Hormone Synthesis and Structure

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Questions and Answers

What is the precursor molecule for the synthesis of steroid hormones?

Cholesterol (correct)

Amino acids

Tyrosine

Glucose

Where are thyroid hormones stored?

Hypothalamus

Pituitary gland

Thyroid gland (correct)

Adrenal medulla

What is the mechanism of hormone secretion from the adrenal medulla?

Endocytosis

Passive transport

Active transport

Exocytosis (correct)

What is the approximate time frame for the full effect of thyroxine and growth hormones?

Months Signup and view all the answers

What is the unit of measurement for the rates of hormones production in an average day?

Micrograms or milligrams Signup and view all the answers

Which of the following hormones is produced in the adrenal medulla?

Epinephrine and norepinephrine Signup and view all the answers

What is the source of amine hormones?

Tyrosine Signup and view all the answers

What is the primary release site of neurotransmitters?

Synaptic junctions Signup and view all the answers

What is the main characteristic of paracrine secretion?

Affects neighboring cells of different types Signup and view all the answers

What is the classification of hormones secreted by the thyroid gland?

Derivatives of the amino acid tyrosine Signup and view all the answers

What is the function of cytokines?

Can work as autocrine, paracrine, and endocrine Signup and view all the answers

What is the characteristic of polypeptide and protein hormones?

Stored in secretory vesicles until needed Signup and view all the answers

How many general classes of hormones exist?

3 Signup and view all the answers

What distinguishes polypeptides from proteins?

Polypeptides have <100 amino acids Signup and view all the answers

What is the primary purpose of negative feedback mechanisms in hormone secretion?

To prevent overactivity or oversecretion of hormones Signup and view all the answers

What type of feedback mechanism is used by Lutenizing hormones to release LH during ovulation?

Positive feedback Signup and view all the answers

What is the term used to describe the periodic variation of hormone release throughout the day?

Circadian rhythm Signup and view all the answers

What is the duration of the cyclical variation of hormone release in females?

28 days Signup and view all the answers

What is the purpose of the surge of hormones during ovulation?

To stimulate ovulation Signup and view all the answers

What is the result of the negative feedback mechanism during ovulation?

Regulation of LH levels Signup and view all the answers

Why are thyroxine and steroid hormones not freely moving in the blood?

They are bound to plasma proteins Signup and view all the answers

What is the purpose of binding hormones to plasma proteins?

To prevent their clearance from the plasma Signup and view all the answers

What is the rate of removal of hormone in the blood referred to as?

Metabolic clearance rate Signup and view all the answers

How is the metabolic clearance rate of a hormone calculated?

By injecting a radioactive substance into the blood stream and measuring the rate of removal Signup and view all the answers

What is the unit of measurement for the metabolic clearance rate of a hormone?

Nanograms per milliliter Signup and view all the answers

What happens to hormones that are freely moving in the blood?

They are cleared through the liver into bile or kidney to urine Signup and view all the answers

What is the consequence of organs malfunctioning in the clearance of hormones?

An accumulation of hormones in the blood Signup and view all the answers

Why are hormones released from plasma proteins when they reach their target tissues?

Because they need to be in their free form to interact with target tissues Signup and view all the answers

Where are peptide hormone receptors typically found?

On the surface cell membrane Signup and view all the answers

What is the main function of hormone receptors?

To bind specific hormones to target tissues Signup and view all the answers

What happens to protein receptors every minute?

They are inactivated or destroyed Signup and view all the answers

Which type of receptors open or close channels for ions such as sodium, potassium, and calcium?

Ion channel–linked receptors Signup and view all the answers

Where are steroid hormone receptors typically found?

In the cytoplasm Signup and view all the answers

What is the role of ion channel–linked receptors in hormone action?

They open or close ion channels Signup and view all the answers

What is the effect of hormone secretion on protein receptors?

They are inactivated or destroyed Signup and view all the answers

What is the location of thyroid hormone receptors?

In the nucleus Signup and view all the answers

Why do most hormones open or close ion channels indirectly?

