Hormone Synthesis and Action of Hormones Flashcards
Types of hormone
water soluble and lipid soluble?
Peptide/protein hormones are water soluble and made from large precursor molecules, called prehormones (so can’t transverse plasma membrane, have to use cell surface receptors)
Steroids and iodinated tyrosines (thyroid hormones) are lipid soluble and made from low molecular weight precursors (can transverse plasma membrane and bind to intracellular receptors)
Protein hormone synthesis
How is the protein made? (outlide the transcription and translation process)
What must first happen to dna to allow all of this?
How is secretory granules made?
What binds to a SRP? where does this bind and what happens to it? Now what?
What is pre-prohormone and pro-hormone?
1) Transcription of DNA to RNA
2) Post transcriptional processing, where the RNA is converted to mature RNA, introns excised, modification of 3’ and 5’ ends
3) Translation of mature RNA into protein using tRNA to transfer the amino acids
4) Post translational processing, cleavage of large pre-hormone, folding of proteins, addition of sugars (glycosylation)
As a reminder, transcription and translation work in essence by the codons being exposed by the unwinding and unfolding of the DNA, adenine will pair with thymine and cytosine will pair with guanine. RNA uses uracil instead of thymine. In transcription the RNA chain is built up by the RNA nuceleotides base pairing with the DNA. In translation the tRNA anticodon pairs with its codon on the mRNA and assembles the correct sequence of amino acids.
The protein being translated needs to be directed inside a cellular membrane so pieces of the membrane can be used to form secretory granules. This occurs by the following mechanism:
There is a signal peptide at the end (N-terminas) of the polypeptide being synthesised by the ribosome (first to be translated) and this binds to a SRP (signal recognition protein). The SRP binds to an SRP receptor on the rough endoplasmic reticulum. The signal sequence is passed through the ER membrane into the lumen of the ER through a protein channel.
The signal peptide sequence is rapidly removed by peptidases and protein synthesis continues.
So the signal peptide allows the protein to be encorporated into the lumen of the ER and subsequently the golgi apparatus.
The pre-prohormone is regarded as the signal sequence plus the prohormone. Then once the signal sequence has been cleaved off, we have the prohormone which consists of the hormone and a redundant sequence.
Synthesis of insulin (example):
Transcription to mRNA -> Excision of introns to messenger RNA -> Removal of signal sequence and formation of disulphide bonds in RER -> pro-preinsulin to proinsulin -> transfer to golgi apparatus, excision of C peptide (a redundant part of protein, between A chain and B chain) and packaging into secretory vesicles.
Then when signal is received to release insulin it is ready to be released.
Control of steroid synthesis from cholesterol
where is cholesterol made?
Where is cholesterol stored? What happens once it is stimulated and what stimulates it?
Where does it go and what does it do there?
What enzymes are involved in steroid synthesis and what genes are involved in synthesising them?
Cholesterol is made in the liver as well as being incorporated into ones diet.
Cholesterol is taken up and stored in lipid stores in steroid synthesising cells (bound to sterol carrier protein). Upon stimulation by a trophic hormone, a cascade will occur including protein kinase A. This will then cause the StAR protein to become active which transports cholesterol to the inner mitochondrial membrane.
Inside the mitochondria there are enzymes to cleave parts of the cholesterol molecule to produce the steroid. Cholesterol can be turned into prognenolone (precursor for many steroid molecules) by the side chain cleavage enzyme P450scc (rate limiting step).
Between the mitochondria and smooth endoplasmic reticulum steroids are synthesised by hydroxylase enzymes.
Each step involves cleavage of the previous molecule.
It is important to know that many enzymes such as dehydrogenases or P450 enzymes are involved in steroid synthesis. These enzymes are synthesised from various CYP genes.
Abnormalities in Steroid Synthesis
What happens if you are deficient in aromatase in men?
What happens to females and males and why?
If an individual is deficient in aromatase it means they cannot synthesis oestrogens from androgens. In men this results in no epiphyseal closure (bone not being regulated) resulting in long stature.
In females they will develop male-type characteristics and boys show early sexual development due to the excess of androgens (pre-cocious puberty). This can be due to problems/mutations in CYP genes.
o Virilisation of XX fetuses
o Clitoromegaly = ambiguous genitalia
Synthesis of Thyroid Hormones
what are they derived from?
What is actively taken up? what happens to this and what is the enzyme involved?
How is the thyroglobulin polypeptide involved and where does this process take placed?
Where is this stored?
How is it released and secreted?
The thyroid hormones are derived from iodinated tyrosine
There first is active uptake of iodide into a follicular cell (in thyroid gland)
There is oxidation of iodide to iodine by thyroid peroxidase (TPO)
There is then iodination of tyrosine residues on the thyroglobulin polypeptide (the protein that tyrosine molecules are incorporated into, it allows the iodination) at the apical-colloid interface
This is stored in a colloid
Finally there is uptake of thyroglobulin droplets into the follicle cell where they can subsequently be released and secreted as T3 and T4 hormones which is stimulated by TSH
Disorder of thyroid hormones
Antibodies to the TSH receptor can act on the thyroid gland and stimulate excess thyroid hormones and can cause an eye disease, called Graves’ disease.
Hormone receptors and cell signalling
How do peptide/protein hormone act? What do they activate?
How do steroid hormones act? What do they activate?
As stated at the beginning, peptide and protein hormones are water soluble so cannot transverse the phospholipid bilayer, so they bind to cell surface receptors. This activates secondary messengers/enzymes in the cell and results in cytoplasmic and nuclear effects.
Steroid hormones are lipophilic and so can transverse the phospholipid bilayer and will bind to intracellular receptors in the cytoplasm or nucleus. Pretty much all of the steroid receptors are transcription factors (which are proteins that bind to DNA and initiate transcription).
