There have been a number of attempts to redesign these enzymes to use the non-phosphorylated

donor, dihydroxyacetone (DHA), by using directed evolution [25] or rational methods using point mutations to redesign the phosphate binding pocket [26•]. In this respect fructose-6-phosphate aldolase (FSA) is of great interest as it has been shown to utilize multiple donor substrates such as dihydroxyacetone (DHA), hydroxyacetone and hydroxybutanone [27]. FSA also provides a route to the production of iminocyclitols which are attractive drug candidates [28]. FSA has been the subject of many studies to alter its substrate specificity Cisplatin manufacturer for different acceptor aldehydes and to increase its affinity for the specific donor DHA [29• and 30]. Another enzyme that uses DHA rather than DHAP is transaldolase (Tal) and, interestingly, FSA activity has been conferred on this enzyme by replacement of a single phenylalanine by tyrosine (F178Y) in the active site [31]. This F178Y variant has also been the subject of further study to increase its activity

with non-phosphorylated acceptor aldehydes. Structure-guided mutagenesis identified residues in the phosphate binding pocket that, when mutated, prevent phosphorylated acceptors from binding. This has produced an enzyme that can synthesize polyhydroxylated, non-phosphorylated compounds and be used in enzymatic cascade synthesis of this type of compound [32]. Many enzymes have Sorafenib solubility dmso been shown to have catalytic promiscuity and as well as Thymidylate synthase using engineering to subvert the substrate specificity of natural aldolases, attempts are now being made to enhance the catalytic promiscuity of other enzyme classes to produce novel aldolases. An early example of the conversion of one enzyme activity into another

type of reaction was the conversion of an alanine racemase into an aldolase by a single active site point mutation [33]. This variant enzyme catalysed a reaction similar to threonine aldolase with rates and specificities comparable with the native enzyme. More recently 4-oxalocrotonate tautomerase (4-OT) was shown to be promiscuous in having low aldolase activity towards the condensation of acetaldehyde and benzaldehyde to yield cinnamaldehyde. This low activity has been enhanced by a single point mutation, F50A, which increased the kcat/KM for the aldolase activity by 600-fold compared to that of the wild-type [ 34•]. Lipases have also been reported to display promiscuous aldolase activity [35 and 36] and recently asymmetric aldol reactions between acetone and 4-nitrobenzaldehyde (catalysed by porcine pancreas lipase) [37] and aromatic and heteroaromatic aldehydes with cyclic ketones (catalysed by chymopapain, nuclease p1, alkaline protease BLAP and acidic protease AUAP) [38 and 39] have been described.