Catalytic capability

Many enzymes contain small nonprotein molecules and metal ions that participate directly in substrate binding or catalysis. Termed prosthetic groups, cofactors, and coenzymes, these extend the repertoire of catalytic capabilities beyond those afforded by the limited number of functional groups present on the aminoacyl side chains of peptides. 

The catalytic capabilities of rhenium compounds burst on the scene about one decade ago, featuring MeRe03 as a catalyst for reactions of hydrogen peroxide. It was quickly verified that peroxorhenium(VII) compounds were the active intermediates. With them, practical reactions and fundamental questions of mechanism could then be resolved. 

While the ruthenium component is important and essential, the reasons for its behaviour are not always well understood. We will give some accounts of the chemical and catalytic properties of the ruthenium tricarbonyl triiodide species, [Ru(CO)3l3 ]", which is often the sole derivative detected in catalytic solutions under reaction conditions. This will contribute to a better understanding of the aspects of the catalytic behaviour and of the scope of catalytic capabilities. 

The major product of nitrite reduction is NO (Denariaz et ai, Iwasaki and Matsubara, 1972 Kakutani et al., 1981 Liu et ai, 1986 Masuko et ai, 1984 Sawada et ai, 1978 Zumft et ai, 1987) just as is the case with the cytochrome cd, type enzyme. As discussed previously in this chapter, the Cu-type enzyme from A. cycloclastes has been reported to reduce nitrite to N2O in the presence of NO (Averill and Tiedje, 1990 Jackson et ai, 1991), a catalytic capability apparently not exhibited by heme-type nitrite reductases. 

The oxygen cathode—for which platinum catalyst due to its outstanding structural and catalytic capability is the rule—is not used as an oxygen evolution anode in the electrolyzer operation mode because oxidation of Pt and fast catalyst deterioration would be the consequence. Therefore an oxygen cathode based on a platinum catalyst must operate as a -evolving cathode in the regenerative mode. 

The catalytic capability of Au in liquid-phase oxidation can be tuned to a wider scope by choosing the support, size of Au, alkaline(with or without) and solvents. The products changed dramatically, depending on the solvents reactant alone without solvent, water, nonpolar and polar organic solvents. 

Column studies have confirmed the catalytic capabilities of Pd. Although laboratory tests have indicated some potential issues with deactivation, field studies offer encouraging results for the Pd technology. With periodic regeneration or optimization of the catalyst material, field... 

An Addenda Stereospecificity of Phospholipase A2. Attack on Phosphoglycerides Chiral at Phosphorus. Only within the past 15 years has any attention been paid to the influence of the phosphorus atom in the catalytic capability of phospholipase A2 on a typical phosphoglyceride such as phosphatidylcholine. However, our current understanding of the influence of the phosphorus configuration on phospholipase A2 activity has derived largely from observations made in the laboratory of Tsai and colleagues, who used chirally modified substrates. In this brief description of these exciting advances, the phosphothioate derivatives will be considered. 

Proteins are macromolecules (molecular weights from about 5000 to 106 [3]). There are thousands of different types of proteins, each with a particular biological function, often extremely specific. Thus, a particular enzyme will often recognize only one or a very narrow class of compounds as reactants and catalyze reaction of those to particular products. Other enzymes, typically those that catalyze hydrolysis or other degradative reactions, recognize a particular bond type but will act upon a broad class of reactants (substrates). These protein catalysts typically operate effectively at ambient temperature and pressure. Unique catalytic capabilities give enzymes their niche in bioprocessing. 

Displacing the Essential Metal Ion in Biomolecules. It is estimated that approximately one third of all enzymes require metal as a cofactor or as a structural component. Those that involve metals as a structural component do so either for catalytic capability, for redox potential, or to confer steric arrangements necessary to protein function. Metals can cause toxicity via substitution reactions in which the native, essential metal is displaced/replaced by another metal. In some cases, the enzyme can still function after such a displacement reaction. More often, however, enzyme function is diminished or completely abolished. For example, Cd can substitute for Zn in the protein famesyl protein transferase, an important enzyme in adding famesyl groups to proteins such as Ras. In this case, Cd diminishes the activity of the protein by 50%. Pb can substitute for Zn in 8-aminolevulinic acid dehydratase (ALAD), and it causes inhibition in vivo and in vitro. ALAD contains eight subunits, each of which requires Zn. Another classic example of metal ions substituting for other metal ions is Pb substitution for Ca in bones. 

To explore the feasibility of such an approach for the design of active catalysts, we have systematically replaced the secondary structural elements in the homodimeric helical bundle chorismate mutase (Fig. 3.18) with binary-patterned units of random sequence. Genetic selection was then used to assess the catalytic capabilities of the proteins in the resulting libraries, providing quantitative information about the robustness of this particular protein scaffold and insight into the subtle interactions needed to form a functional active site [119]. 

Key to the Origin of Life - Ribozymes Ribozymes are nucleic acids with catalytic capabilities. The discovery of ribozymes led to the RNA world hypothesis for the origin of life. 

An RNA that exhibits enzyme-Hke activity is called a ribozyme. The discovery of ribozymes had a great impact on research into the origins of life. Identifying catalytic capabilities in RNA, an information molecule, led to a new theory the RNA world hypothesis. This suggests that RNA was the first life form on Earth, and when it first evolved it performed both catalytic and enzymatic functions. The natural selection process associated with evolution eventually caused the RNA to evolve into the highly sophisticated supramolecular systems observed in the complex life forms present today. 

RNA has three basic roles in the cell. First, it serves as the intermediate in the flow of information from DNA to protein, the primary functional molecules of the cell. The DNA is copied, or transcribed, into messenger RNA (mRNA), and the mRNA is translated into protein. Second, RNA molecules serve as adaptors that translate the information in the nucleic acid sequence of mRNA into information designating the sequence of constituents that make up a protein. Finally, RNA molecules are important functional components of the molecular machinery, called ribosomes, that carries out the translation process. As will be discussed in Chapter 2, the unique position of RNA between the storage of genetic information in DNA and the functional expression of this information as protein as well as its potential to combine genetic and catalytic capabilities are indications that RNA played an important role in the evolution of life. 

M. Nakamura, T. Tatsumi, and H.-O. Tominaga, Synthesis and Catalytic Capability of Zeolite-encapsulated Iron and Manganese Tetramethylporphine Complexes. Bull. Chem. Soc. Jpn., 1990, 63, 3334-3336. 

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