Effector sites

Figure 6.25 Schematic diagram of the structure of one dimer of phosphofructokinase. Each polypeptide chain is folded Into two domains (blue and red, and green and brown), each of which has an oi/p structure. Helices are labeled A to M and p strands 1 to 11 from the amino terminus of one polypeptide chain, and respectively from A to M and 1 to 11 for the second polypeptide chain. The binding sites of substrate and effector molecules are schematically marked In gray. The effector site of one subunit is linked to the active site of the other subunit of the dimer through the 6-F loop between helix F and strand 6. (Adapted from T. Schlrmer and P.R. Evans, Nature 343 140-145, 1990.)...

The basic kinetic properties of this allosteric enzyme are clearly explained by combining Monod s theory and these structural results. The tetrameric enzyme exists in equilibrium between a catalytically active R state and an inactive T state. There is a difference in the tertiary structure of the subunits in these two states, which is closely linked to a difference in the quaternary structure of the molecule. The substrate F6P binds preferentially to the R state, thereby shifting the equilibrium to that state. Since the mechanism is concerted, binding of one F6P to the first subunit provides an additional three subunits in the R state, hence the cooperativity of F6P binding and catalysis. ATP binds to both states, so there is no shift in the equilibrium and hence there is no cooperativity of ATP binding. The inhibitor PEP preferentially binds to the effector binding site of molecules in the T state and as a result the equilibrium is shifted to the inactive state. By contrast the activator ADP preferentially binds to the effector site of molecules in the R state and as a result shifts the equilibrium to the R state with its four available, catalytically competent, active sites per molecule. 

FIGURE 15.9 Monod-Wyman-Changeux (MWC) model for allosteric transitions. Consider a dimeric protein that can exist in either of two conformational states, R or T. Each subunit in the dimer has a binding site for substrate S and an allosteric effector site, F. The promoters are symmetrically related to one another in the protein, and symmetry is conserved regardless of the conformational state of the protein. The different states of the protein, with or without bound ligand, are linked to one another through the various equilibria. Thus, the relative population of protein molecules in the R or T state is a function of these equilibria and the concentration of the various ligands, substrate (S), and effectors (which bind at f- or Fj ). As [S] is increased, the T/R equilibrium shifts in favor of an increased proportion of R-conformers in the total population (that is, more protein molecules in the R conformational state). 

Muscle glycogen phosphorylase is a dimer of two identical subunits (842 residues, 97.44 kD). Each subunit contains a pyridoxal phosphate cofactor, covalently linked as a Schiff base to Lys °. Each subunit contains an active site (at the center of the subunit) and an allosteric effector site near the subunit interface (Eigure 15.15). In addition, a regulatory phosphorylation site is located at Ser on each subunit. A glycogen-binding site on each subunit facilitates prior association of glycogen phosphorylase with its substrate and also exerts regulatory control on the enzymatic reaction. 

FIGURE 15.15 (a) The structure of a glycogen phosphorylase monomer, showing the locations of the catalytic site, the PLP cofactor site, the allosteric effector site, the glycogen storage site, the tower helix (residnes 262 throngh 278), and the snbnnit interface. 

Chemokine-mediated DC migration determines disease outcome. In the future, steady-state migration and more complicated migration within effector sites (LNs) will be clarified also by chemokine research. Such studies would provide novel therapeutic strategies against various immune diseases. 

The main site of the mucosal immune system in the gut is referred to as gut-associated lymphoid tissue (GALT), which can be divided into inductive and effector sites. In the small intestine, the inductive sites are in the Peyer s patches, which consist of large lymphoid follicles in the terminal small intestine. The contact with external antibodies triggers a series of cascade events in the body based on immune response (Brandtzaeg et al., 1999). 

Substrates can affect the conformation of the other active sites. So can other molecules. Effector molecules other than the substrate can bind to specific effector sites (different from the substrate-binding site) and shift the original T-R equilibrium (see Fig. 8-9). An effector that binds preferentially to the T state decreases the already low concentration of the R state and makes it even more difficult for the substrate to bind. These effectors decrease the velocity of the overall reaction and are referred to as allosteric inhibitors. An example is the effect of ATP or citrate on the activity of phosphofructokinase. Effectors that bind specif-... 

Neurotransmission in autonomic ganglia is more complex than depolarization mediated by a single transmitter 190 Muscarinic receptors are widely distributed at postsynaptic parasympathetic effector sites 190 Stimulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber 191 Competitive blocking agents cause muscle paralysis by preventing access of acetylcholine to its binding site on the receptor 191... 

