Most receptor proteins that convey environmental information into the cell interior fall into three classes: seven-transmembrane-helix receptors, dimeric receptors that recruit protein kinases, and dimeric receptors that are protein kinases. We begin by considering the largest class of receptors, the seven-transmembrane-helix receptors.

The seven-transmembrane-helix (7TM) receptors transmit information initiated by signals as diverse as photons, odorants, tastants, hormones, and neurotransmitters (Table 13.1). Several thousand such receptors are known, and the list continues to grow. Indeed, approximately 50% of the drugs that we use alter receptors of this class. Mutations in these receptors and their associated components cause a host of diseases, some of which are listed in Table 13.2. As the name indicates, these receptors contain seven helices that span the membrane bilayer (Figure 13.3). An example of a 7TM receptor that responds to chemical signals is the β-adrenergic receptor. This protein binds epinephrine (also called adrenaline), a hormone responsible for the “fight or flight” response. We will address the biochemical roles of this hormone in more detail later (Chapters 16 and 24).

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The 7TM receptors undergo a change in conformation in response to ligand binding. The binding of a ligand on the outside the cell induces a conformational change in the 7TM receptor that can be detected inside the cell. As we shall see, 7TM receptors also have in common the next step in their signal-transduction cascades.

Let us focus on the β-adrenergic receptor as a model of the 7TM receptor class. What is the next step in the pathway after the binding of epinephrine by the β-adrenergic receptor? The conformational change in the cytoplasmic domain of the receptor activates a GTP-binding protein. This signal-coupling protein is termed a G protein (G for guanyl nucleotide). The activated G protein stimulates the activity of adenylate cyclase, an enzyme that increases the concentration of the second messenger cAMP by forming it from ATP (Figure 13.4).

How do these G proteins operate? In the unactivated state, the guanyl nucleotide bound to the G protein is GDP. In this form, the G protein exists as a heterotrimer consisting of α, β, and γ subunits; the a subunit (referred to as G_α) binds the nucleotide (Figure 13.5). The α and γ subunits are usually anchored to the membrane by covalently attached fatty acids. The exchange of the bound GDP for GTP is catalyzed by the ligand-bound receptor. The ligand–receptor complex interacts with the heterotrimeric G protein and opens the nucleotide-binding site so that GDP (guanosine diphosphate) can depart and GTP (guanosine triphosphate) can bind. The α subunit simultaneously dissociates from the βγ dimer (G_βγ; Figure 13.4). The dissociation of the G-protein heterotrimer into G_α and G_βγ units transmits the signal that the receptor has bound its ligand. The human genome encodes 15 genes for G_α, one of which (G_αs) is expressed ubiquitously while the others are expressed in specific cell-types.

A single ligand–receptor complex can stimulate nucleotide exchange in many G-protein heterotrimers. Thus, hundreds of G_α molecules are converted to their GTP-bound forms from their GDP-bound forms for each bound molecule of hormone, giving an amplified response. All 7TM receptors appear to be coupled to G proteins, and so the 7TM receptors are sometimes referred to as G-protein-coupled receptors or GPCRs.

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As just described, the formation of the ligand–receptor complex activates a G protein. How does the G protein propagate the message that the ligand is present? It does so by a variety of means, depending on the specific type of G protein. We will begin by examining one target of a G protein, the enzyme adenylate cyclase (Figure 13.6). The adenylate cyclase enzyme that is activated by the β-adrenergic signaling pathway is a membrane protein that contains 12 presumed membrane-spanning helices. The G_α protein binds to adenylate cyclase on the G_α surface that had bound the βγ dimer when the G_α protein was in its GDP form. G_αs (“s” stands for “stimulatory”) stimulates adenylate cyclase activity, thus increasing cAMP production. The net result is that the binding of epinephrine to the receptor on the cell surface increases the rate of cAMP production inside the cell.

