Page 59 - Anatomy and Physiology of Farm Animals, 8th Edition

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Binding of a ligand may also activate
produce any biologic response. However, receptors that function as protein kinases.
VetBooks.ir because the β‐blocker occupies the A kinase is an enzyme that adds phosphate
receptor, the binding of any other ligand
groups (phosphorylation) to specific amino
is prevented. Thus, β‐blockers are used
when it is desirable to reduce the biologic acid residues, typically serine/threonine or
activity of β‐adrenergic receptor stimula- tyrosine. Conversely, a phosphatase is an
tion. The binding of the β‐blocker to the enzyme that removes phosphate groups.
β‐receptor prevents normal endogenous Insulin binding to the dimerized insulin
agents (epinephrine and norepinephrine) receptor, a transmembrane receptor tyrosine
from binding to the receptor and bring- kinase, initiates tyrosine phosphorylation
ing about a biologic response. within the cytoplasmic portion of the
The binding of a ligand to a membrane receptor, as well as phosphorylation of other
receptor is only the first step in the process proteins that are recruited to facilitate the
by which a chemical in the extracellular biological effect of the LR interaction.
fluid can alter the function of a cell without Another way that the function of a cell can
entering it. The second step depends on the be changed after a ligand binds membrane
particular type of membrane receptor. In receptors involves another specific group of
some cases, the membrane receptor is also a membrane proteins known as G proteins. G
membrane channel. The binding of a ligand proteins are heterotrimeric guanine nucleo­
to these types of receptors is associated with tide‐binding proteins composed of an alpha
a change in the permeability of the channel; (α), beta (β), and gamma (γ) subunit within
hence, these channels are described as being the cell membrane and are closely associated
ligand‐gated. Other membrane receptors with certain receptor proteins. The receptors
are also enzymes that are activated by the associated with the G proteins are termed G
binding of a ligand. These enzymes have protein coupled receptors (GPCRs) and are
active sites facing the interior of the cell, so unique in that they possess seven transmem­
intracellular functions are changed as a brane domains (Fig. 2‐17). A GPCR can be
result of the LR interaction. Substances that classified based on the intracellular response
bind to receptors or channels on the exte­ that is elicited by ligand binding to the recep­
rior surface of the cell are typically hydro­ tor. After ligand binding, the G proteins act as
philic and are unable to cross the plasma intermediates in a chain of events between
membrane of the cell. the GPCR and the cellular response

Hormone
Receptor
Extracellular
uid

Cytoplasm

G-protein GDP GTP GTP activated
(inactive) G-protein target protein
(active) (enzyme)
Figure 2-17. Ligands for G protein coupled receptors are typically hydrophilic substances that are
unable to cross the plasma membrane, including many protein and peptide hormones. Ligand binding to
the receptor causes the inactive α, β, and γ G protein complex to associate with the receptor and become
activated with the exchange of a guanosine trisphosphate (GTP) for a guanine diphosphate (GDP). The
GTP‐bound α subunit then dissociates from the other G protein subunits and is able to interact with
membrane‐bound target proteins (enzymes) that initiate intracellular signals. Source: adapted from
Guyton and Hall, 2006. Reproduced with permission from Elsevier.