GROWTH HORMONE
Victoria Brown
Structure of hormone
191 amino acids long
Protein structure 4 helices that
help it bind its receptor
2 strong sulfide bonds hold the structure together
Structure of receptor
Single-chain glycoprotein receptor 2 binding receptors for each molecule of
growth hormone: GHRI GHRII
The 2 receptor molecules bind to two structurally distinct sites on opposite sides of a single growth hormone molecule
Binding of the two receptors is sequential, not simultaneous
Where does it come from?
Synthesized and secreted by somatotroph cells (anterior pituitary) in response to growth-hormone releasing hormone (GHRH) from the hypothalamus Activates gene transcription by cyclic
adenosine monophosphate (cAMP) mechanisms
What happens in the cell?
The hormone binds on the outside of the cell, bringing two receptors together
The interaction of growth hormone with its receptor leads to activation of cytoplasmic tyrosine kinases, triggering several enzymatic reactions and signaling processes that stimulate growth
Growth hormone travels through the blood and stimulates the liver to produce a protein called insulin-like growth factor (IGF-1)
In children, IGF-1 stimulates chondrocytes to multiply in the cartilage at the ends of long bones (epiphyseal plate) This leads to growth in the length of the bones
and increases the child's height In adults, growth hormone plays an important
role inrepair and maintenance of the body’s tissues
IGF-1 also acts on immature muscle cells to increase muscle mass
Role in growth
Role in metabolism
Protein metabolism: Increased amino acid uptake Increased protein synthesis
Fat metabolism: Triglyceride breakdown in adipocytes (lipolysis)
Carbohydrate metabolism: Helps maintain blood glucose levels Suppresses insulin to prevent uptake of
glucose in peripheral tissues Glucose synthesis in the liver
(gluconeogenesis)
Primary Target tissues
Bone Muscle Fat
Regulation
2 hormones from the hypothalamus: Growth hormone-releasing hormone (GHRH)
Stimulates both the synthesis and secretion of growth hormone
Somatostatin (SS) Inhibits growth hormone release from somatotroph
cells 1 hormone from the stomach:
Ghrelin Binds to receptors on somatotroph cells and potently
stimulates secretion of growth hormone
Negative Feedback
IGF-I Directly suppresses the somatotroph cells Stimulates the release of somatostatin from
the hypothalamus Growth hormone
Directly suppresses the somatotroph cells Inhibits GHRH secretion
Feedback
Normal blood values
1 - 9 ng/mL (male) 1 - 16 ng/mL (female) Natural levels of growth hormone
fluctuate during the day, depending on: Stress Exercise Nutrition Sleep
Too much growth hormone?
Usually from a tumor on the pituitary
Begins before puberty Gigantism
Robert Wadlow At birth: 8.5 lbs 5 years old: 5’ 4” and
105 lbs Adult: 8’ 11” and 490 lbs
Begins after puberty Acromegaly
Enlarged face, hands, and feet
High blood pressure and heart disease
A number of metabolic derangements, including hyperglycemia
Too little growth hormone?
Begins before puberty: Dwarfism
Begins after puberty: Reduction of
muscle/bone strength and mass among other debilitating factors
“There are no great limits to growth because there are no limits of human intelligence, imagination, and wonder.”
—Ronald Reagan