Enzyme-metal catalysis

Cofactors serve functions similar to those of prosthetic groups but bind in a transient, dissociable manner either to the enzyme or to a substrate such as ATP. Unlike the stably associated prosthetic groups, cofactors therefore must be present in the medium surrounding the enzyme for catalysis to occur. The most common cofactors also are metal ions. Enzymes that require a metal ion cofactor are termed metal-activated enzymes to distinguish them from the metalloenzymes for which metal ions serve as prosthetic groups. 

Catalytic transformation based on combined enzyme and metal catalysis is described as a new class of methodology for the synthesis of enantiopure compounds. This approach is particularly useful for dynamic kinetic resolution in which enzymatic resolution is coupled with metal-catalyzed racemization for the conversion of a racemic substrate to a single enantiomeric product. 

Fig. 13.8 An early example of combined enzyme and metal catalysis the one-pot cascade conversion of glucose into mannitol [17, 18].

Asymmetric Transformations by Coupled Enzyme and Metal Catalysis Dynamic Kinetic Resolution... 

To understand the catalytic system under investigation as a whole, we tried to transfer experimental methods and the corresponding/orma/ism from inorganic chemistry (see Chap. 3) and enzyme chemistry to metal catalysis. To get an insight into the... 

The manifold intermediates in homogeneous transition-metal catalysis are certainly metal complexes and therefore show a behaviour like ordinary coordination compounds associations of phosphorus donors open up multifarious additional controls. Both, substrates and P ligands are Lewis bases that we have to consider and that compete at the coordination centers of the metal, leading to competitive, non-competitive or uncompetitive activation or inhibition processes in analogy to the terminology of enzyme chemistry... 

Some metal ions participate as acids to assist the enzyme in catalysis. 

The concept presented in Fig. 6 could use also other type of ordered mesoporous membranes, based on silica for example. As discussed before, oxides such as Ti02 provide better multi-functionalities for the design of such a type of nanofactory catalysts. Worth to note is that in the cover picture of the recent US DoE report Catalysis for Energy a very similar concept was reported. This cover picture illustrates the concept, in part speculative, that to selectively convert biomass-derived molecules to fuels and chemicals, it is necessary to insert a tailored sequence of enzyme, metal complexes on metal nanoparticles in a channel of a mesoporous oxide. 

To elucidate the difference between the enzymatic and nonenzymatic participation of metal ions, it is clearly desirable to be able to compare the effect of a large number of metal ions upon the same reaction both in the presence and absence of the enzyme. For such a study to be feasible it is necessary to work with a metal-activated enzymatic reaction, which will also take place when the metal, but not the enzyme, is omitted. Such a reaction is the decarboxylation of oxaloacetic acid. The mechanism of metal catalysis of this reaction is similar to that assumed for carboxypeptidase, and can be represented as follows (44). 

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