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Cyclodextrin glycosyltransferase (CGTase) enzymes from various bacteria catalyze the formation of cyclodextrins from starch. The Bacillus stearothermophilus maltogenic a-amylase (G2-amylase is structurally very similar to CGTases, but converts starch into maltose. Comparison of the three-dimensional structures revealed two large differences in the substrate binding clefts. (i) The loop forming acceptor subsite +3 had a different conformation, providing the G2-amylase with more space at acceptor subsite +3, and (ii) the G2-amylase contained a five-residue amino acid insertion that hampers substrate binding at the donor subsites -3/-4 (Biochemistry, 38 (1999) 8385). In an attempt to change CGTase into an enzyme with the reaction and product specificity of the G2-amylase, which is used in the bakery industry, these differences were introduced into Thermoanerobacterium thermosulfurigenes CGTase. The loop forming acceptor subsite +3 was exchanged, which strongly reduced the cyclization activity, however, the product specificity was hardly altered. The five-residue insertion at the donor subsites drastically decreased the cyclization activity of CGTase to the extent that hydrolysis had become the main activity of enzyme. Moreover, this mutant produces linear products of variable sizes with a preference for maltose and had a strongly increased exo-specificity. Thus, CGTase can be changed into a starch hydrolase with a high exo-specificity by hampering substrate binding at the remote donor substrate binding subsites. (C) 2003 Elsevier B.V. All rights reserved.

An acidic chitinase (SE) was found to accumulate in leaves of sugar beet (Beta vulgaris) during infection with Cercospora beticola. Two isoforms, SE1 and SE2, with MW of 29 kDa and pI of approximately 3.0 were purified to homogeneity. SE2 is an endochitinase that also exhibits exochitinase activity, i.e., it is capable of hydrolyzing chito-oligosaccharides, including chitobiose, into N-acetyl-glucosamine. Partial amino acid sequence data for SE2 were used to obtain a cDNA clone by polymerase chain reaction. The clone was used to isolate a cDNA clone encoding SE2. The deduced amino acid sequence for SE2 is 58-67% identical to the class III chitinases from cucumber, Arabidopsis, and tobacco. A transient induction of SE2 mRNA during the early stages of infection with C. beticola is much stronger in tolerant plants than in susceptible plants. Transgenic tobacco (Nicotiana benthamiana) plants constitutively accumulate SE2 protein in the intercellular space of their leaves. In a preliminary infection experiment, the transgenic plants did not show increase in resistance against C. nicotianae.

International audience; The nutrient content of food and animal feed may be improved through new knowledge about enzymatic changes in complex carbohydrates. Enzymatic hydrolysis of complex carbohydrates containing alpha or beta glycosidic bonds is very important in nutrition and in several technological processes. These enzymes are called glycosidases (Enzyme Class 3.2.1) and include amylases, pectinases and xylanases. They are present in many foods such as cereals, but their microbial analogues are often produced and added in many food processes, for instance to improve the shelf-life of bakery products, clear beer, produce glucose, fructose or dextrins, hydrolyse lactose, modify food pectins, or improve processes. However, many plant foods also contain endogenous inhibitors, which reduce the activity of glycosidases, in particular, proteins, peptides, complexing agents and phenolic compounds. The plant proteinaceous inhibitors of glycosidases are in focus in this review whose objective is to report the effect and implications of these inhibitors in industrial processes and applications. These studies will contribute to the optimisation of industrial processes by using modified enzymes not influenced by the natural inhibitors. They will also allow careful selection of raw material and reaction conditions, and future development of new genetic varieties low in inhibitors. These are all new and very promising concepts for the food and feed sector.