Introduction 1 Flashcards
3 Types of Hormones
- protein/polypeptide
- steroid
- Miscellaneous
Protein hormone synthesis first Step
Synthesised as a prohormone that is cleaved to form the hormone
Prohormone for Adrenocorticotropic Hormone (ACTH)
Pro-opiomelanocortin (POMC)
Where are protein hormones synthesised?
by cortiocotroph cells. All anterior pituitary hormones are polypeptides
Describe Steps for Protein Hormone Synthesis
1) prohormone transcribed and mRNA binds to rER
2) Blood supply provides amono acids for translation
3) Prohormone endocytosed into Golgi apparatus
4) Golfi adds enzymes to vesicles for cleavage
5) vesicles with active hormone accumulate near the surface for exocytosis
6) when signal arrives, exocytosis occurs and active hormone e.g. ACTH is released
What are all steroid hormones derived from and where?
Cholesterol and from adrenal glands or gonads
Describe Delivery and storage of Cholesterol
Delivered to cells (e.g. adrenal cortical cells) as Low Density Lipoproteins (LDLs) and stored as Fatty Acid Esters that appear as Fat droplets
Describe synthesis of steroid hormones
1) fatty acid esters are broken down by esterase to access the cholesterol
2) Steroid hormones are made in the mitochondria
3) Cholesterol enters the Mitochondria by Steroidogenic Acute Regulatory Proteins (StAR Proteins). This is the rate limiting step
4) Enzymes in the mitochondria convert to the desired steroid hormone. This is cell dependent e.g. adrenal cortical cells produce cortisol
5) Cortisol secreted into the cytoplasm and diffuses straight to the blood stream
Major difference between polypeptide and steroid hormones.
Polypeptide hormones can be stored in vesicles for release from the cell whereas steroids are secreted immediately
Describe Protein hormone transport
- travel unbound to the target site due to storage in cells/tissues that produce them. As such, they have very short half lifes (usually a matter of minutes)
Describe Steroid hormone transport
- They are stored in blood so bind to a number of Plasma Proteins
- Albumin is a plasma protein that can bind to a number of steroid hormones through non-specific interactions
- There are other plasma proteins that bind more specifically/strongly e.g. Cortisol and Cortiocsteroid Binding Globulin (CBG)
- small amount of free steroid hormones in the blood
General Hormone Transport Ideal
Only free hormones are biologically active
- Thyroxine Binding Globulin (TBG) binds to thyroid hormones
- Sex Hormone Binding Globulin (SHBG) binds to oestrodiol and testosterone
Describe Hormone Equilibrium
There is an equilibrium between free hormones/plasma proteins to plasma protein bound hormones. This is adapted depending on needs of tissues
Describe Polypeptide Hormone Action e.g. ACTH
They are not lipid soluble and therefore have to bind to receptors
1) receptors usually G protein Receptors
2) binding of ACTH to ACTH receptor (in the adrenal gland) causes activation of adenylate cyclase
3) This causes an increase in cAMP production from ATP
4) This cAMP activates protein kinase
5) Therefore, esterase is phosphorylated allowing cholesterol liberation
6) StAR proteins are also activated during phosphorylation
Describe Steroid Hormone Action
- only free hormones can enter the cells
- very lipid soluble so can diffuse through the plasma membrane
- They bind to intracellular receptors
- They then translocate to the nucleus where they have huge effects on DNA transcription and subsequent translation
