the hydrolysis of GTP to GDP. In their resting state, the G‐proteins exist in the cell membrane as heterotrimeric complexes of α, β and γ subunits. The β and γ subunits associate with high affinity, creating functional units of Gα and Gβ/γ. Hormone occupancy results in a conformational change in the receptor. In turn, this causes a conformational change in the α‐subunit, leading to an exchange of GDP for GTP. The acquisition of GTP causes the α‐subunit to dissociate from the Gβ/γ subunits and bind to a downstream catalytic unit, either adenylate cyclase in the generation of cAMP or phospholipase C (PLC) to produce DAG/IP3 from phosphatidylinositol (Figures 3.10 and 3.11). The energy to activate these target enzymes comes from the cleavage of one phosphate from GTP. This regenerates Gα‐GDP, which no longer associates with adenylate cyclase or PLC, thus switching off the cascade and recycling the Gα‐GDP back to the start.
There are over 20 isoforms of the Gα subunit that can be grouped into four major subfamilies (Box 3.7). These are differentially involved with different hormone receptor signalling pathways (Table 3.1). More than half of GPCRs can interact with different Gα subunits providing contrasting, and sometimes opposing, intracellular second messenger systems (Figure 3.12). In part, this promiscuity can be attributed to the extent of hormonal stimulation or according to the activation of different receptor subtypes. For instance, at low concentrations, TSH, calcitonin and LH receptors associate with Gsα to activate adenylate cyclase, whereas higher concentrations recruit Gqα to activate PLC. Calcitonin receptor subtypes are differentially expressed according to the stage of the cell cycle. Defects in the G‐protein signalling pathways result in many endocrine disorders (Box 3.8; Figures 3.13 and 3.14).
Figure 3.10 G‐protein–coupled receptors. The extracellular domain is ligand‐specific and, hence, less conserved across family members (e.g. only 35–45% for the TSH, LH and FSH receptors). The transmembrane domain has a characteristic heptahelical structure, most of which is embedded in the cell membrane and provides a hydrophobic core. Conserved cysteine residues can form a disulphide bridge between the second and third extracellular loops. The cytoplasmic domain links the receptor to the signal‐transducing G‐proteins and, in this example, is linked to membrane‐bound adenylate cyclase. The activation of adenylate cyclase is depicted by the conversion of C to C*.
Figure 3.11 Second messengers that mediate G‐protein–coupled receptor signalling. The symbol P is the abbreviation for a phosphate group. Carbon atoms are numbered in their ring position. R1 and R2 represent fatty acid chains.
Box 3.7 Sub‐families of Gα protein subunits
Gsα: activates adenylate cyclase
Giα: inhibits adenylate cyclase
Gqα: activates PLC
Goα: activates ion channels
Second messenger pathways
Cyclic adenosine monophosphate
Activation of membrane‐bound adenylate cyclase catalyzes the conversion of ATP to the second messenger cAMP (Figure 3.11). cAMP interacts with protein kinase A (PKA), which unmasks its catalytic site to allow phosphorylation of serine and threonine residues on a transcription factor called cAMP response element‐binding protein (CREB) (Figure 3.15). CREB then translocates to the nucleus where it binds to a short palindromic sequence in the regulatory regions of cAMP‐regulated genes. This signalling pathway controls major metabolic pathways, including those for lipolysis, glycogenolysis and steroidogenesis.
The cAMP response is terminated by a large family of phosphodiesterases, which can be activated by a variety of systems, including phosphorylation by PKA, in effect providing a negative feedback loop. Phosphodiesterases rapidly hydrolyze cAMP to the inactive 5′‐AMP. In addition, activated PKA can phosphorylate serine and threonine residues of the GPCR to cause receptor desensitization.
Table 3.1 Use of different G‐protein α‐subunits by various hormone signalling pathways
| Hormone | Dominant G‐protein α‐subunit(s) |
|---|---|
| Thyrotrophin‐releasing hormone (TRH) | Gqα |
| Corticotrophin‐releasing hormone (CRH) | Gsα |
| Gonadotrophin‐releasing hormone (GnRH) | Gqα |
| Somatostatin (SS) | Giα/Gqα |
| Thyroid‐stimulating hormone (TSH) | Gsα/Gqα |
| Luteinizing hormone (LH)/human chorionic gonadotrophin (hCG) | Gsα/Gqα |
| Follicle‐stimulating hormone (FSH) | Gsα/Gqα |
| Adrenocorticotrophic hormone (ACTH) | Gsα |
| Oxytocin | Gqα |
| Vasopressin | Gsα/Gqα |
| Catecholamines (β‐adrenergic) | Gsα |
| Angiotensin II (AII) | Giα/Gqα |
| Glucagon | Gsα |
| Calcium | Gqα/Giα |
| Calcitonin | Gsα/Giα/Gqα |
