Chemoenzymatic Syntheses of Linear and Branched Hemithiomaltodextrins as Potential Inhibitors for Starch-Debranching Enzymes

Oligosaccharides embodying the S‐maltosyl‐6‐thiomaltosyl structure have been readily synthesised by using convergent chemoenzymatic approaches. The key steps for the preparation of these molecules involved: 1) transglycosylation reactions of maltosyl fluorides onto suitable acceptors catalysed by the bacterial transglycosylase, cyclodextrin glycosyltransferase (CGTase), and 2) the SN2‐type displacement of a 6‐halide from acetylated acceptors by activated 1‐thioglycoses. The target molecules, which were obtained in good overall yields, proved to be useful for investigating substrate binding in the active sites of several enzymes that act upon the α‐1,6‐linkage of pullulan and/or amylopectin. The compounds exhibit Ki values in the 2.5–1350 μM range with the different enzymes, and the highest affinity found by using these molecules was seen for the pullulanase from Bacillus acidopullulyticus. Both barley‐malt limit dextrinase and pullulanase type II from Thermococcus hydrothermalis only recognised the longest linear thiooligosaccharide, while a branched heptasaccharide was the strongest inhibitor of pullulanase from Klebsiella planticola. Enzymatic and chemical combinatorial assembly of few disaccharidic synthons allowed the preparation of 6‐thiooligosaccharides, analogues to the products resulting from the enzymatic degradation of amylopectin (see cartoon).