Oxynitrilases substrate acceptance

The Sorghum (S)-oxynitrilase exclusively catalyzes the addition of hydrocyanic acid to aromatic aldehydes with high enantioselectivity, but not to aliphatic aldehydes or ketones [519, 526], In contrast, the Hevea (S)-oxynitrilase was also found to convert aliphatic and a,/ -unsaturated substrates with medium to high selectivity [509, 527]. The stereocomplementary almond (R)-oxynitrilase likewise has a very broad substrate tolerance and accepts both aromatic, aliphatic, and a,/ -unsaturated aldehydes [520, 521, 523, 528, 529] as well as methyl ketones [530] with high enantiomeric excess (Table 9). It is interesting to note that this enzyme will also tolerate sterically hindered substrates such as pivalaldehyde and suitable derivatives 164 which are effective precursors for (R)-pantolactone 165 [531],... 

The nitrilase mediated DKR route to enantiomerically pure 2-hydroxycarboxylic acids is restricted to the (R)-enantiomers because, to our knowledge, no (S)-selec-tive nitrilases for cyanohydrin substrates are commonly available [11]. We reasoned that a fully enzymatic route to the (S)-acids should be possible by combining an (S)-selective oxynitrilase (hydroxynitrile lyase, EC 4.1.2.10, (S)-hydroxynitrile lyase) and a non-selective nitrilase in a bienzymatic cascade (see Figure 16.3). Besides being more environmentally acceptable than chemical hydrolysis, the mild reaction conditions of the combined enzymatic reaction would be compatible with a wide range of hydrolysable groups. 

Interesting new enzymes can of course also be isolated from non-microbial sources. A recent example of a useful novel enzyme or plant origin is the peptide amidase from orange flavedo (Fig. 5) discovered by Steinke and Kula [130, 131]. This enzyme, which has an extremely wide substrate range, is useful for C-terminal enzymatic deprotection in peptide synthesis under very mild conditions. Substrates of this peptide amidase are protected and unprotected peptide amides, N-protected amino acid amides. The enzyme is stereoselective with regard to the C-terminal position only L-amino acid amides are accepted as substrates, with the exception of proline. Other interesting enzymes of plant origin are the oxynitrilases (see below). 

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