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Section IBasic principlesG-proteins bind GDP and GTP, hence the name ‘G-protein’. In the inactive form
GDP is bound to theαsubunit but on interaction with an activated GPCR GTP
replaces GDP, giving a complex ofα-GTP-βγ. Theα-GTP subunit then dissociates
from theβγdimer and activates or inhibits an effector protein, either an enzyme,
such asadenylyl cyclaseorphospholipase C(Figure3.2)oranion channel. For
example,β-adrenergic agonists activate adenylyl cyclase and opioid receptor ago-
nists, such as morphine, depress transmission of pain signals via inhibition of N-type
Ca^2 +channels through G-protein mechanisms. In some systems, theβγdimer can
also activate intermediary mechanisms.
Theα-subunit itself acts as a GTPase enzyme, splitting the GTP attached to it
to regenerate an inactiveα-GDP subunit. This then reforms the entire inactive
G-protein complex by recombination with anotherβγdimer.
Theαsubunit of the G-proteins shows marked variability, with at least 17 molecular
variants arranged into three main classes. Gstype G-proteins haveαsubunits that
activate adenylyl cyclase, Gihaveαsubunits that inhibit adenylyl cyclase and Gq
haveαsubunits that activate phospholipase C. Each GPCR will act via a specific
type of G-protein complex and this determines the outcome from ligand-receptor
coupling. It is known that the ratio of G-protein to GPCR is in favour of the G-
proteins in the order of about 100 to 1, permitting signal amplification. Regulation of
GPCR activity involves phosphorylation at the intracellular carboxyl-terminal that
encourages binding of a protein,β-arrestin, which is the signal for removal of the
receptor protein from the cell membrane. The binding of an agonist may increase
phosphorylation and so regulate its own effect, accounting for tachyphylaxis seen
withβ-andrenergic agonists.
Adenylyl cyclase catalyzes the formation of cAMP, which acts as a final common
pathway for a number of extracellular stimuli. Allβ-adrenergic effects are mediated
through Gsand opiate effects through Gi. The cAMP so formed acts by stimulating
protein kinase A, which has two regulatory (R) and two catalytic (C) units. cAMP binds
to the R unit, revealing the active C unit, which is responsible for the biochemical
effect, and it may cause either protein synthesis, gene activation or changes in ionic
permeability.
cAMP formed under the regulation of the G-proteins is broken down by the action
of the phosphodiesterases (PDEs). The PDEs are a family of five isoenzymes, of
which PDE III is the most important in heart muscle. PDE inhibitors, such as theo-
phylline and enoximone, prevent the breakdown of cAMP so that intracellular levels
rise.
Therefore, in the heart, positive inotropy is possible by either increasing cAMP
levels (with aβ-andrenergic agonist or a non-adrenergic inotrope such as glucagon),
or by reducing the breakdown of cAMP (with a PDE III inhibitor such as milrinone).
Phospholipase C is also under the control of the G-proteins, but theαsubunit is of
the Gqtype. Activation of the Gq-proteins by formation of an active ligand–receptor
complex promotes the action of phospholipase C. This breaks down a membrane