Substrate, accessible surface area specificity

Shin and Kim [39] used the accessible surface area of essential amino acid residues of the amine pyruvate aminotransferase and various amino donors and acceptors to explore the active site structure. Their results suggested a model consisting of two pockets, one large and the other small. The size difference between the binding pockets and the strong repulsion for a carboxylate in the small pocket were key determinants of the substrate specificity and stereoselectivity. 

Specific, surface confined reactions not only directly involve catalysis but also the built-up of sdf-assembled multilayers (see Fig. 9.1 (3)) with co-functionalities for more complex (bio-) catalytic systems such as proteins or the directed deposition of active metals. Furthermore, SAM on flat substrates can be used for the study and development of e.g. catalytic systems, but are not useful for large scale applications because they have very limited specific surface. Here, nanoparticle systems covered with 3D-SAMs are the ideal solution of combining the advantages of high surface area, defined surface composition and accessibility of proximal active catalytic centers. 

Despite the high specific surface areas, the amount of accessible catalyst remains low due to the limited thickness of the porous catalytic layer dictated by considerations such as the adhesion to the substrate. The susceptibility of the fine channels to blockage with solid impurities or deposits formed in the reaction, together with the problems of integrating connections with the external macroenvironments and ensuring uniform gas distribution between the individual channels, a prerequisite for numbering up, represent further questions that have to be resolved for the industrial application of microreactors to become practicable. 

The study of both heterogeneous catalysts and enzymes is dominated by the concept of the active site. Specifically, in enzymes the active site is known to represent only a small portion of the large protein molecule that is the enzyme [6], The active site may lie at or near the surface, but it may also be buried in an active site groove or crevice that limits access of all but the desired substrate. Clearly, the total surface area of the protein is significantly larger than that of the active site. 

---