Because they couple with G protein–linked or enzyme-linked receptors Signup and view all the answers

Where do steroid hormones bind with specific receptor proteins?

In the cytoplasm of the cell Signup and view all the answers

What is the ultimate result of the sequence of events in steroid hormone function?

Increased protein synthesis Signup and view all the answers

How do G-protein linked hormone receptors regulate the activity of target proteins?

By coupling with G-proteins Signup and view all the answers

What is the characteristic of the action of steroid hormones?

Delayed and prolonged Signup and view all the answers

What is the role of the receptor protein in steroid hormone function?

To bind to the hormone and initiate the sequence of events Signup and view all the answers

What is the site of action of aldosterone?

Renal tubular cells Signup and view all the answers

What is the result of the combination of the receptor protein and hormone binding to DNA?

Activation of the transcription process Signup and view all the answers

Where do G-proteins bind to in the cell membrane?

On the inner surface of the cell membrane Signup and view all the answers

What is the ultimate function of the proteins synthesized in response to steroid hormones?

To act as enzymes or transport proteins Signup and view all the answers

What is the difference between the actions of peptide and amino acid–derived hormones and steroid hormones?

The speed of action Signup and view all the answers

Study Notes

Steroid Hormones

Steroid hormones are synthesized from cholesterol and are not stored.
They have a similar structure to cholesterol.
They are produced from plasma.

Amine Hormones

Amine hormones are derived from tyrosine.
Thyroid and adrenal medullary hormones are formed from tyrosine.

Thyroid Hormones

Thyroid hormones are stored in the thyroid gland.
They are formed by the actions of enzymes in cytoplasmic compartments of glandular cells.
Hormones are secreted from thyroglobulin, released into the blood, and combine with plasma proteins to reach target areas.

Adrenal Medulla Hormones

Adrenal medulla forms epinephrine (four times more) and norepinephrine.
Catecholamines are stored in vesicles, waiting to be released.
Proteins hormones are stored in secretory granules.

Hormone Secretion and Clearance

Hormones are secreted in response to a stimulus.
The duration of action varies among different hormones.
Some hormones, like norepinephrine and epinephrine, take effect in seconds to a minute after gland stimulation.
Hormones like thyroxine and growth hormones take months to achieve full effect.

Hormone Concentrations and Secretion Rates

Hormone concentrations in the circulating blood are extremely small, measured in micrograms or milligrams.
Hormone production rates are also very small, with a daily average production rate of micrograms or milligrams.

Hormone Secretion and Classification

Neurotransmitters are released by axon terminals of neurons into synaptic junctions.
Endocrine hormones are secreted by glands directly into the circulating blood.
Neuroendocrine hormones are secreted by neurons into the circulating blood.
Paracrine secretion involves cells releasing substances into extracellular fluid, affecting neighboring cells of different types.
Autocrine secretion involves cells releasing substances into extracellular fluid, affecting the function of the cell that produces them.
Cytokines are secreted by cells into extracellular fluid and can work as autocrine, paracrine, and endocrine hormones.

Hormone Classification

Three general classes of hormones exist: proteins and polypeptides, steroids, and derivatives of the amino acid tyrosine.
Proteins and polypeptides are secreted by: • Posterior and anterior pituitary gland • Pancreas (insulin and glucagon) • Parathyroid gland (parathyroid hormone) • Other glands
Steroids are secreted by: • Adrenal cortex (cortisol and aldosterone) • Ovaries and placenta (estrogen and progesterone)
Derivatives of the amino acid tyrosine are secreted by: • Thyroid (thyroxine and triiodothyronine)

Hormone Storage

Polypeptide and protein hormones are stored in secretory vesicles until needed.
Polypeptides with >100 amino acids are classified as proteins.

Feedback Control of Hormone Secretion

Negative feedback mechanism prevents overactivity of hormone systems by suppressing further production of hormones to prevent oversecretion.
Negative feedback can work on target tissues rather than secretion rates.

Role of Negative Feedback in Hormone Regulation

Prevents over secretion of hormones, such as LH (Lutenizing hormone) during ovulation in females.
Acts as a counterbalance to positive feedback mechanisms, which can cause surges of hormones.

