Hormones act as chemical messengers that bind to specific receptors and elicit responses in target cells. There are two main classes of hormones based on receptor type: Class 1 hormones like steroids and thyroid hormones act through intracellular receptors and directly influence gene expression. Class 2 hormones like peptides and catecholamines act through transmembrane receptors and induce intracellular second messengers to elicit a response. The mechanisms of action generally involve hormone binding, receptor activation, signal transduction through second messengers for Class 2, and modulation of gene expression for Class 1.

1. Hormone Action and Signal Transduction
part 1
DR. AHMAD SAAD
MD, PHD
LECTURER OF MEDICAL BIOCHEMISTRY & MOLECULAR BIOLOGY ,
FACULTY OF MEDICINE CU

2. Agenda
 Learning objectives
 Basic definitions
 Classification of Hormones
 Mechanism of action of class 1 hormones
 Mechanism of action of class 2 hormones

3. Learning objectives
 Explain what is meant by chemical messengers and signal transduction and signal
termination.
 Know the types of hormone receptors and their role in signal transduction.
 Classify the hormones according to the types of receptor.
 Recognize the general features of the two main classes of hormones.
 Outline the general signal transduction pathway of hormones of class I.

4. Basic Concepts
Hormones
Chemical Messenger
Receptors
Signal transduction
Signal termination

6. Hormones and Chemical Messenger
 Hormone:
 chemical messengers that invoke profound changes
within target cells
 Chemical messenger:
 Secreted from one cell in response to stimulus
 Travel to a target cell
 Bind to a specific receptor
 Elicit a response
Chemical messenger
Hormones

7. Endocrine
CNS
Immune
Hormones
Signaling
Molecules Neurotransmitters
Cytokines
Endocrine
Paracrine
Autocrine

8. Receptors
 Receptors:
 proteins that contain a binding site specific
for a single chemical messenger and another
binding site involved in transmitting the
message.
 Two types:
 Plasma membrane Receptors
 Intracellular Receptors

9. Signal Transduction
 Signal transduction:
 It is intracellular response.
 Two fundamental
mechanisms:
 Modulation of Gene Expression
 Activation of enzymes and other
dynamic molecules

10. Signal Termination
 After response to a signal, the signaling
process must be terminated
 If not >> the cell loses its responsiveness to
new signals
 Moreover >> signaling processes that fail to
terminate properly can lead to uncontrolled
cell growth

12. Classification of
hormones
according to the
type of receptor

16. Mechanism of
Action of
Group I Hormones

17. Receptor
 Group 1 hormones:
 steroid, thyroid hormones, calcitriol and retinoids
 Receptors:
 Steroid hormones act on cytoplasmic receptors
 Thyroid, retinoids act on nuclear recptors
 function as ligand dependent transcription factors.

18.  Structure of Intracellular
Receptors:
 single polypeptide chain
 The carboxy-terminus or ligand-binding
domain >> Binds to hormone
 DNA binding zinc finger domain >> Binds to
DNA (HRE)
 The amino-terminus >> interact with other
transcriptional mationary

19. Mechanism of Action of
Group I Hormones
Step 1: Dissociation of hormone from the circulatory transport protein
Step 2: Transport of the free hormone into cell cytosol or nucleus
Step 3: Binding of hormone to the un-activated nuclear or cytosolic receptor.
Step 4: Activation of hormone-receptor complex.
Step5: Transport of the cytosolic hormone-receptor complex to nucleus
Step 6: Activated hormone-receptor complex binds DNA (HRE}.
Step7: DNA transcribed into messenger RNA.
Step 8: Newly made proteins elicit biological response

20. Mechanism of Action
of Hormones with
Cell Surface
Receptors

21. Receptor
 Group 2 hormones:
 Protein and peptide hormones, catecholamines like epinephrine, and eicosanoids
such as prostaglandins.
 Receptors:
 Transmembrane protein >> span the cell membrane and thus haveboth
extracellular and intracellular components.
 generation of so-called second messengers within the cell (signal transduction).

22.  Structure of Cell Surface
Receptors:
 Transmembrane protein consists of 3 main
parts:
 Extracellular domains: hormone or ligand-
binding domain.
 Transmembrane domains: serve to anchor
the receptor in the membrane.
 Intracellular domains: lead to generation of
second messengers.

23. Mechanism of Action of Group II
Hormones
 Reception of the primary messenger >>
Binding of hormone to receptor
 Delivery of the Message Inside the Cell
by the Second Messenger
 Activation of effectors That directly alter
the physiological response
 Termination of the Signal

25.  Fate of hormone receptor
complex:
 Internalization cell surface
receptors
 then either destruction by
lysosomes or recycled