Active site group theories

Uncovering of the three dimentional structure of catalytic groups at the active site of an enzyme allows to theorize the catalytic mechanism, and the theory accelerates the designing of model systems. Examples of such enzymes are zinc ion containing carboxypeptidase A 1-5) and carbonic anhydrase6-11. There are many other zinc enzymes with a variety of catalytic functions. For example, alcohol dehydrogenase is also a zinc enzyme and the subject of intensive model studies. However, the topics of this review will be confined to the model studies of the former hydrolytic metallo-enzymes. 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

Table 5.2 compares the dissolution rate of various Al-minerals. The differences are remarkable. At pH = 3, the half life of surface sites of different aluminum (hydroxides varies from 2 years (corundum) to 20 hours (bayerite). The large difference in rates must be due to different coordinative arrangements of the active surface groups. Although no detailed theory is available, it is perhaps reasonable to assume, that the dissolution rate increases with the frequency of surface groups which be present as endstanding =AI-OH groups. 

As already mentioned, choosing the active space for CASSCF calculations is not always a trivial matter. In the systems under consideration, there is a plane of symmetry (that of the phenylene linker), which helps in classifying the MOs as CT and tt (or A and A", using group-theory notation). Experience shows that a reasonably balanced active space is made of the -ir system of the linker and one CT-orbital and one -ir-orbital per reactive site (carbene or nitrene) (Fig. 2). 

In the framework of general BRC theory in the example of PPFe3+0H/Al203, the unified picture of two-proton transfer to acidic-basic groups of the carrier (A1203) with electron transfer to the active site (PPFe3+OH) is observed. Finally, the substrate is redox converted. It is typical that in enzymatic catalysis conditions without acidic-basic groups redox processes are suppressed. 

Thus, the probable mechanism of catalytic redox transformation of the substrate within one catalytic domain is described in the framework of the BRC theory. It is implemented owing to two-proton transfer to the acidic-basic carrier (A1203) groups with electron transfer to active site perFTPhPFe(III)OH. In this context, of special attention are data on studies of qualitative and quantitative parameters of surface acidic-basic sites of inorganic matrices, including A1203 [8],... 

The Marcus Theory can also be applied for heterogeneous electron transfer reaction at electrode surfaces [24 and references therein]. The electronic coupling between the protein and the electrode can be varied using different self-assembled monolayers controlling the orientation of the redox active protein on the surface and the distance between the redox active site of the protein and the electrode. The driving force is related to the appHed potential and the redox potential of the protein. In many cases the rate of electron transfer across the protein-electrode interface is limited by conformational reorganization. This has focussed the efforts of many groups on tailored interaction between proteins and enzymes and electrode surfaces. 

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