Anticooperativity in binding

In the presence of the strong electrostatic forces contributed by Ca ", the two residues (Glu-43 and Arg-87) exert various effects anticooperativity in binding the substrate and cooperativity in binding the transition state. This combination of effects is presumed to lower the kinetic barrier to catalysis by 8.1 kcal/mol (7). 

These template polymerizations suffer from three fundamental problems (i) In most cases the binding of the polymer to the template is stronger than the binding of the monomer due to the cooperativity of the interaction between the polymers. As a consequence the newly formed macromolecules are not released from the template and multiple replication is not possible without multiple separation steps, (ii) We lack the possibility to start the polymerisation reaction at the terminal group of the monomer-template complex, (iii) While a weak interaction between the template and the monomer is favourable to allow easy separation of the template and the newly formed macromolecule, it leads to incomplete complexation of the template and interraption of the polymerisation along the chain. A solution of these problems would require a relatively strong complexation of the monomers in combination with sufficient anticooperativity in the complexation of the polymer. The latter however would inevitably impede the polymerisation reaction and require therefore a living polymerisation mechanism which does not suffer from a slowed down rate of polymerisation. 

Similar results were obtained in Yon s laboratory with aspartate amino transferase from pig heart cytosol, which is a dimer with one PLP bound per subunit. A strong anticooperativity for binding of pyridoxamine phosphate (PMP) was reported (Arrio-Dupont, 1972). In the haloenzyme-apoenzyme hybrid an important decrease of reactivity of Cys 190 (which is entirely accessible in apoenzyme and buried in holoenzyme) also indicated strong coupling between protomers in the enzyme molecule (Cournil, 1975 Cournil and Arrio-Dupont, 1975). Dissociation of the apoenzyme by dilution was... 

An anticooperative mode of interactions was assumed in case of concave-shaped Scatchard plots, as alrea% proposed by other authors (Mattai Kwak, 1986 Gamier et al, 1994). A convexe curvature of the plots indicated a cooperative binding process (figure 4). 

Figure 5.26 (a) The isomeric anticooperative open (HF)3 structure (fully optimized), and (b) the leading np interaction with one of the two equivalent Lewis-acid monomers (with the second-order stabilization energy in parentheses). The net binding energy is 7.92 kcal mol-1. 

Figure 1. Illustration of lone electron pair preferences in alcohol dimers, cooperative and anticooperative binding sites for a third monomer, ring strain and steric repulsion in alcohol trimers, alternation of residues in alcohol tetramers, and chain, branch, and cyclic hydrogen bond topologies in larger clusters.

In some earlier publications the term cooperativity is used for positive cooperativity and anticoop-erativity is used for negative cooperativity. In this book cooperativity is used whenever g 1. Sometimes, when there is positive cooperativity one says that a ligand at a supports or favors the binding of a ligand at b, and vice versa. 

Binding of additional ligands to a small substrate is often hindered for steric reasons (anticooperative binding). In most of the protein associations, however, the first association step favors the following binding steps. This behavior, called cooperative binding, leads to the formation of more highly coordinated species in the analyte. 

A smaller secondary microscopic association constant compared to the first binding constant (k, = k2 > k2, = kl2) results in anticooperative binding behavior in cases A and B. If k, = k2 < k2i = kl2, as in cases D and E, the cooperative binding yields a higher amount of higher-order complexes. 

When Kab is small (no "mixed" dimer) Eq. 7-16 also simplifies to Eq. 7-45 for completely cooperative binding with the value K given by Eq. 7-17. On the other hand, if KAB is large compared to KAA and KBB, anticooperativity (negative cooperativity) will be observed. The saturation curve will contain two separate steps just as in the binding of protons by succinate dianion (Fig. 7-5). 

Alternatively, the exchange may be related to the multimeric nature of the enzyme. It may be that the binding mechanism is defined by extreme anticooperative binding, such that in the multimeric enzyme only half the binding sites may be occupied, while exchange is allowed via transient species that never accumulate. Such a binding scheme based on a dimeric structure is illustrated in Scheme II, where UDPR is either UTP or UDPglucose. 

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