The property of loose stereochemical control at aldol products from aldolases helped to synthesize multiple polyhydroxylated compounds with nonnatural stereoconfiguration. In this study, we discovered for the first time that some fructose 1,6-diphosphate aldolases (FruA) and tagatose 1,6-diphosphate (TagA) aldolases lost their strict stereoselectivity when using L-glyceraldehyde and synthesized not only L-sorbose but also a high proportion of L-psicose. Among the aldolases tested, TagA A ldolases are remarkably useful and have been widely investigated for catalyzed COC bond formation (1). The ability to catalyze aldol addition reactions from small chiral polyfunctional molecules and generate up to two new stereogenic centers allows aldolases to synthesize a broad range of both natural and novel polyhydroxylated compounds. Among the aldolase families, dihydroxyacetone-phosphate (DHAP)-dependent aldolases, which utilize DHAP as the donor substrate and accept a broad range of acceptor aldehydes, are widely investigated. Well-known members of this class include fuculose 1-phosphate aldolase (FucA), rhamnulose 1-phosphate aldolase (RhaD), fructose 1,6-diphosphate aldolase (FruA), and tagatose 1,6-diphosphate aldolase (TagA) (2). These enzymes have been used to synthesize several new deoxy or phosphorylated sugars and iminocyclitols (3-5).
DHAP-dependent aldolases create two new stereogenic centers at C atoms 3 and 4 ([3S,4R], [3S,4S], [3R,4S], [3R,4R]). FucA, FruA, RhaD, and TagA generally form (3R,4R), (3S,4R), (3R,4S), and (3S,4S) stereoconfigurations, respectively (1, 2). However, some aldolases sometimes lost their strict stereoselectivity at the C-4 stereocenter with some aldehydes as substrates. For example, RhaD aldolase from Escherichia coli catalyzed DHAP and L-glyceraldehyde to form L-fructose (3R,4S). However, D-sorbose (3R,4S) and D-psicose (3R,4R) were formed when using D-glyceraldehyde (6-8). Moreover, FucA aldolase from E. coli catalyzed DHAP and L-glyceraldehyde to form the single product L-tagatose (3R,4R). However, FucA from Thermus thermophilus HB8 generated L-tagatose (3R,4R) and L-fructose (3R,4S) simultaneously from the same substrates. This finding indicated that aldolases had a high level of stereocontrol at C-3; however, the configuration of the stereocenter at C-4 in some cases depended on the particular enzyme and acceptor structure. This aldolase property can be well utilized to synthesize various types of rare sugars. Thus far, no reports have shown this property for FruA aldolase. In addition, TagA aldolases form diastereomer mixtures of product, in which instead of the natural configuration (3S,4S), those of the configuration (3S,4R) predominated (Ͼ90%) (4).L-Sugars, which are one large group of rare sugars, often hold enormous potential for applications in the pharmaceutical industry (9-11). For example, several L-sugars can be used to produce L-nucleoside analogues, which showed increased antiviral activity, better metabolic stability, and more favorable toxicological profiles (12, 1...