Allosteric effector site

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. 

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ATP lowers its oxygen affinity more than DPG, and GTP lowers it more than ATP, just as in teleost fish (R. Baumann, unpublished observations).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. 

C.R. McCudden and S.G. Powers-Lee. 1996. Required allosteric effector site forJV-acetylglutamate on carbamoylphosphate synthetase I Biol. Chem. 271 18285-18294. (PubMed)... 

VlcCudden, C R., and Powers-Lee, S. G. 1996. Required allosteric effector site tor N acelylglulamatc on carbamoyl-pho.sphatc synthetase I./ Bio/. C7iem. 271 18285-18294. 

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. 

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. 

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-... 

X-ray crystallographic analyses alone could not sort out the reason for differences between the RSR, MM and other weak acting series. Figure 17.2a is a stereo diagram showing the overlap of four allosteric effectors that bind at the same deoxy-Hb site but differ in their allosteric potency. Only small differences in atomic positions are apparent when comparing the strong RSR molecules with the MM molecules. 

Finally, the activity of key enzymes can be regulated by ligands (substrates, products, coenzymes, or other effectors), which as allosteric effectors do not bind at the active center itself, but at another site in the enzyme, thereby modulating enzyme activity (6 see p. 116). Key enzymes are often inhibited by immediate reaction products, by end products of the reaction chain concerned feedback inhibition), or by metabolites from completely different metabolic pathways. The precursors for a reaction chain can stimulate their own utilization through enzyme activation. 

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