Structural complexity, active sites

Figure 4.6 displays the structure of the complexed active site of citrate synthase complexed with L-malate and acetyl-CoA. This active site is rather solvent exposed... 

Besides these rather complex coenzyme-dependent enzymes, the none-coenzyme requiring protease subtilisin is the most extensively mutated enzyme. The substrate specificity of the enzyme as well as its dependence on pH and its stability were altered by site-directed mutagenesis [72-78]. As the knowledge about exact details of the structure and active site of the enzyme is essential for the application of this method, progress in this field is difficult to achieve. Site-directed mutagenesis as a means of catalyst improvements will be used only after extensive application of conventional optimization procedures. 

The current general understanding of the mechanism operating in cycloolefin metathesis polymerisation leads us towards the acceptance of the structure of active sites in systems with catalysts belonging to the aforesaid three major groups as one that alternates between metal carbene complexes and metallacyclobutanes. 

Due to the complex enzyme structure the active site components are generally far apart in the linear aminoacid sequence. 

As described here, it is understandable that the activity and selectivity of methanol synthesis over Cu-based catalysts depend upon not only the Cu dispersion but also the starting salts for preparation and the residual ions on the catalysts and their effects on the structure of active sites. As the candidate of new catalyst system for methanol synthesis, the following catalysts have been vigorously studied in recent years (1) Cu-ZnO containing new components (2) precious metal such as Pd, Pt, Au, and multicomponent catalysts containing precious metals (3) transition metal except for Cu and their complex oxides (4) homogeneous catalysts consisting of Ru, Ni, etc. 

PEI can be applied as a polymer matrix because the PEI molecules contain amino groups which are capable of bonding with group VIII metal salts. Catalytic properties of coordination compounds of PEI have been thoroughly studied [57-62] The structure or active sites of PEI complexes with nickel, cobalt, rhodium and palladium ions is presented as a five-membered chelate ring ... 

A catalytic antibody raised to bind the hydrophobic dye 2-bromoaceta-mido-l,5-naphthalenedisulfonate was also found to catalyze the Kemp decarboxylation, with a rate acceleration, defined as fecat/feuncat, of 10, which was roughly 1000-fold more than the acceleration brought about by the simple CTAB micelles (fecat was found to be equal to 17 min at pH 8. 0, 20 °C). It was proposed that partitioning of the substrate into the significantly hydrophobic antibody active site resulted in s pificant rate acceleration. However, subsequent structural analyses su ested a more complex active site of the antibody, with an alternation of polar and nonpolar residues. This result is in agreement with the findings in simpler systems, such as the rate acceleration brought about by the simple cationic... 

Enzymes are highly stereospecific in binding chiral substrates and in catalyzing reactions. This stereospecificity arises because enzymes are made of L-amino acids and form asymmetric active sites. Similar to homogeneous and heterogeneous catalysis, chemical reactions proceed on active sites of enzymes, which represent a small part of the total protein. Due to the complex enzyme structure, the active site components are generally far apart in the linear amino acid sequence. 

Noble M E M, R K Wierenga, A-M Lambeir, F R Opperdoes, W H Thunnissen, K H Kalk, H Groendijk and W G J Hoi 1991. The Adaptability of the Active Site of Trypanosomal Triosephosphate Isomerase as Observed in the Crystal Structures of Three Different Complexes. Proteins Structure, Function and Genetics 10 50-69. 

Living systems contain thousands of different enzymes As we have seen all are structurally quite complex and no sweeping generalizations can be made to include all aspects of enzymic catalysis The case of carboxypeptidase A illustrates one mode of enzyme action the bringing together of reactants and catalytically active functions at the active site... 

The Protein Data Bank, PDB ID 1A71. Colby, T. D., Bahnson, B. J., Chin, J. K., Klinman, J. P, Goldstein, B. M., Active Site Modifications in a Double Mutant of Liver Alcohol Dehydrogenase Structural Studies of Two Enzyme-Ligand Complexes. To be published. 

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