U1v: Compare + contrast peptide & steroid hormones

Hormone = chemical messenger produced & secreted by an endocrine cell which circulates in the blood & acts on specific target cells

Example

Peptide

APG hormones (LH, FSH, ACTH, TSH, GH, PTH, Insulin)

Steroid

Cortisol, Aldosterone, O, P, Testosterone

Amines

Thyroxine

Catecholamines

Structure

Peptide

Vary from 3 α-acids to glycoproteins

Steroid

3 cyclohexal rings + 1 cyclopental ring

Amines

THYROID HORMONE = α double tyrosine with 4 iodine atoms

CATECHOLAMINES = derived from Tyrosine consists of a catechol. + amine group.

Precursor

Peptide

Prehormones

Steroid

Cholesterol

Amines

Tyrosine

Synthesis + Storage

Peptide

mRNA transcription → ribosomal translation → production of polypeptide in ER

Steroid

Cholesterol transported from cytoplasm → mitochondria (rate limiting step)

→ in mitochondria, cholesterol is converted to PREGNENOLONE (rate limiting step)

→ precursor for all steroid hormones

→ not stored/ immediately released

Amines

THYROID HORMONE

Thyroglobulin scaffold is source of Tyrosine (1 thyroglobulin = 134 tyrosine) + Iodine (taken up by thyroid cells)

→ THYROSINE PEROXIDASE iodinates the Tyrosine on Thyroglobulin

→ Thyroid hormone accumulates in colloid on surface of Thyroid epithelial cell

→ Lysosomes excise + release into blood

  • Amines → Catechol synthesis + storage
    • Tyrosine taken up by Chromaffin Cells in Medulla →
Tyrosine
  • Stored in electron dense granules with ATP
  • Preganglionic symp. fibres release Ach → activate chromaffins cells of adrenal medulla → release of catecholamines

Transport

Peptide

Dissolved in plasma → diffuse to target site

Steroid

No storage

Circulate bound to plasma proteins (this is their ‘reservoir’)

Amines

Catechols → released into blood + bound to proteins

* Free thyroid hormone enter plasma + bind to proteins for transport to target

Kinetics

Peptide

Bind receptor on cell surface of target site

→ Act via 2nd msg

→ Induce rapid response which is short-lived

Steroid

Bind receptor in cytosol/nucleus of target cell

→ bind DNA/modify transcription

→ Change is slower but more permanent

Amines

Catechols → act on catechol receptors

Β → ↑cAMP

α → activate the phosphatidylinositol system

Thyroid hormones → cross membrane

→ Binds nucleus

→ ↑gene transcription

→ alters cell metabolism

Secondary Messenger Systems

cAMP

    • Hormone binds receptor on cell membrane
    • Activates adenyl cyclase
    • Activated AC converts ATP → cAMP
    • cAMP activates intracellular protein kinases
    • Activated protein kinases phosphorylate proteins that then go on to mediate cellular reactions

Phosphatidylinositol (PIP) System

    • Hormone binds to receptor, causing:
      • Breakdown of membrane phospholipid phosphatidylinositol biphosphate (PIP2) by phospholipase C
      • PIP2 is broken into → IP3 + DAG
      • IP3 → mobilises Ca2+ from ER & mitochondria

→ Smooth m. contraction, cellular secretion, ciliary activity

    • DAG → activates Protein Kinase C → promotes cell division + proliferation

Calmodulin

    • Peptide hormones ↑intracellular Ca2+ (opening Ca2+ channel or activating IP3)
    • Ca2+ binds calmodulin
    • Ca2+ – calmodulin complex activates various enzymes i.e. activates Myosin Light Chain Kinase → phosphorylates myosin → smooth m. contraction