Binding equilibria dimerizing protein

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). 

Let us carry over the binding sequence in a dimeric protein in Fig. 16 to a dimeric enzyme operating under the rapid equilibrium conditions, in order to express the binding sequences in terms of all three types of constants the substrate binding constant, Ks, the transformation constant, Kt> and the interaction constants, ICaa, and iCgB (Eq. (13.79)). 

Often, DNA-binding proteins must associate to become active. For example, CD was used to study the concentration-dependent unfolding of bacteriophage P22 Arc repressor, which is associated with a monomer-dimer equilibrium [191], One major class of DNA-... 

Interferon-7-inducible protein 10 kDa (IP-IO/GXGLIO) is a chemokine that in solution, exists in equilibrium between monomer and dimer. A solution structure of a monomeric variant has been solved by NMR (Booth et al, 2002). However, like MGP-1, it has also been crystallized in different space groups, revealing tetrameric oligomerization states (Swaminathan et al, 2003). One of the forms is similar to the PF4 and MGP-1 tetramers. The other two tetramers form a novel twelve-stranded /3-sheet containing structure (Fig. 9), which the authors postulate could represent structures induced by binding of different GAGs. 

Tyrosine kinase receptors These receptors exist in a monomer-dimer equilibrium. The dimer, which is sta-bihzed upon ligand binding, is the signaling structure. Dimer formation stimulates catalytic activity and results in intermolecular autophosphorylation within the dimer and triggers signahng cascades that lead to the phosphorylation of cytoplasmic substrates (insulin receptor as example. Figure 3.1). The increase in phosphorylation of tyrosine residues of intracellular proteins either increases or decreases their activity, particularly that of protein kinases or protein phosphatases that often play a crucial role in the regulation of cellular function. 

Fig. 5. Model for sHsp chaperone activity. The sHsp oligomer (T Hspl6.9 shown here) is in rapid equilibrium with a smaller species (possibly a dimer). Heat-denatured substrates bind hydrophobic sites exposed on the sHsp subunits to form soluble sHsp/substrate complexes, preventing formation of insoluble aggregates of denatured proteins. The sHsp/substrate complexes may also be in rapid equilibrium, and when dissociated, the denatured substrate can be picked up and refolded in an ATP-dependent fashion by the Hsp70 or DnaK (plus cochaperone) machinery. Note that sHsp/substrate complexes can also become larger and insoluble, and the fate of these latter complexes is unknown.

Cassman and King have reported cooperative binding of NADH to beef heart s-MDH (67). They also found evidence for the existence of s-MDH monomer from sedimentation equilibrium analysis at low protein concentrations (67). The fluorescence studies of the binding of NADH to beef heart s-MDH were interpreted in terms of a model in which both monomer and dimer bind NADH with cooperative binding to the dimer (67). The dissociation constant for the first site was about 25 /Jkf and for the second site about 0.18 iiM. 

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