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Technical Discussion Hormones are chemical
messengers that invoke profound changes within target cells. How is
this accomplished? There are two fundamental mechanisms by which
such changes occur:
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Lay Interpretation
Here we examine the way a hormone actually makes
something else happen. |
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Activation of enzymes and other dynamic
molecules: Most enzymes shuttle between conformational
states that are catalytically active versus inactive, on
versus off. Many hormones affect their target cells by
inducing such transitions, usually causing an activation of one
of more enzymes. Because enzymes are catalytic and often serve
to activate additional enzymes, a seemingly small change induced
by hormone-receptor binding can lead to widespread consequences
within the cell.
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Enzymes are highly reactive chemicals, normally making a process
occur or bringing another process into commencement. Hormones
often trigger activation of one or more enzymes within a cell, thus
a limited number of receptor sites being activated can still bring
about huge changes. |
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Modulation of gene expression:
Stimulating transcription of a group of genes clearly can alter
a cell's phenotype by leading to a burst of synthesis of new
proteins. Similarly, if transcription of a group of previously
active genes is shut off, the corresponding proteins will soon
disappear from the cell.
More specifically, when a receptor becomes bound to
a hormone, it undergoes a conformational change which allows it to
interact productively with other components of the cells, leading
ultimately to an alteration in the physiologic state of the cell.
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A fascinating way hormones can act on a cell is genetic switching.
some hormones switch on and off parts of the genetic package of a
cell. This can radically alter the function and purpose of a
cell.
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Considerable information about a how a hormone
acts can be gained by knowing the type of receptor it uses.
Despite the molecular diversity of hormones, all hormone receptors
can be categorized into one of two types, based on their location
within the cell:
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Location of Receptor |
Classes of Hormones |
Principle Mechanism of
Action |
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Cell surface receptors (plasma
membrane) |
Proteins and peptides, catecholamines and
eicosanoids |
Generation of second messengers which
alter the activity of other molecules - usually enzymes -
within the cell |
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Intracellular receptors (cytoplasm
and/or nucleus) |
Steroids and thyroid hormones |
Alter transcriptional activity of responsive
genes |
Thus, if introduced to a new steroid hormone, one
can quickly deduce that it has an intracellular receptor and acts
upon its target cells by affecting transcription.
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With respect to the next two pages, all hormones except steroids and
thyroid hormones react to cell surface receptors and are usually
enzyme reactive. Steroids and Thyroid hormones act on the genetic
level and target receptors within a cell, rather than on its
surface. |
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