Page 60 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 45

(Fig. 2‐18). In contrast, ligands that bind to signal pathway or the phosphatidylinositol
signal pathway. Adenyl cyclase is a cell mem­
intracellular receptors are only able to initiate
VetBooks.ir gene transcription (Fig. 2‐19). These ligands, brane enzyme whose activity is regulated by
LR interactions. In some cases, the enzyme
which are often steroid hormones, are hydro­
phobic and are able to readily move across the also acts as a cell membrane receptor, but in
plasma membrane after being transported other cases, the enzyme is linked to a receptor
through the blood and extracellular fluid by via a G protein. Adenyl cyclase catalyzes the
a carrier protein. Diversity of intracellular intracellular formation of cAMP, which in
responses, including second messengers and turn can activate intracellular kinases. These
gene transcription, as well as amplification of activated kinases can activate other intracel­
the signal created by the LR interaction are lular proteins to change cellular function.
hallmarks of cell surface receptors, including In this general scheme, cAMP is said to be a
protein kinase receptors and GPCRs. second messenger in that it transfers a chem­
GPCRs are classified as modifying either the ical signal from the membrane to the interior
cyclic adenosine monophosphate (cAMP) of the cell. The association of particular

Extracellular Hormone
fluid

Cytoplasm

Adenylyl
GTP
cyclase

cAMP ATP
Active Inactive
cAMP- cAMP-
dependent dependent
protein protein
kinase kinase

Protein – PO4 + ADP Protein + ATP

Biological response

Figure 2-18. After ligand binding, the G proteins associated with the membrane receptor can facilitate an
increase or decrease in membrane‐associated enzyme activity, opening or closing of membrane channels, with
numerous intracellular effects as a result of the second messengers elicited by the ligand–receptor interaction.
In this figure, the G protein is stimulating the adenylyl cyclase‐cAMP system and is called a G protein as the
αs
α subunit is stimulatory. The G protein catalyzes the conversion of cytoplasmic adenosine trisphosphate
αs
(ATP) into cyclic adenosine monophosphate (cAMP). This activates cAMP‐dependent protein kinase, which
phosphorylates specific proteins within the cell and triggers a biological response to the ligand from the cell.
ADP, adenosine diphosphate; GTP, guanosine trisphosphate. Source: adapted from Guyton and Hall, 2006.
Reproduced with permission from Elsevier.