Effector binding

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. 

Furthermore, peptidomimetic SH2 domain inhibitors for Src, such as AP-22408 have been designed that interfere with effector binding and thereby disrupt signal transduction. AP-22408 decreases bone resorption in animal studies and may be a promising drug to treat osteoporosis and other bone diseases, such as Paget s disease and osteolytic bone metastasis. 

Kd 50 nM for GIRK [G-protein-activated inwardly rectifying K+ channel] activation) and persistent in the absence of competing Ga-GDP, Py-effector binding is not irreversible. 

ALLOSTERIC EFFECTORS bind specifically to either the T or the R states. 

Here biophysical methods contributed a reductionistic approach. By analyzing a limited number of actors under well defined conditions, the mechanisms of nucleotide exchange, intrinsic and stimulated GTP-hydrolysis, effector binding, and membrane attachment have been elaborated to present a comprehensive model. 

ALLOSTERIC EFFECTORS bind specifically to either the T or the R states. Heterotropic (nonsubstrate) activators bind to and stabilize the R state, while heterotropic inhibitors bind preferentially to the T state. 

F. Hucho, S. Stengelin, G. Bandini (1979). Effector binding sites and ion channels in excitable membranes. In F. Gualtieri, M. Gianella, C. Melchiorre (Eds.). Recent Advances in Receptor Chemistry. New York Elsevier/North Holland, pp. 37-58. 

The binding of an effector molecule to the effector binding site leads to a coupled conformational change to fixate either a high affinity R-form or a low affinity T-form. 

Definition of allosteric enzymes and their function Allosteric enzymes are multi-subunit proteins that are regulated by molecules called effectors. The effectors bind noncovalently at a site other than the active site, and can alter the affinity of the enzyme for its substrate, modify the maximal catalytic activity of the enzyme, or both. Allosteric enzymes frequently catalyze the committed step early in a pathway. 

Stevens, R. C., J. E. Gouaux, and W. N. Lipscomb, Structural consequences of effector binding to the T state of aspartate carbamoyltransferase Crystal structures of the unligated and ATP- and CTP-complexed enzymes at 2.6 A resolution. Biochem. 29 7691, 1990. 

Aspartate carbamoyltransferase is an allosteric enzyme in which the active sites and the allosteric effector binding sites are on different subunits. Explain how it might be possible for an allosteric enzyme to have both kinds of sites on the same subunit. 

In all unicellular organisms and in most cells of multicellular organisms, these regulatory enzymes are sensitive to inhibition and activation by small molecules that reflect the metabolic state of the cell. When an effector binds to the regulatory site of an enzyme, it encourages the enzyme to... 

Allosteric site. Location on an allosteric enzyme where the allosteric effector binds. 

Although Ga proteins exhibit considerable selectivity, effector-binding surfaces can nevertheless tolerate substantial variation. That is why it has been possible to use chimeric Ga proteins in structural studies of G protein effector complexes. Some Ga isoforms are difficult to express in heterologous... 

Effector binding, per se, is the most direct output of Ga activation, and this event may be sufficient to account for the mechanism of PDE activation, in which Gat binds and sequesters the inhibitory subunit of the enzyme complex. Thus, the C-terminal residues of PDEy responsible for inhibition of the catalytic PDE a/) heterodimer are sequestered in the interaction with Gat (Brown, 1992 Lipkin et al., 1988). In other systems, Ga are allosteric activators or inhibitors, or act in more subtle and less well-understood roles as organizing centers of G protein-signaling complexes. 

Switch segments of Ga proteins serve as critical determinants for catalytic and effector-binding activity, thereby providing a mechanism for kinetic coupling of GTP binding to effector activity and GTP hydrolysis to signal termination. 

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