Catalysis/catalysts enzyme role

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

Enzymes are a subset of proteins whose role in life is catalysis. Catalysts are agents that increase the rates of chemical reactions without undergoing chemical change themselves. Let s consider a model reaction m which some substrate, S, is converted into some product, P. We can write this simply as S P. The point is the difference in the rate of this reaction in the absence and presence of a catalyst. If we make the necessary measurements, we will find that the rate m the presence of the catalyst is greater, perhaps very much greater, than in its absence. At the same time, the catalyst. 

The award of the Nobel Prize in Chemistry in 2001 to William R. Knowles and Ryoji Noyori for their work on metal-catalyzed enantioselective hydrogenation reactions and to K. Barry Sharpless for his work on catalyzed enantioselective oxidation reactions was a landmark in chiral catalysis studies. Enzymes and biocatalysts have also played a pivotal role as asymmetric catalysts [16]. 

These catalysts, their structures, modes of action, and uses, are discussed in the rest of the book. Both synthetic small-molecule catalysts as well as some of Nature s finest enzymes are discussed and the role of hydrogen bonding in catalysis is described in detail. 

Frequently enzymes act in concert with small molecules, coenzymes or cofactors, which are essential to the function of the amino acid side chains of the enzyme. Coenzymes or cofactors are distinguished from substrates by the fact that they function as catalysts. They are also distinguishable from inhibitors or activators in that they participate directly in the catalyzed reaction. Chapter 10, Vitamins and Coenzymes, starts with a description of the relationship of water-soluble vitamins to their coenzymes. Next, the functions and mechanisms of action of coenzymes are explained. In the concluding sections of this chapter, the roles of metal cofactors and lipid-soluble vitamins in enzymatic catalysis are briefly discussed. 

In this chapter we focus on supramolecular chemical reactivity. In particular this means predominantly the role supramolecular chemistry plays in accelerating or understanding chemical reactions. There are close parallels between artificial, abiotic supramolecular reactivity and biochemistry, for example in the study of enzymes, Nature s catalysts - described in Section 2.6. Synthetic catalysts can both model natural ones and allow the design of new, different kinds of reactions. Supramolecular catalysis sits somewhere between chemical catalysis (transition metal and organocatalysis) and biology. Some considerations within various kinds of catalysis are summed up in the chart shown in Figure 12.1. 

The area between enzymatic and chemical catalyses, associated with simulation of biochemical processes by their basic parameters, is accepted as mimetic catalysis. The key aspect of the mimetic catalyst is diversity of enzyme and biomimetic function processes, which principally distinguishes the mimetic model from traditional full simulation. Based on the analysis of conformities and diversities of enzymatic and chemical catalysis, the general aspects of mimetic catalysis are discussed. An idealized model of the biomimetic catalyst and the exclusive role of the membrane in its structural organization are considered. The most important achievements in the branch of catalysis are shown, in particular, new approaches to synthesis and study of biomimetic catalase, peroxidase and monooxidases reactions. 

Apart from enzymes and catalytic antibodies, chemical catalysis has been shown to be of utility for such a transformation by using either metal-containing complexes or purely organic molecules. In both cases, the catalyst plays a key role in activating both the pronucleophilic carbonyl compound and the electrophilic aldehyde it also imparts effective stereoinduction. The most representative metal-ligand complexes and organocatalysts yet reported in this context [7] are depicted in Figs. 1 and 2, respectively. 

This new style of synthetic catalysis will of course not replace all normal synthetic methods. For many purposes, the standard methods and rules - e.g. aldehydes are more easily reduced than are ketones - will continue to dominate organic synthesis. However, when we require a synthetic transformation that is not accessible to normal procedures, as in the functionalization of unactivated carbons remote from functional groups, artificial enzymes can play a role. They must compete with natural enzymes, and with designed enzyme mutants, but for practical large-scale industrial synthesis there can be advantages with catalysts that are more rugged than proteins. 

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