Receptors
Fig 3.16 Top-down view of a hormone receptor with an adrenaline molecule bound between the membrane spanning segments.
The receptor is viewed from the extracellular surface. The N-terminal domain is glycosylated. The two catechol hydroxyls of adrenaline interact with the serine residues present in the membrane spanning -helix E. The binding serines, S204 and S207, are separated by three residues (one turn) and so they are both projected towards the ligand, shifted by about 0.15 nm along the helix axis. The ligand amino group binds to an aspartate residue on the membrane spanning -helix C.
Adapted from Ostrowski et al.46
N-terminal segment and on the exposed loops linking the transmembrane helices (Figure 3.17b). The receptors for glycoprotein hormones have been adapted by elongated N-terminal chains that extend well out into the extracellular aqueous environment (Figure 3.17c).
Calcium sensors and metabotropic receptors
The sites of attachment for neurotransmitters (such as glutamate and-aminobutyrate) on metabotropic receptors, and for Ca2 ions on Ca2 -sensing receptors, are on specialized extracellular N-terminal extensions of up to 600 residues47 (Figure 3.17d). Binding of Ca2 causes a pincer-like conformational change in the lobes of the extended extracellular domain. This exposes residues which then interact with the transmembrane core of the receptor.
In this way, the extracellular domain acts as an auto-ligand. The Ca2 -sensing receptor confronts a particular problem since it has to sense and then respond to very small changes in Ca2 concentration against a high basal level ( 0.025 mmol l 1 in 2.5 mmol l 1). The cells of the parathyroid gland react by secretion of parathyroid hormone whenever the concentration of circulating
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Signal Transduction
Fig 3.17 Ligand binding sites. The 7TM receptor is a jack of many
trades, regulating a variety of effectors and responding to diverse ligands with sizes that range from 40 Da (Ca2 )
to more than 100 kDa. (a) Most of the common low-molecular-mass hormones (such as adrenaline and acetylcholine) bind to sites within the hydrophobic core. (b and c) Peptide
and protein ligands are accommodated on the exterior face of the receptor.
(d)Although of low molecular mass, Ca2 and the amino acids glutamate and GABA bind to large N-terminal extensions. This induces a
conformational change in the extension, which then interacts with the receptor.
(e)Proteinase-activated receptors
are cleaved and the newly exposed N-terminus acts as an auto-ligand. The freed peptide may also interact separately with another receptor.
Adapted from Ji et al.41
Ca2 dips below about 2.2 mmol l 1. Obviously, the affinity of the sensor has to be very low or it would be fully saturated at all times and under all conditions. On the other hand, the system must be sensitive to proportional changes in concentration that are minute compared with those sensed by conventional hormone receptors which react to changes ranging over orders
of magnitude. The Ca2 sensor is endowed with an extracellular domain which acts as a low affinity chelator, binding or releasing Ca2 as its concentration varies within the (extracellular) physiological range.47 It may operate as
a dimer, the two components being joined by a disulfide bond between cysteine residues present in the extracellular domain.48 A monomer–dimer equilibrium could underlie the special binding properties of this receptor.49
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Receptors
Proteinase-activated receptors (PARs)
Although blood platelets respond to many ligands (e.g. collagen exposed at sites of tissue damage, ADP released from damaged cells and, more importantly, secreted from activated platelets), the most potent activator is thrombin. Platelet-dependent arterial thrombosis triggers most heart attacks and strokes. Thrombin activates blood platelets, causing them to aggregate within seconds. It also has numerous longer-term functions related to inflammation and tissue repair which are mediated by a wide range of cell types. Thrombin is a serine proteinase enzyme related to trypsin and chymotrypsin and also to acetylcholine esterase. It has a unique specificity, cleaving peptide chains between arginine and serine, only if they are embedded in particular peptide sequences. As with the other serine esterases, the proteolytic activity of thrombin can be inhibited by the organophosphorus compounds mentioned earlier, and its action in stimulating blood platelets is also inhibited by compounds of this class. By cleaving the N-terminal exodomain of the thrombin receptor a new
N-terminus is revealed which itself acts as a tethered ligand interacting with the exposed loops of the receptor (Figure 3.17). A synthetic pentapeptide, equivalent to the five N-terminal amino acids revealed after thrombin cleavage, also has agonistic activity for the thrombin receptor.50 In addition to the tethered ligand, the cleaved 41-residue peptide acts as a strong agonist for platelets.51 Activation results in the generation of prostaglandin E2, causing bronchodilation.
Although conventional in the sense that these PARs are coupled to G proteins, there is the particular problem that their activation by proteolytic cleavage is necessarily irreversible. Of course, for the functioning of platelets there is no problem since stimulation initiates a sequence of events which terminates in their demise. For PARs in other tissues, specialized mechanisms ensure their desensitization and removal.56 In addition to the usual processes of receptor phosphorylation and endocytosis, these include cleavage of the tethered ligand in lysosomes. Because the receptors undergo cleavage both as a consequence of stimulation and again in the process of desensitization, resensitization of the system necessarily occurs by de novo protein synthesis.
The thrombin receptor is now recognized as a
member of a larger family of protease activated receptors (PAR1–4), some of which are additionally activated
by trypsin,52,53 In the epithelia of the upper intestine, PAR2 receptors confer protection against self-digestion by proteolytic enzymes through the production of prostaglandins.54
Similarly, in the bronchial airways, the presence
of proteolytic enzymes released by inflammatory cells (mast cells) appears to be signalled by PARs.55
PARs should not be confused with Par proteins (see page 385)
The adhesion receptor subfamily
Also called the LNB-7TM (large N-terminal family B) family of receptors, these are characterized by N-terminal extensions of 200–2800 residues that contain motifs characteristic of those present in cell-surface adhesion molecules.
For example, the N-terminal extensions of the EMR group of 7TM molecules expressed predominantly in leukocytes all contain multiple EGF-like domains. In addition there is an RGD (arginine–glycine–aspartate) motif, a mucin-like
EMR: epidermal growth factor module-containing mucin-like receptor.
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