Catalytic reactions biocatalysis

Abstract This chapter discusses the potential usefulness of ionic liquids with respect to biocatalysis by illustrating the stability and activity of enzymes in ionic liquids in the presence or absence of water. Ionic liquids are a class of coulombic fluids composed of organic cations like alkyl-substituted imidazolium, pyrrolidin-ium, and tetraalkylammonium ions and anions such as halides, tetrafluoroborates, hexafluorophosphates, tosylates, etc. The possibility of tunable solvent properties by alternation of cations and anions has made ionic liquids attractive to study biocatalysis which warrants an understanding of enzyme stability and activity in ionic liquids. This chapter systematically outlines the recent studies on the stability of enzymes and their reactivity toward a wide range of catalytic reactions. A careful approach has been taken toward analysis of relationship between stabil-ity/activity of enzymes versus chaotropic/kosmotropic nature of cations and anions of ionic liquids. 

Catalysis is one of the most important and rapidly expanding areas in modem organic chemistry. Catalytic reactions can be achieved by either chemocatalysis or biocatalysis. The former field is dominated by transition metal catalysis, whereas in biocatalysis the use of isolated enzymes dominates over whole-cell transformations. 

Biocatalysis Chemical reactions mediated by biological systems (microbial communities, whole organisms or cells, cell-free extracts, or purified enzymes aka catalytic proteins). 

In the enzyme design approach, as discussed in the first part of this chapter, one attempts to utilize the mechanistic understanding of chemical reactions and enzyme structure to create a new catalyst. This approach represents a largely academic research field aiming at fundamental understanding of biocatalysis. Indeed, the invention of functional artificial enzymes can be considered to be the ultimate test for any theory on enzyme mechanisms. Most artificial enzymes, to date, do not fulfill the conditions of catalytic efficiency and price per unit necessary for industrial applications. 

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. 

In each of these areas the relative merits of biocatalysis versus other catalytic methodologies will be assessed. Note that the text is given an asterisk ( ) when mention is made of a catalyst for a reduction or oxidation reaction that is featured in the later experimental section of this book. 

The notion that RNA existed prior to enzymes and that RNA molecules can be catalytically active is by now well accepted. Indeed, RNA molecules have been observed to catalyze phophodiester bond breaking and synthesis (as occurs during replication and splicing) but also reactions more distant to its stmcture, such as amide bond formation (Wiegand, 1997). RNA thus provided the means to assemble peptides which may have led to the protein world of today (Zhang and Cech, 1998). Reactions catalyzed by RNA molecules have thus far not been employed in biocatalysis and it is unlikely that... 

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