The increased concentration of cAMP can affect a wide range of cellular processes, depending on the cell type. For example, it enhances the degradation of storage fuels, increases the secretion of acid by the gastric mucosa in the cells of the stomach and intestines, leads to the dispersion of melanin pigment granules in skin cells, diminishes the aggregation of blood platelets, and induces the opening of chloride channels in the pancreas. How does cAMP affect so many cellular processes? Is there a common denominator for its diverse effects? Indeed there is. Most effects of cAMP in eukaryotic cells are mediated by the activation of a single protein kinase. This key enzyme is called protein kinase A (PKA). Kinases are enzymes that phosphorylate a substrate at the expense of a molecule of ATP. PKA consists of two regulatory (R) subunits and two catalytic (C) subunits. In the absence of cAMP, the R₂C₂ complex is catalytically inactive (Figure 13.7). The binding of cAMP to the regulatory subunits releases the catalytic subunits, which are enzymatically active on their own. Activated PKA then phosphorylates specific serine and threonine residues in many targets to alter their activity. The alteration in activity is due to structural and ionic changes that result from the introduction of the large negatively charged phosphate functional group. The cAMP cascade is turned off by cAMP phosphodiesterase, an enzyme that converts cAMP into AMP, which does not activate PKA. The C and R subunits subsequently rejoin to form the inactive enzyme.

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The ability to shut down signal-transduction pathways is as critical as the ability to turn them on. How is the signaling pathway initiated by activated 7TM receptors switched off? G_α subunits have intrinsic GTPase activity, hydrolyzing bound GTP to GDP and P_i (inorganic orthophosphate) and thereby deactivating itself. This hydrolysis reaction is slow, however, requiring from seconds to minutes and thus allowing the GTP form of G_α to activate downstream components of the signal-transduction pathway before GTP hydrolysis deactivates the subunit. In essence, the bound GTP acts as a built-in clock that spontaneously resets the G_α subunit after a short time period. After GTP hydrolysis and the release of P_i, the GDP–bound form of G_α then reassociates with G_βγ to reform the heterotrimeric protein (Figure 13.8).

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The ligand-bound activated receptor must be reset as well to prevent the continuous activation of G proteins. A key step in the inactivation of the receptor rests on the fact that the receptor–ligand interaction is reversible (Figure 13.9). When the ligand dissociates, the receptor returns to its initial, unactivated state. The likelihood that the receptor remains in its unbound state depends on the concentration of ligand in the environment.

Cyclic AMP is not the only second messenger employed by 7TM receptors and the G proteins. We turn now to another ubiquitous second-messenger cascade used by many hormones to evoke a variety of responses. The phosphoinositide cascade, like the adenylate cyclase cascade, converts extracellular signals into intracellular ones. The intracellular messengers formed by activation of this pathway arise from the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP₂), a membrane phospholipid. The binding of a hormone such as vasopressin, which regulates water retention, to its 7TM receptor, leads to the activation of phospholipase C. The G_α protein that activates phospholipase C is called G_αq. The activated enzyme then hydrolyzes the phosphodiester linkage joining the phosphorylated inositol unit to the acylated glycerol moiety. The cleavage of PIP₂ produces two messengers: inositol 1,4,5-trisphosphate (IP₃), a soluble molecule that can diffuse from the membrane, and diacylglycerol (DAG), which stays in the membrane (Figure 13.11).

What are the biochemical effects of the second messenger IP₃? Unlike cAMP, IP₃ does not cause a cascade of phosphorylation to elicit a response from the cell. IP₃ directly causes the rapid release of Ca²+ from intracellular stores—the endoplasmic reticulum and, in muscle cells, the sarcoplasmic reticulum. IP₃ associates with a membrane protein called the IP₃-gated channel or IP₃ receptor to allow the flow of Ca²⁺ from the endoplasmic reticulum into the cell cytoplasm. The elevated level of Ca²⁺ in the cytoplasm then triggers a variety of biochemical processes such as smooth-muscle contraction, glycogen breakdown, and vesicle release.

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The lifetime of IP₃ in the cell is very short—less than a few seconds. It is rapidly converted into derivatives that have no effect on the IP₃-gated channel. Lithium ion, widely used to treat bipolar affective disorder, may act by inhibiting the recycling of IP₃, although the details of lithium action remain to be determined.

Diacylglycerol, the other molecule formed by the receptor-triggered hydrolysis of PIP₂, also is a second messenger that, in conjunction with Ca²⁺, activates protein kinase C (PKC), a protein kinase that phosphorylates serine and threonine residues in many target proteins (Figure 13.12).