Multifunctional cooperativity

While in ancient times, the sensorial properties of a flavour for foodstuffs were of major importance, modern flavours have to perform like multifunctional systems. Physical form, chemical and mechanical stability and controlled release mechanisms are meanwhile essential criteria for the flavour quality. All these properties have to be addressed by a flavourist in close cooperation with technologists. Therefore, knowledge about food product properties must lead to a careful and intelligent evaluation of the flavour system as an important driver for the success of the final product. 

In systems developed by Rhodia [31] the main component is Sn02 (cassiterite), which is inactive in the reaction of propane ammoxidation, while it acts as the carrier for the active components V/Sb/O and SbOx. Tin oxide facilitates the dispersion of the active components, and yields a multifunctional catalyst in which the various species can effectively cooperate in the reaction. 

The properties of a dendrimer are determined not only by the specific properties of its functional units, but also by their number and structural variety, and by cooperative effects between different functional units. A number of special applications and the development of substances suitable for mimicking biological systems [53] require such multifunctional dendritic systems with more than one kind of functionality. 

Another important point in enzyme catalysis is that an enzyme is a multiple catalyst in vdiidi many catalytic groiqts cooperate intramolecularly (i). In other words, an enzyme is an intramolecular multiple catalyst or a multifunctional catalyst. On this aspect of ens me catalysis some investigations have been carried out through the cooperative functions of pendant catalytic groups of cyclic peptides. These enz mie... 

Catalysts containing multiple types of active centres were also described. These catalysts may be used to perform several steps in a sequence or to alter the characteristics of a single reaction such as rate and selectivity by a synergic cooperation. This approach, which was inspired by the action of enzymes as the best examples of multifunctional catalysts, deserves future intensive studies since in the last few years it has brought a series of very exciting results. 

Myxobacterial secondary metabolites are frequently hybrid structures derived from the linkage of carboxylic acids and amino acids. In the majority of cases, the compounds are formed by complex, multistep biosynthetic processes catalyzed by giant multifunctional enzymes called PKSs and NRPSs. Hybrid systems in which PKS and NRPS multienzymes cooperate are also known, and occasionally these machineries even appear within the same polypeptide. " A detailed understanding of the mechanisms involved in PKS, NRPS, or PKS-NRPS biosynthesis is a prerequisite for optimizing product yields and for manipulating the biosynthetic pathways in order to generate altered natural products. 

Some activities show a lag phase (no reaction takes place in the first hours), which is a typical behaviour of multifunctional enzymes and is ascribed to conformational changes hysteresis or time cooperativity or kinetic cooperativity. 

The multifunctional enzyme V2 is (like the enzymes VI and V3) a regulatory (allosteric) and indeed a hysteretic (8) and, more precisely, a mnemonic enzyme. Hysteresis (kinetic cooperativity or chronological cooperativity) (9) is the phenomenon of slow conformational changes of enzymes by isomerization, association-dissociation or by the binding of an effector. 

So far, most approaches to enzyme-like activity have used just one of the functional groups which are present in enzymes. However, many enzymes only operate by a cooperation of functional groups (see for instance the catalytic triade in peptidases). There, the enzyme s functional groups perform a multifunctional catalysis. Therefore in (organo) catalysis, bifunctional catalysis has been developed, too. In the field of concave reagents, first bifunctional catalysts have been constructed (Figure 7.28), and future will tell how capable they are to catalyse reactions with their acidic and basic functionalities. 

Kunitake, T. Okahata, Y. Multifunctional hydrolytic catalyses. 7. Cooperative catalysis of the hydrolysis of phenyl esters by a copolymer of iV-methacryloyl hydroxamic acid and 4-vinyl imidazole. J. Am. Chem. Soc. 1976, 98, 7793-7799. 

Figure17.4 Three-dimensional multifunctional microstructures of PANI self-assembled from ID nanofibers by a cooperative effect of micelles as soft-templates and molecular interactions as driving forces [64-67].

Fig.l. Fatty acid biosynthesis, Cooperative action of the proteinsof fatty acid synthase, which may be a multienzyme compiex, or a single multifunctional protein (see Table2). The zig-zags represent the movable pantetheine arm of the acyl carrier protein. The central circle represents acyl carrier protein the other enzymes are represented by the six peripheral circles (see Table 1). This purely diagrammatic representation does not necessarily show the true juxtaposition of the proteins. 

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