G-coupled proteins

Several human receptors for the neurohypophyseal hormones have been cloned and the sequences elucidated. The human V2 receptor for antidiuretic hormone presumably contains 371 amino acids and seven transmembrane segments and activates cycHc AMP (76). The oxytocin receptor is a classic G-protein-coupled type of receptor with a proposed membrane topography also involving seven transmembrane components (84). A schematic representation of the oxytocin receptor stmcture within the membrane is shown in Eigure 4 (85). 

G-Protein Coupling. The heterotrimeric guanosine triphosphate (GTP) binding proteins, known as G-proteins, are a principal family of proteins serving to couple membrane receptors of the G-protein family to ionic and biochemical processes. This topic is reviewed in References 63—67. 

The G-proteins are heterotrimers made of three families of subunits, a, P, and y, which can interact specifically with discrete regions on G-protein-coupled receptors. This includes most receptors for neurotransmitters and polypeptide hormones (see Neuroregulators). G-protein-coupled receptors also embrace the odorant receptor family and the rhodopsin-linked visual cascade. 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

Kenakin, T. P. (1997). Differences between natural and recombinant G-protein coupled receptor systems with varying receptor/G-protein stoichiometry. Trends Pharmacol. Sci. 18 456-464. 

From this equation it can be seen that for a given receptor density systems can spontaneously produce physiological response and that this response is facilitated by high G-protein concentration, high-affinity receptor/ G-protein coupling (low value of KG), and/or a natural tendency for the receptor to spontaneously form the active state. This latter property is described by the magnitude of L, a thermodynamic constant unique for every receptor. 

The ternary complex model followed by the extended ternary complex model were devised to describe the action of drugs on G-protein-coupled receptors. 

One target type for which the molecular mechanism of efficacy has been partly elucidated is the G-protein-coupled receptor (GPCR). It is known that activation of GPCRs leads to an interaction of the receptor with separate membrane G-proteins to cause dissociation of the G-protein subunits and subsequent activation of effectors (see Chapter 2). For the purposes of binding, this process can lead to an aberration in the binding reaction as perceived in experimental binding studies. Specifically, the activation of the receptor with subsequent binding of that... 

The effect of G-protein coupling on observed agonist affinity (4.6.8)... 

The Effect of G-protein Coupling on Observed Agonist Affinity... 

Christopoulos, A. (2000). Quantification of allosteric interactions at G-protein coupled receptors using radioligand assays. In Current protocol in pharmacology, edited by Enna, S. J., pp. 1.22.21-1.22.40. Wiley and Sons, New York. 

Lazareno, S., and Birdsall, N. J. M. (1995). Detection, quantitation, and verification of allosteric interactions of agents with labeled and unlabeled ligands at G protein-coupled receptors Interactions of strychnine and acetylcholine at muscarinic receptors. Mol. Pharmacol. 48 362-378. 

In constitutively active receptor systems (where the baseline is elevated due to spontaneous formation of receptor active states, see Chapter 3 for full discussion), unless the antagonist has identical affinities for the inactive receptor state, the spontaneously formed active state, and the spontaneously G-protein coupled state (three different receptor conformations, see discussion in Chapter 1 on receptor conformation) it will alter the relative concentrations of these species—and in so doing alter the baseline response. If the antagonist has higher affinity for the... 

Wildoer, M., Fong, J., Jones, R. M., Lunzer, M. M., Sharma, S. K., Kostensis, E., Portoghese, P. S., and Whistler, J. L. (2005). A heterodimer-selective agonist shows in vivo relevance of G-protein coupled receptor dimmers. Proc Natl. Acad. Sci. USA 102 9050-9055. 

Heptahelical receptors, another name for 7 TM receptors or G-protein-coupled receptors. It refers to the motif of the helices of the protein crossing the cell membrane seven times to form intracellular and extracellular domains. 

Furchgott method, 92, 95, 97-98 G-protein coupling effects on, 76 partial, 89-91, 97, 124, 260-261 potency and, 80f Agonist occupancy, 115 Aikake s information criteria, 243, 243f Alleles, 6... 

See G-protein = coupled receptors historical studies of, 2-4 operational model of, 45—47, 46f,... 

Adenosine is produced by many tissues, mainly as a byproduct of ATP breakdown. It is released from neurons, glia and other cells, possibly through the operation of the membrane transport system. Its rate of production varies with the functional state of the tissue and it may play a role as an autocrine or paracrine mediator (e.g. controlling blood flow). The uptake of adenosine is blocked by dipyridamole, which has vasodilatory effects. The effects of adenosine are mediated by a group of G protein-coupled receptors (the Gi/o-coupled Ai- and A3 receptors, and the Gs-coupled A2a-/A2B receptors). Ai receptors can mediate vasoconstriction, block of cardiac atrioventricular conduction and reduction of force of contraction, bronchoconstriction, and inhibition of neurotransmitter release. A2 receptors mediate vasodilatation and are involved in the stimulation of nociceptive afferent neurons. A3 receptors mediate the release of mediators from mast cells. Methylxanthines (e.g. caffeine) function as antagonists of Ai and A2 receptors. Adenosine itself is used to terminate supraventricular tachycardia by intravenous bolus injection. 

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