Industrial Biotransformations

54
2 The Enzyme Classification
2.2.4
EC 4 Lyases
From a commercial perspective, these enzymes are an attractive group of catalysts as
demonstrated by their use in many industrial processes (see Chapter 4). The reactions
catalyzed are the cleavage of C–C, C–O, C–N and some other bonds. It is important to
mention that this bond cleavage is different from hydrolysis, often leaving unsaturated
products with double bonds that may be subject to further reactions. In industrial processes
these enzymes are most commonly used in the synthetic mode, meaning that the
reverse reaction – addition of a molecule to an unsaturated substrate – is of interest. To
shift the equilibrium these reactions are conducted at very high substrate concentrations,
which results in very high conversions into the desired products. For instance, a specific
type of lyase, the phenylalanine ammonia lyase (EC 4.3.1.5), catalyzes the formation of an
asymmetric C–N bond yielding the l-amino acid dihydroxy-l-phenylalanine (l-DOPA).
This amino acid is produced on a ton scale and with very high optical purities (see the
process on page 460).
Systematic denomination of these enzymes should follow the pattern substrate grouplyase.
To avoid any confusion the hyphen should not be omitted, e.g., the term hydro-lyase
should be used instead of hydrolyase, which actually looks fairly similar to hydrolase.
In the recommended names, terms such as decarboxylase, aldolase or dehydratase
(describing the elimination of CO 2, an aldehyde or water) are used. If the reverse reaction
is much more important, or it is the only one known, the term synthase may be used.
EC 4.1 Carbon–carbon lyases
R 2
R 1
OH
R 3
R5
4
R
R 2
R 1,2,3,4,5 = hydrogen, organic residue
R 1
R 3
H
+
R 4
O
If the substrate is a carboxylic acid, one of the products will be carbon dioxide. If the substrate
is an aldehyde, carbon monoxide could be a product.
EC 4.2 Carbon–oxygen lyases
R 1
H
R 2
OR 5
R4
3
R
R 1,2,3,4,5 = hydrogen, organic residue
R 1
R 2
R 4
R 3
+ HOR 5
R 5