Cell surface receptors for protein and peptide hormones
What are the two main types of receptors? What do they both have in common in terms of structure?
How does a G-coupled receptor work?
How does a phosphoinositide pathway work?
RAF/MEK/ERK1/2 signalling pathway - what is it involved in? what does it do?
Phosphatidylinositol kinase (PI-3)/AKT signalling pathway - involved in?
JAK/STAT signalling pathway - what does it do?
These are either G-protein coupled receptors or receptors with or associated with tyrosine kinase domains (which will phosphorylate secondary messengers).
Both these receptors will be composed of an extracellular part to bind the ligand and an intracellular part which will involve a secondary messenger cascade.
There are 5 main different cell signaling pathways for protein and peptide hormones that are intra-cellular—
G-protein coupled:
Adenyl cyclase and cAMP signaling pathway = The ligand binds and it causes the alpha subunit to bind GTP instead of GDP, the alpha subunit detaches and binds to adenyl cyclase which then uses ATP to produce cAMP. cAMP then activates PKA which then phosphorylates a binding protein that initiates transcription.
Phosphoinositide signalling pathway = Similar mechanism, with downstream signalling, releasing calcium from intracellular stores, PKC is also activated which activates cytosolic enzymes associated with the phosphorylation of transcription factors
Tyrosine receptor:
RAF/MEK/ERK1/2 signalling pathway = Involved in cell growth, ligand binds, and downstream phosphorylation, ERK 1/2 can go into nucleus and activate transcription
Phosphatidylinositol kinase (PI-3)/AKT signalling pathway = Involved in proteins synthesis
JAK/STAT signalling pathway = Also activates transcription through downstream phosphorylation
Second messengers in G-protein linked receptors (again)
Adenyl cyclase -> cAMP -> protein kinase A
Phospholipase c -> PIP2 -> DAG -> Protein kinase C
PIP2 -> IP3 -> Ca2+
Kinases activate enzymes or can activate transcription factors
It is important to know about receptors and their signalling pathways in order to understand endocrine disorders and to provide targets for the development of new drugs.
Endocrine disorders associated with mutations in the receptor or the associated G protein
What does activation of g coupled protein lead to?
What is testotoxicosis?
What is McCune Albright syndrome?
A defective G-protein coupled receptor could lead to:
Thyroid adenoma due to TSH receptor mutation (activating mutation, meaning overactivated receptor)
Precocious puberty due to LH receptor (activating mutation)
Hypergonadotrophic hypogonadism (inactivating mutation of LH receptor)
Defective G-protein could lead to:
Pseudohypoparaythyroidism due to PTH receptor (inactivating mutation)
McCune Albright syndrome (activating mutations)
Combined precocious puberty and hypoparathyroidism due to LH receptor (activating) and PTH receptor (inactivating)
Activating mutation of LH receptor (also a G-protein coupled receptor) can lead to testotoxicosis, high levels of testosterone will stimulate penile growth and growth of pubic hair in young children.
Steroid Hormone Receptors
What are steroid hromones?
What is the c domain of the receptor?
How manu domains are there?
Which domains have transcriptional activity?
Basic structure of steroid hromones? What do you need to allow it to lock into the DNA?
What are transcriptional factors normally bound to in the cytoplasm? How are they displaced and why? What happens next and where does it bind?
If receptors not located in the cytoplasm, where are they located? What are bound to these receptors to keep them inactive? What happens after steroid hormones bind to these receptors?
Steroid hormone receptors are a family of transcription factors (proteins which bind to the DNA and sit in nucleus or cytosol usually in an inactive state until activated).
There are different functional regions of the receptor and these are defined as domains A-F. The C domain is the DNA binding region and is highly conserved.
Both the A/B domains and E/F domains have transcriptional activity.
Different steroid receptors are continually being discovered.
Basic Structure of steroid receptors:
Zinc fingers which are part of the DNA binding domain (domain C) of the transcription factor (i.e. the steroid receptor), allow it to lock into the DNA.
The C domain – The DNA binding region is made up of two zinc fingers which can slot into the helix of the DNA
Series of events in steroid cell signalling—
Hormone crosses cell membrane
Generally the transcription factors (steroid receptors) are found in an inactive state in the cytoplasm. They are bound to heat shock proteins (acting as chaperone protein). This can be displaced when the hormone enters the cytoplasm as the hormone has a higher affinity for it.
Dimerization occurs.
The dimer receptor (hormone-receptor complex) translocates to the nucleus where it binds to the hormone response element of the DNA.
Along with other transcription factors being involved, transcription is initiated
Protein synthesis occurs and target protein produced
Note some receptors are located within the nucleus not the cytoplasm. These would follow a similar pattern in that they remain in an inactive state until stimulated.
Steroids (ligands) will bind to receptors (nuclear receptors), which will usually be kept inactive by represser proteins. When binding occurs, these repressor proteins are released and along with other co-activators and co-factors. The chromatin will be opened up and allow for gene transcription.
Metabolic syndrome
4 features of metabolic syndrome
causes?
o Central obesity – waist circumference over 100cm in men, 88cm in women
o Dyslipidaemia – increased triglycerides (over 1.6nM) and less HDLs (below 0.9M in males and 1M in females)
o Insulin resistance – fasting plasma glucose above 6.1M
o Hypertension – BP greater than 140/90mmHg
• Visceral adiposity
o Fat in abdominal region associated with greater health risks than those in peripheral regions
o Causes of metabolic syndrome:
- Ageing
- Lifestyle
- Genetics
Diabetes
the two different types
o Type 1 – autoimmune destruction of pancreatic islets leading to an absolute insulin deficiency
o Type 2 – insulin resistance leading to partial loss of insulin production (insulinopaenia), often associated with OBESITY
