Stereoselective nitrile converting enzymes

Wieser, M. and Nagasawa, T. (2000) Stereoselective nitrile-converting enzymes, in Stereoselective Biocatalysis (ed. R.N. Patel), Marcel Dekker, New York, Chapter 17. 

Arylpropionitriles and -amides have been frequently tested as substrates for stereoselective nitrile-converting enzymes because the acid products, also known as profens, constitute an important class of anti-inflammatory drugs [41] with only the iS) isomer being biologically active [42]. In this class, (5)-naproxen (Figs. 11 and 18), (5)-ibuprofen... 

Among stereoselective nitrile-converting enzymes, the combined action of a stereoselective nitrile hydratase and a stereoselective amidase has been often described, and stereospecific nitrile conversions by whole cells are frequently found in the patent literature [59,60] however, careful analysis has usually revealed stereoselectivity in the amidase and not or only to a low extent in the hydratase [61,62]. If both enzymes were stereosj ific, nitrile hydratase and amidase have been described to act either synergistically or antagonistically regarding their enantioselectivity. 

Amidases catalyze the second step in die two-enzyme step conversion of nitriles to the corresponding acids, the hydrolysis of amides. Among stereoselective nitrile-converting enzymes, the highest enantioselectivities were found in amidases. Amidases have been mostly characterized as thiol-dependent homodimers (aa) sharing amino acid sequence homologies with other amidases. The functional relationship between nitrile hydratases and amidases is structurally documented by the close proximity of their genes [71,72]. In this chapter, stereospecific amidases alone or with nonselective nitrile hydratases are described. 

Besides the use of stereoselective nitrile-converting enzymes as described above, useful chiral building blocks have also been obtained by stereoselective nitrile-forming enzymes. The main product class of nitrile-forming enzymes are cyanohydrins (a-hydroxynitriles, 1-cyanoalkanols), which are versatile synthons in organic synthesis that are readily convertible to a-hydroxy acids [90], a-hydroxy aldehydes [91], ethanolamines [92], amino alcohols, pyrethroid insecticides [93], imidazoles, and heterocycles [94]. Examples of valuable bioactive products derived from chiral cyanohydrins are (i )-adrenaline, L-ephedrin, and (5)-amphetamines [95]. For the synthesis of chiral cyanohydrins, stereoselective enzymes from both plant and bacterial sources have been used. 

Predominantly Rhodococcus species have been isolated as biocatalysts bearing stereoselective nitrile-converting enzymes. It has been assumed that rhodococci generally contain a nitrile hydratase/amidase system [128], but to regard this as a rule might be... 

First enantioselective nitrile conversions were recently industrialized such as the preparation of (jR)-mandelic acid (Fig. 6) or are intended to be industrialized such as the synthesis of (5)-piperazine-2-carboxylic acid (Fig. 31). There are various other commercial processes in which the stereospecific conversion of nitriles is desirable. The increasing number of reports on stereoselective nitrile-converting enzymes in recent years shows that biological mechanisms hold much potential for such processes and indicates that nitrileconverting enzymes might be as useful as esterases and lipases for the synthesis of chiral building blocks. 

---