Glycosyltransferase

Glycosyltransferases (GTFs, Gtfs) are enzymes (EC 2.4) that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar (also known as the 'glycosyl donor') to a nucleophilic glycosyl acceptor molecule, the nucleophile of which can be oxygen- carbon-, nitrogen-, or sulfur-based.Glycosyltransferases can be segregated into 'retaining' or 'inverting' enzymes according to whether the stereochemistry of the donor's anomeric bond is retained (α→α) or inverted (α→β) during the transfer. The inverting mechanism is straightforward, requiring a single nucleophilic attack from the accepting atom to invert stereochemistry.The recent discovery of the reversibility of many reactions catalyzed by inverting glycosyltransferases served as a paradigm shift in the field and raises questions regarding the designation of sugar nucleotides as 'activated' donors.Sequence-based classification methods have proven to be a powerful way of generating hypotheses for protein function based on sequence alignment to related proteins. The carbohydrate-active enzyme database presents a sequence-based classification of glycosyltransferases into over 90 families. The same three-dimensional fold is expected to occur within each of the families.In contrast to the diversity of 3D structures observed for glycoside hydrolases, glycosyltransferase have a much smaller range of structures. In fact, according to the Structural Classification of Proteins database, only three different folds have been observed for glycosyltransferases Very recently, a new glycosyltransferase fold was identified for the glycosyltransferases involved in the biosynthesis of the NAG-NAM polymer backbone of peptidoglycan.Many inhibitors of glycosyltransferases are known. Some of these are natural products, such as moenomycin, an inhibitor of peptidoglycan glycosyltransferases, the nikkomycins, inhibitors of chitin synthase, and the echinocandins, inhibitors of fungal b-1,3-glucan synthases. Some glycosyltransferase inhibitors are of use as drugs or antibiotics. Moenomycin is used in animal feed as a growth promoter. Caspofungin has been developed from the echinocandins and is in use as an antifungal agent. Ethambutol is an inhibitor of mycobacterial arabinotransferases and is used for the treatment of tuberculosis. Lufenuron is an inhibitor of insect chitin synthases and is used to control fleas in animals.The ABO blood group system is determined by what type of glycosyltransferases are expressed in the body.Glycosyltransferases have been widely used in the both targeted synthesis of specific glycoconjugates as well as the synthesis of differentially glycosylated libraries of drugs, biological probes or natural products in the context of drug discovery and drug development (a process known as glycorandomization). Suitable enzymes can be isolated from natural sources or produced recombinantly. As an alternative, whole cell-based systems using either endogenous glycosyl donors or cell-based systems containing cloned and expressed systems for synthesis of glycosyl donors have been developed. In cell-free approaches, the large-scale application of glycosyltransferases for glycoconjugate synthesis has required access to large quantities of the glycosyl donors. On the flip-side, nucleotide recycling systems that allow the resynthesis of glycosyl donors from the released nucleotide have been developed. The nucleotide recycling approach has a further benefit of reducing the amount of nucleotide formed as a by-product, thereby reducing the amount of inhibition caused to the glycosyltransferase of interest – a commonly observed feature of the nucleotide byproduct.