Hormones are derived from amino acids or lipids. Amine hormones originate from the amino acids tryptophan or tyrosine. Larger amino acid hormones include peptides and protein hormones. Steroid hormones are derived from cholesterol.
Steroid hormones and thyroid hormone are lipid soluble. All other amino acid–derived hormones are water soluble. Hydrophobic hormones are able to diffuse through the membrane and interact with an intracellular receptor. In contrast, hydrophilic hormones must interact with cell membrane receptors. These are typically associated with a G protein, which becomes activated when the hormone binds the receptor. This initiates a signaling cascade that involves a second messenger, such as cyclic adenosine monophosphate (cAMP). Second messenger systems greatly amplify the hormone signal, creating a broader, more efficient, and faster response.
Hormones are released upon stimulation that is of either chemical or neural origin. Regulation of hormone release is primarily achieved through negative feedback. Various stimuli may cause the release of hormones, but there are three major types. Humoral stimuli are changes in ion or nutrient levels in the blood. Hormonal stimuli are changes in hormone levels that initiate or inhibit the secretion of another hormone. Finally, a neural stimulus occurs when a nerve impulse prompts the secretion or inhibition of a hormone.
adenylyl cyclase
membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation
cyclic adenosine monophosphate (cAMP)
second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade
diacylglycerol (DAG)
molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade
downregulation
decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone
first messenger
hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system
G protein
protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone–receptor binding
hormone receptor
protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response
inositol triphosphate (IP3)
molecule that initiates the release of calcium ions from intracellular stores
phosphodiesterase (PDE)
cytosolic enzyme that deactivates and degrades cAMP
phosphorylation cascade
signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein
protein kinase
enzyme that initiates a phosphorylation cascade upon activation
second messenger
molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein
upregulation
increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone
1. A newly developed pesticide has been observed to bind to an intracellular hormone receptor. If ingested, residue from this pesticide could disrupt levels of ________.
A) melatonin
B) thyroid hormone
C) growth hormone
D) insulin
B
2. A small molecule binds to a G protein, preventing its activation. What direct effect will this have on signaling that involves cAMP?
A) The hormone will not be able to bind to the hormone receptor.
B) Adenylyl cyclase will not be activated.
C) Excessive quantities of cAMP will be produced.
D) The phosphorylation cascade will be initiated.
B
3. A student is in a car accident, and although not hurt, immediately experiences pupil dilation, increased heart rate, and rapid breathing. What type of endocrine system stimulus did the student receive?
A) humoral
B) hormonal
C) neural
D) positive feedback
C
1. Compare and contrast the signaling events involved with the second messengers cAMP and IP3.
In both cAMP and IP3–calcium signaling, a hormone binds to a cell membrane hormone receptor that is coupled to a G protein. The G protein becomes activated when the hormone binds. In the case of cAMP signaling, the activated G protein activates adenylyl cyclase, which causes ATP to be converted to cAMP. This second messenger can then initiate other signaling events, such as a phosphorylation cascade. In the case of IP3–calcium signaling, the activated G protein activates phospholipase C, which cleaves a membrane phospholipid compound into DAG and IP3. IP3 causes the release of calcium, another second messenger, from intracellular stores. This causes further signaling events.
2. Describe the mechanism of hormone response resulting from the binding of a hormone with an intracellular receptor.
An intracellular hormone receptor is located within the cell. A hydrophobic hormone diffuses through the cell membrane and binds to the intracellular hormone receptor, which may be in the cytosol or in the cell nucleus. This hormone–receptor complex binds to a segment of DNA. This initiates the transcription of a target gene, the end result of which is protein assembly and the hormonal response.