Receptor For Steroid Hormones

Intracellular receptors or intra nuclear receptors have several functional domains.

1. Site for binding of hormone

2. Site that bind to specific DNA binding region

3. Site that concern to activates or repress the gene transcription on DNA

4. High affinity binding to DNA

The Receptor For Water Soluble Hormone-Cellular surface receptors

Classified into three types:
1. Ion- channel linked receptors → Transmitter gated ion channels Eg.Ach, Na+,Ca+

2. G- Protein linked receptors → The interaction between the receptor and target protein is mediated by third protein called G Protein

3. Enzyme linked receptors → When activated, either function directly as enzyme or associated with enzymes.

Membrane Receptors & Nuclear Receptors

1. Imbedded in target cell membrane; integral proteins/ glycoproteins; penetrate through membrane.

2. For protein & charged hormones (peptides or neurotransmitters)

3. 3 major groups: Serpentine = 7 transmembrane domains, Growth factor/cytokine = 1 transmembrane domain, Ion channels.

4. Nuclear Receptors
Nuclear proteins that usually act in pairs & bind to specific Hormone Recognition Elements (HREs) = sequences on the DNA in the promoter regions of target genes.
For small, hydrophobic molecules (steroids, thyroid hormones)

What is a hormone receptor?

1. Hormone Receptors are cellular proteins that bind with high affinity to hormones & are altered in shape & function by binding; they exist in limited numbers.

2. Binding to hormone is non-covalent & reversible.

3. Hormone binding will alter binding to other cellular proteins & may activate any receptor protein enzyme actions.

RECEPTORS

1. The first step in action of hormone is its binding to a specific molecules called receptors. All are protein in nature.


2. There are two types of receptors:

a) The receptor for water soluble hormone- Cell surface receptors
b) The receptor for fat soluble hormones – Intracellular receptors.

CLASSIFICATION OF HORMONE BY GENERAL MECHANISM ACTION

1. Hormones that bind to intracellular receptors.


2. Hormones that bind to cell surface receptors

a) Second messenger is c AMP
Eg. ACTH, ADH, PTH

b) Second messenger is c GMP Eg. ANP,NO

c) Second messenger is calcium or phosphoinocitol or both Eg.Ach, α ADRENERGIC,TRH

d) Second messenger is a kinase or phosphatase cascade Eg. GROWTH FACTORS,INSULIN

Factors influencing hormone production

There are many factors that influence hormone levels:

1. Time of day (cortisol - diurnal variation)

2. Time of month (LH/FSH/estradiol in menstrual cycle)

3. Time of year (melatonin levels higher in winter)

4. Age (T3 lower in elderly subjects, sex steroid levels lower in pre pubertal children)

5. Sex (testosterone/estradiol levels in men and women)

6. Stress (cortisol, adrenaline increase during stress )

7. Diet (insulin , growth hormone, cortisol change in fasting/feeding states)

8. Illness (thyroid function tests in non thyroidal illness)

9. Drugs (amiodarone/contraceptive pill effect TFT results)