CASE FILES: BIOCHEMISTRY
Figure 7-1. The hormonal-signaling process.
Hormones can be grouped into three main classifications based on their
chemical structure and how they are synthesized. Peptide hormones include
the polypeptide hormones insulin and glucagon, as well as the smaller pep-
tides such as thyrotropin-releasing hormone (TRH) and the enkephalins. Some
peptide hormones, like thyrotropin (TSH) are glycoproteins. Amino acid-
derived hormones are synthesized from amino acid precursors. The cate-
cholamines and serotonin are included in this grouping, as are the thyroid
hormones, T3 and T4. The third classification is the steroid and steroid-like
hormones, which includes the progestogens, corticosteroids, androgens,
estrogens, and calcitriol (the active form of vitamin D).
The cell-associated receptors are the molecular entities primarily respon-
sible for recognizing the hormonal signal. When the hormone binds to the
receptor, the hormone-receptor complex initiates the events that culminate in
the cellular response. These cell-associated hormone receptors can be classi-
fied into two main categories, nuclear receptors and cell-surface receptors.
Nuclear receptors are intracellular proteins present in either the cytosol or
the nucleus that bind hormones which cross the cell membrane by simple dif-
fusion. When nuclear receptors bind their cognate hormone, they undergo a
conformational change that enables them to bind to DNA at specific sites
called cis-response elements. When the hormone-nuclear receptor binds
specifically to these response elements, they influence transcription of the
DNA genome into messenger RNA, either by activating or repressing it.