ABSTRACT
Leonie Engels and Lothar Elling Laboratory for Biomaterials, Institute of Biotechnology and Helmholtz-Institute for Biomedical Engineering, RWTH Aachen University, Germany L.elling@biotec.rwth-aachen.de
6.1 INTRODUCTIONCarbohydrate molecules are involved in many biological processes. They are found in well-known biomolecules such as glycoproteins, glycolipids, and proteoglycans. Together with glycosylated bioactive natural products, antibiotics, and antitumor drugs they belong to the important class of glycoconjugates. Their complex sugars and oligosaccharides have a great impact on the distinct bioactivity of natural products or the detoxification of xenobiotics, and play an active role as multifunctional information carrier in cell-cell and cell-matrix communication.Studies on these biological processes require the synthesis of glycoconjugates as potential targets for therapeutic or pharmaceutical applications. Due to the structural diversity of glycoconjugates the Carbohydrate-Modifying Biocatalysts Edited by Peter Grunwald Copyright © 2012 Pan Stanford Publishing Pte. Ltd. www.panstanford.com
chemical synthesis of these biomolecules is still challenging because strategies for the protection, reactivity, and deprotection of functional groups results in multiple time-consuming steps and moderate yields. However, significant progress in carbohydrate chemistry has been made by fast-automated oligosaccharide synthesis overcoming some of the aforementioned restrictions (Seeberger, 2009). Together with chemical methods for nucleotide sugar synthesis (Wagner et al., 2009) these approaches demonstrate high flexibility for the chemical synthesis of modified glycoconjugates. In contrast, enzymatic routes are more attractive when synthetic preparative strategies for natural glycoconjugates have to be developed (Hancock et al., 2006; Murata and Usui, 2006; Rich and Withers, 2009; Thayer and Wong, 2007). However, they are dependent on access to Leloir-glycosyltransferases as well as enzymes for the synthesis of their donor substrates, the nucleotide sugars. Leloir glycosyltransferases sequentially transfer monosaccharides from an activated nucleotide sugar to an aglycon or a growing oligosaccharide chain. These enzymes show high substrate specificity and absolute stereo-and regioselectivity for the glycosidic linkage (Lairson et al., 2008). Nucleotide sugars, such as UDP-a-d-glucose, UDP-a-d-glucuronic acid, UDP-a-d-xylose, UDP-a-d-galactose, UDP-N-acetyl-a-d-glucosamine, UDP-N-acetyla-d-galactosamine, GDP-a-d-mannose, GDP-a-l-fucose, and CMP-N-acetyl-b-d-neuraminic acid are mostly required as donor substrates (Fig. 6.1). With the exception of CMP-b-d-Neu5Ac, these compounds derive from precursors directly synthesized from monosaccharide-1-phosphates and nucleoside triphosphate (NTP) substrates (Scheme 6.1). Multienzyme routes for the synthesis of nucleotide sugars have been reported and advanced applications including the combination of nucleotide sugar synthesis or regeneration with glycosyltransferases were established (Rupprath et al., 2005; Sauerzapfe and Elling, 2008, and references cited therein).
