Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/ |
Chemical Signaling: A Primer
I.
Overview
A. General: Recall that cells/organisms require the
ability to relay internal messages from one cell or part of the individual to
another. Communication can occur via:
electrical signals/messages - quick; transient; mostly animals
chemical signals/messages - slow; longer-lasting; animals & plants
The function of these messages is to:
B. Response System/cascade
The generalized system by which animals and plants respond to
signals/messages can be diagrammed as follows:
signal → target tissue/cell → receptor → signal
amplification (transducing mechanism) → response
II.
Internal Chemical Signals (= hormones)
Hormones are defined as: organic compound,
active in low concentration, that is produced in one part of the cell/organism
and has it action in another. Thus, there is a separation of the site of
production and synthesis. The degree of separation varies: in
plants, usually little separation; in animals - in some cases there is little (autocrine),
minimal (paracrine) or lots (endocrine).
In animals - exocrine glands secrete hormones into the blood stream (e.g., hypothalamus, pituitary, kidneys, pancrease, ovaries, testis); endocrine glands secrete hormones in ducts other than circulatory system (pancreas, salivary glands)
III. Evidence for the existence of hormones
A. Plants - check out the Case Studies in phototropism and fruit set in soybeans
B. Animals - check out the secretin example in dogs (Case Study)
IV.
Criteria to establish that a chemical is a hormone (check out the
Case
Study)
Essentially, four criteria need to be met:
V.
Chemistry of Hormones
A variety of molecules are used as hormones.
These include:
VI.
Mechanism of Action
A. General
Hormones typically bind to receptors to initiate a response.
The receptors are usually proteins and can be located in the cell membrane (for
hydrophilic, water-soluble hormones) or somewhere in the interior of the cell
(lipid-soluble hormones like steroids). The receptor then initiates a
series of responses that amplifies the hormonal signal ultimately resulting in
an increase in transcription and/or translation and/or protein activation.
The amplification process often involves protein kinases that add/remove a
phosphate from an enzyme to activate/inactivate the enzyme (enzyme-inactive + Pi
→ enzyme-active). A general scheme includes:
hormone → receptor → amplification via protein kinases, G proteins, other → transcription, translation, enzyme activation, other → response
B. Example 1 - Estradiol
Estradiol is a steroidal sex hormone and readily crosses
membranes. Estradiol enters the nucleus and binds to a protein receptor
(we know it is a protein because when a cell extract is treated with proteases
there is no response) the activated protein/hormone complex binds to DNA at the
promoter region activating transcription and ultimately translation.
Proteins that bind to DNA typically have regions called zinc-fingers.
C. Example 2
The water-soluble hormones, like noradrenaline, typically
bind to a protein receptor embedded in the membrane (transmembrane protein).
The membrane protein then activates a cascade/amplification of events that can
either:
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Last updated: March 24, 2009 � Copyright by SG Saupe