ENZYME-CATALYZED REACTIONS IN CONCENTRATE D SUCROSE SOLUTIONS - HYDROLYSIS AND TRANSFER

The wide variety of available enzymes acting on sucrose allows to use this sugar as well as a substrate for hydrolysis reactions or as a sub strate for synthesis reactions involving glycosyl residue transfer. Su crose hydrolysis is catalyzed by invertase. When this enzyme is operat ed under concentrated sucrose solution conditions (up to 2.7 mol/l), t wo unexpected phenomena can be noticed: the kinetic behaviour of inver tase, and particularly its susceptibility to excess substrate inhibiti on, is related to a modification of the structure of the sucrose molec ule occuring when sucrose concentration is increased; invertase stabil ity is greatly enhanced in concentrated sucrose solutions. This observ ation has led to a deepened study of the mechanism of enzyme stabiliza tion by various additives. Synthesis reactions are obtained by transfe r of D-glucosyl or D-fructosyl residues from sucrose acting as a donor . This reaction can be catalyzed by several enzymes: dextransucrase, a lternansucrase, amylosucrase, etc. These enzymes are able to synthesiz e as well high molecular weight carbohydrate polymers as controlled lo w molecular weight oligosaccharides, when efficient ''acceptor'' molec ules are added to the reaction medium. Dextransucrase (DS), for exampl e, catalyzes the synthesis of glucooligosaccharides (GOS) in the prese nce of maltose as acceptor. Such oligosaccharides either contain only additional alpha(1-->6) osidic bonds (DS from Leuconostoc mesenteroide s B-512F) or mixed alpha(1-->6) and alpha(1-->2) bonds (DS from L. mes enteroides B-1299). The presence of such alpha(1-->2) bonds at the GOS non reducing end results in a very high resistance to the attack of b oth human and animal digestive enzymes. In addition, such GOS are spec ifically metabolized by the intestinal or cutaneous (BioEcolia(R)) sap rophyte microbial flora. Structure/function relationships are under st udy at the level of D-glucosyltransferases in view to develop new and efficient enzymatic tools for stereo- and regio-selective glycosylatio n.