Allosterism and Cooperativity

Cooperativity Observed when the reaction of one substrate molecule with a protein has an effect on the reaction of a second molecule of the substrate with another active site of the protein. 

Positive The binding of the first substrate makes the reaction of the next substrate easier. 

Allosterism The binding of an effector molecule to a seperate site on the enzyme affects the or Vmax of the enzyme. 

The separation between allosteric effectors and cooperativity lies in the molecule doing the affecting. If the effector molecule acts at another site and the effector is not the substrate, the effect is deemed allosteric and heterotropic. If the effector molecule is the substrate itself, the effect is called cooperative and/or homotropic. 

TABLE 10.2 Responses of multiple-site receptors to various binding plots 

Klotz Scatchard Hm V vs A V/A vs V logv/(n - V) vs log A Linear Linear Linear, h = 1 Concave up/down Concave down/up Linear, 1 h n Nonlinear Nonlinear Nonlinear 

Therefore, n, Xn Kj (or Xni/Ki) and TlKi can be evaluated. An insight into the relative magnitude of the binding constants is feasible if m is small and known. 

Each subunit (protomer) in the protein is capable of existing in either of two conformational states, namely T (tight) and R (relaxed) states. 

The conformational symmetry is maintained such that all protomers of the protein must be in the same conformation, either all T or all R at any instance. Therefore a protein with n subunits is limited to only two conformational states, T and Rn. Thus only a single eqnilibrium constant L = [Tn]/[Rn] is sufficient to express the equilibrium between them. 

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The MWC model says that in the R state, all the active sites are the same and all have higher substrate affinity than in the T state. If one site is in the R state, all are. In any one protein molecule at any one time, all subunits are supposed to have identical affinities for substrate. Because the transition between the R and the T states happens at the same time to all subunits, the MWC model has been called file concerted model for allosterism and cooperativity. The MWC model invokes this symmetry principle because the modelers saw no compelling reason to think that one of the chemically identical subunits of a protein would have a conformation that was different from the others. Alternative models exist that suggest that each subunit can have a different conformation and different affinities for substrate. Experimentally, examples are known that follow each model. 

Two theoretical models for allosteric effects have been proposed to explain the mechanism for ligand-protein cooperative interactions the concerted (or symmetry) model of Monod, Wyman, and Changeux and the sequentially induced-fit model of Koshland. The nomenclature associated with allosterism and cooperativity originated from the concerted model. Both models assume that... 

Allosteric and cooperative effects may be of different kind and Table 1 shows the main types encountered in enzymology. 

Enzyme activity can also be affected by binding of substrate and nonsubstrate Mgands, which can act as activators or inhibitors, at a site other than the active site. These enzymes are called allosteric. These responses can be homotropic or heterotropic. Homotropic responses refer to the allosteric modulation of enzyme activity strictly by substrate molecules heterotropic responses refer to the allosteric modulation of enzyme activity by nonsubstrate molecules or combinations of substrate and nonsubstrate molecules. The allosteric modulation can be positive (activation) or negative (inhibition). Many allosteric enzymes also display cooperativity, making a clear differentiation between allosterism and cooperativity somewhat difficult. 

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