
32-a.a. peptide hormone secreted by the thyroid gland. Reduces blood calcium, opposing the effects of parathyroid hormone
calcitonin gene related peptide
37-a.a. peptide primarily secreted in nervous system. Vasodilator, implicated in sensing pain
alternative splicing enables _____ of a hormone receptor
synthesizing variants
- eg there are two splice variants of the insulin receptor
T/F: protein and mRNA are degraded at variable rates (often fast for signaling molecules like hormones)
true, 5’ mRNa cap and length of 3’ poly a tail affect mRNa half life
regulation at the level of a hormones receptor is also an important…
point of endocrine function control
- eg increasing or decreasing receptor synthesis in target cells
hormone receptors (as well as specific blood binding proteins) are comprised of proteins…
therefore encoded by genes
- subject to the same types of regulatory control of their genes transcription/translation as peptide hormones
- some hormones affect the synthesis of their own receptors ( feed back regulation)
– hormones might regulate their target genes at transcriptional, post- transcriptional, and/or post-translational levels
stages of peptide hormone translation (mRNa to protein)
- initiation stage
- elongation stage
-termination stage
mature mRNA
template for assembling amino acids via tRNAs
translation of mRNA to protein starts at…
specific site on mRNa = start codon (usually AUG, encodes methionine)
translation will continue until…
a stop codon is reached
initiation stage
ribosome (small subunit first) and initiation factors assemble at 5’ mRNA cap, and scan for start codon; the first tRNA [with methionine] is attached at the start codon
elongation stage
peptide bond between methionine and subsequent amino acid, and assembled ribosome translocates three nucleotide along the mRNA
termination stage
when stop codon is reached, ribosomal subunits dissociate and releases the polypeptide and mRNA
translation =
energetically costly, driven by ATP/GTP hydrolysis
initiation usually involves…
interactions of certain key proteins ( the initiation factors) with 5’ mRNA cap. these proteins bind the small (40S) ribosomal subunit and hold the mRNA in place
peptide hormones are stored and secreted from
secretory vesicles - therefore, must be synthesized within the rough ER
first amino acids that are translated from the ___ template form a _____
mRNA, signal sequence
-> causes the nascent peptide chain to enter a secretory pathway. 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 are synthesized…
as large precursors, which then undergo several modifications during and/or after translation
-> eg preprohormone (w/ signal sequence) -> prohormone -> mature hormone
after the transfer of ___ to the ER membrane…
ribosomes, the growing polypeptide chain enters the ER lumen as translation continues, and the signal sequence is rapidly cleaved enzymatically
additional modifications that may occur during translation in 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)
- disulphide bond formation
once translation is terminated and proper folding and assembly have occurred…
the hormone or prohormone is transferred to golfi apparatus (membranous regions)
in the golgi further processing may incl…
proteolytic cleavage, glycosylation/modifications of carbohydrates, acylation, acetylation, phosphorylation
incorporated into _____, often as ______: cleavage by a prohormone convertase (PC) needed to produce ______
secretory granules (membrane vesicles), inactive prohormone, active hormone
proglucagon is processed in a
tissue specific manner, depending in part on the local dominant prohormone convertase (PC)
in pancreate alpha cells, glucagon is
synthesized (‘opposes insulin: raises blood glucose)
enteroendocrine cells synthesize
glucagon-like peptides (GLP)
-> GLP-1: a peptide hormone which amplifies glucose-stimulated insulin secretion (among other effects)
post-translational modifications in the ER and golgi apparatus:
can regulate bioactivity and or synthesis of different hormones
insulin post translational processing
preproinsulin -> proinsulin -> c-peptide + insulin
preproinsulin
110-a.a., biologically inactive. In ER.
proinsulin
86-a.a., biologically inactive.
- In ER:folding, disulphide bonds
- In Golgi apparatus:further modifications (glycosylation)
c-peptide
31 aa, cleaved in secretory granules
insulin
51 aa
- biologically active as monomer. in secretory granules
peptide hormones are 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 (eg hormones) into extracellular space
what happens during sorting of exocytosis
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 help mediate this.
what happens during budding of exocytosis
Proteins that coat membrane-bound transport vesicles gather and cause vesicle to break free. These coat proteins mediate transfer of vesicles within the cell.
what happens during trafficking
vesicles moved via motor proteins along cytoskeleton (e.g. microtubules); requires energy.