Cyclical Variations in Hormone Release

Hormone release follows a cyclical pattern, influenced by circadian clocks (daily rhythms).
Examples of cyclical variations:
- Growth hormone production is higher during early stages of sleep and decreases towards the end of sleep.
- The 28-day ovulation cycle in females, preparing the body for breeding.

Positive Feedback Mechanism

Can cause surges of hormones, such as LH in ovulation.
Works in conjunction with negative feedback to regulate hormone release.

Transport of Hormones in the Blood

Thyroxine and steroid hormones are not freely moving in the blood because they are bound to plasma proteins.
Binding to plasma proteins is necessary for clearance of hormones from plasma.
When hormones reach their target tissues from capillaries, they are released from plasma proteins.

Clearance of Hormones from the Blood

Clearance of hormones is influenced by two factors:
- Rate of hormone production in the blood
- Rate of removal of hormone in the blood (metabolic clearance rate)
Metabolic clearance rate is calculated by dividing the rate of plasma removal in the blood by the plasma concentration of the hormone.
Units of metabolic clearance rate are typically nanograms per milliliter.

Measuring Metabolic Clearance Rate

To calculate metabolic clearance rate, a purified solution of hormone is tagged with a radioactive substance and slowly injected into the bloodstream at a steady rate.
When the plasma concentration of the hormone reaches a steady state, the rate of plasma removal is divided by the plasma concentration of the hormone to calculate the metabolic clearance rate.

Fate of Hormones in the Blood

Hormones are cleared from the blood through various routes, including:
- Liver: hormones are excreted into bile
- Kidney: hormones are excreted into urine
- Tissue binding: hormones bind to specific tissues
- Destruction: hormones are broken down in tissues
Malfunction of these organs can lead to accumulation of hormones, which can have negative consequences.

Hormone Receptors and Their Activation

Hormone receptors are specific to binding hormones and are found in three locations:
- Surface cell membrane (proteins, catecholamines, peptide hormones)
- Cytoplasm (many steroid hormones)
- Nucleus (thyroid hormones)
Number and sensitivity of hormone receptors are regulated through:
- Inactivation or destruction of protein receptors every minute
- Hormone secretions to high levels

Intracellular Signaling After Hormone Receptor Activation

Hormones require their receptors to affect target tissues
Ion channel-linked receptors:
- Combine with receptors in the postsynaptic membrane, causing a change in receptor structure
- Open or close channels for specific ions (e.g., sodium, potassium, calcium)
- Altered ion movement causes subsequent effects on postsynaptic cells
Most hormones that open or close ion channels do so indirectly through:
- G protein–linked receptors
- Enzyme-linked receptors

G-Protein Linked Hormone Receptors

Hormones activate receptors that indirectly regulate activity of target proteins by coupling with G proteins.
G proteins are groups of cell membrane proteins called heterotrimeric guanosine triphosphate (GTP)-binding proteins.

Steroid Hormones and Protein Synthesis

Steroid hormones cause protein synthesis in target cells.
Proteins produced can function as enzymes, transport proteins, or structural proteins.
The sequence of events in steroid hormone function:
- Steroid hormone diffuses across the cell membrane and binds with a specific receptor protein in the cytoplasm.
- The combined receptor protein-hormone diffuses into or is transported into the nucleus.
- The combination binds to specific points on DNA strands in chromosomes, activating transcription process of specific genes to form mRNA.
- mRNA diffuses into the cytoplasm, promoting translation process at ribosomes to form new proteins.

Example of Steroid Hormone Function: Aldosterone

Aldosterone is a hormone secreted by the adrenal cortex that binds with the mineralocorticoid receptor in renal tubular cells.
The sequence of events cited above ensues, leading to the production of proteins that promote sodium reabsorption from the tubules and potassium secretion into the tubules.
The full action of the steroid hormone is characteristically delayed for at least 45 minutes, with effects lasting several hours or even days.

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Description

This quiz covers the synthesis and structure of steroid and amine hormones, including their derivation from cholesterol and tyrosine, and their storage and release mechanisms.