Consequences of acetylcholinesterase inhibition differ with effector site 197... 

TABLE 11-1 Predominance of sympathetic or parasympathetic tone at effector sites effects of autonomic ganglionic blockade... 

Consequences of acetylcholinesterase inhibition differ with effector site. At postganglionic parasympathetic effector sites, AChE inhibition enhances or potentiates the action of administered ACh or ACh released by nerve activity. In part, this is a consequence of diffusion of... 

Effector site Now draw a second, similar curve to the right of the first. This shows the concentration of the drug at its site of action. The degree of displacement to the right of the first curve is determined by the factors mentioned above. 

Key points When both curves are drawn, mark a fixed concentration point on the y axis and label it C. Demonstrate that the plasma concentration curve crosses this value twice, at times tx and t2. At time f, the concentration in the plasma is rising and at t2 it is falling. The crucial point now that enables you to define hysteresis is to demonstrate that the effector site concentration is different at these two times depending on whether the plasma concentration is rising (giving concentration Ej) or falling (giving concentration E2). 

Since this structure was first proposed, Braunitzer and co-workers have determined the amino acid sequence of rhinoceros hemoglobin (23a). Its allosteric effector site shows only a single substitution compared to that of human hemoglobin—His NA2/8 — Glu—yet ATP lowers its oxygen affinity more than DPG, and GTP lowers it more than ATP, just as in teleost fish (R. Baumann, unpublished observations). This observation supports the hydrogen bond between N-6 of the adenine and Glu NA2 proposed in Fig. 6 in fact it can hardly be explained without that bond. 

All the mechanisms we have encountered, which affect the way in which a drug is handled by the body, are important for one major reason - they all work together to determine how much drug is present at any given time at the point in the body where the drug acts - its effector site. Commonly this is at a receptor site in particular cells and organs, and the... 

Too little drug at the effector site means no therapeutic effect, too much may cause toxic effects to appear. So there is commonly a range of plasma concentrations between which the desired effect is obtained without toxicity - often called the therapeutic window or therapeutic range (Fig. 5). 

The first difference is their size. As protein macromolecules they have molecular weights exceeding 1000 daltons (Da) - some as high as 250 kDa. Remember the criteria for medicines to cross biological membranes and you will realise that proteins are likely to have big problems in getting to their effector site unless there is some form of transport mechanism that can take them across cell membranes. 

Third, they can act as antigens and generate an immune response which may result in a lower effective concentration of the protein at its effector site (because some of it is bound to the antibody) or occasionally in a clinical allergic syndrome - most particularly if the protein has been derived in whole or in part from non-human DNA (mouse DNA is incorporated with human in some production systems and this tends to produce more common immunological... 

If the poisoning is due to an organophosphate, prompt administration of pralidoxime chloride will result in dephosphorylation of cholinesterases in the periphery and a decrease in the degree of the blockade at the skeletal neuromuscular junction. Since pralidoxime is a quaternary amine, it will not enter the CNS and therefore cannot reactivate central cholinesterases. In addition, pralidoxime is effective only if there has been no aging of the phosphorylated enzyme. Pralidoxime has a greater effect at the skeletal neuromuscular junction than at autonomic effector sites. 

Katzenellenbogen JA, O Malley BW, Katzenellenbogen BS. Tripartite steroid hormone receptor pharmacology interaction with multiple effector sites as a basis for the cell- and promoter-specific action of these hormones. Mol Endocrinol. 10, 119-131, 1996. 

Activated T and B cells emigrate from the inductive environment via lymphatic drainage, circulate through the bloodstream, and target the mucosal effector sites. B cells expressing IgA differentiate during their spread to mucosal sites to become IgA-producing plasma cells. IgG also reaches these areas by passive diffusion from the bloodstream (Corthesy, 2007 Rojas and Apodaca, 2002). 

The mobile receptor model was proposed by Cuatrecasas and by De Haen in an attempt to explain why so many different drugs, hormones, and neurotransmitters can activate adenylate cyclase. According to classical concepts, a recognition site is permanently associated with an effector site, and will regulate its operation on a one-to-one or some other stoichiometric basis. The recognition site is, of course, specific. 

Enzymes that are regulated by effector molecules in an allosteric maimer possess, apart from the binding site for the substrate, a specific binding site for the effector molecule. The binding of effector molecules to the effector site leads to a shift in equilibrium between the various conformations and thus to a change in activity (see 2.3). 

Figure 1 Model of NMDA receptor tetramer complex including the different pharmacological effector sites (adapted from Danysz and Parsons (1998))...

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