what does trafficking require
energy (ATP, also myosin required)
what happens during docking/fusion
mediated by interactions with docking proteins at the destination site
exocytosis is
regulated
-> is not continuous but rather acutely triggered in response to a stimulus
what is exocytosis triggered by
changes in calcium concentrations in the cytoplasm that affects secretory granule fusion
exocytosis results in ____ secretion…
acute, allowing for a rapid release of a large amount of hormones
what factors may stimulate exocytosis
metabolites, other hormones, or neuropeptides or direct nerve innervations
secretory granule release is
an important control point for peptide hormones
secretion of peptide hormones is
pulsatile, may be rhythmic
why is pulsatile release important
crucial for physiological function due to the fluctuating levels of hormones during secretion
how does the freq of pulses vary among diff peptide hormones
some exhibiting intervals of 4-30 minutes, while others may have longer intervals of 45-180 minutes
what about amplitude of pulsatile release?
it varies, for anterior pituitary hormones, it may undergo 1000-fold changes during secretion
what patterns may hormones exhibit in terms of rhythmic changes
rhythmic changes, such as bursts every hour, every ~24 hours (circadian), or even longer intervals
what factors could influence the rhythmic changes in hormone secretion?
by environmental stimuli, such as the light-dark cycle, or an internal biological clock
example of hormone with notable circadian rhythm
Adrenocorticotropic hormone (ACTH), which exhibits a circadian rhythm characterized by high levels during the early morning hours
what hormones, associated w the menstrual cycle have peaks approx every 30 days
Gonadotropins, specifically Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), have peaks every ~30 days during the menstrual cycle.
characteristics of peptide hormones in terms of solubility
most are soluble in aqueous solutions, making them hydrophilic
how do peptide hormones travel in blood stream
do not require blood binding proteins bcuz are soluble in aq solutions
what is consequence from solubility of peptide hormones
makes them vulnerable to rapid degradation, resulting in short plasma half life and duration of action
how do blood binding proteins influence the release and stability of hormones?
affect the controlled release and stability of the hormone pool, providing a reservoir and influencing solubility
what increases in hormone solubility important
blood binding proteins, benefit hydrophobic hormones like steroid hormones
what does growth hormone (GH)-insulin-like growth factor 1 (IGF-1) axis regulate
linear growth in children and adolescents
what is the structure of GH and where is it synthesized
191 aa polypeptide synthesized by anterior pituitary
-> approx half in circulation is bound to specific GH-binding proteins, reducing oscillations and prolonging half-life
what is the structure of IGF-1 and where is it synthsized
70 aa polypeptide, synthesized by many tissues, acting in a paracrine/autocrine manner
-> most circulating is produced by liver and its bioavailabity is determined by specific IGF binding proteins
protein/peptide hormones are often ____ after being _____ by target cells
degraded, internalized
-> eg receptor mediated clearance: hormone degraded in intracellular lysosomes)
how else can peptide hormones be degraded
by non target cells, or cleared/filtered by kidney
- also by extracellular proteases
many peptide hormones circulate in
free form (exceptions: GH, IGF-1), therefore rapid degradation/short duration of action
type 2 diabetes
Initially, failure of target cells to adequately respond to insulin; in later stages, insulin biosynthesis in β-cells is impaired
GLP-1
a peptide hormone which amplifies glucose- stimulated insulin secretion, among other effects
control points for hormone levels
important for healthy physiological function (changes in hormone levels due to environmental & internal changes mean that hormones to carry out their intended function of communicating/ causing appropriate responses within the body)
what happens when a hormone interacts w its specific receptor on a target cell
triggers cascade of biochemical rxns in the target cell, eventually modifying the cells function or activity
what are the main types of cell surface receptors for peptide hormones
- G-protein-coupled receptors
(or “7-transmembrane receptors”) - Receptor tyrosine kinases
(or “growth factor receptors;” have tyrosine kinase domain) - Cytokine receptors
(assoc. with an accessory protein with a tyrosine kinase domain) - Guanylyl cyclase receptors (have guanylyl cyclase domain)
interactions btwn hormones and their receptors depend on…
- number of receptors
- affinity of the hormone
- concentration of circulating hormone
regulation at the level of the receptor is also important point of endocrine function control…
- increasing or decreasing receptor synthesis
- internalization vs cell membrane localization of cell surface receptors
- desenitization of receptors (‘uncouple’ from signal transduction pathway due to such mechs as phosphorylation of receptor)
define these: [H], [R], [HR], k+1, k-1
[H] = concentration of free hormone
[R] = concentration of free receptor
[HR] = concentration of hormone-receptor complex
(hormone bound to receptor)
k+1 = rate constant for [HR] formation
k-1 = rate constant for [H] + [R] formation
cells with more receptors will have…
a greater response (more [HR]) at a given concentration of hormone
higher concentration of hormone will lead to more
[HR]
what is kd
equilibrium dissociation constant that defines the affinity of the hormone receptor binding
- lower kd the higher affinity of receptor for the hormone
what is 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.
