Leloir transferases

The second prerequisite of the Leloir-transferases are the necessary donor substrates which are nucleotide mono- or diphosphate sugars. Particularly in micro organisms, there seems to be an almost unlimited number of donor sugars occurring in Nature. However, mammals use only a highly restricted set of donors. The description of the preparative use of mammalian transferases in combination with non-natural donor sugars and non-natural acceptor substrates is the main topic of this article. 

In mammalian systems, the Leloir-transferases play the central role in the biosynthesis of glycosidic bonds [33]. These enzymes transfer a monosaccharide unit from a nucleotide activated donor regio- and stereoselectively onto a specific OH-group of an acceptor sugar (Fig. 1). 

Fig. 1. General scheme for enzymatic glycosylations by Leloir-transferases...

Sucrase-type (non-Leloir-type) enzymes that operate both regio- and stereo-selectively, using sucrose as a cheap substrate, or, in some cases (such as cyclodextrin (CD) transferases) starch these enzymes are, however, limited to the transfer of only glucose or fructose... 

Both transferases from the Leloir and non-Leloir pathways have been employed for the in vitro synthesis of oligosaccharides and glycoconjugates. Glycosidases have also been exploited for synthesis. The function of glycosidases in vivo is to cleave glycosidic bonds,... 

Glycosyl transferases are essential enzymes for oligosaccharide biosynthesis. These enzymes can be classified as enzymes of the Leloir pathway and those of the non-Leloir pathway. The glycosyltransferases of the Leloir pathway are involved in the biosynthesis of most iV-linked... 

Highly selective enzymatic synthesis using Leloir glycosyl transferases (GTF), but restricted due to limited availability of enzymes, and nucleotide activated substrates at high prices... 

Some transferases can directly use and transform oligosaccharides as donors without further activation. The following paragraph will discuss some reactions with non-Leloir glycosyltransferases which are performed on a preparative level. For example, cyclodextrin-al-4-glucosyltransferase (CGT) from Klebsiella pneumonia or from Bacillus macerans are rather versatile enzymes which can catalyze various reactions. 

Inulin is not biosynthesised by a Leloir pathway involving a nucleotide mono- or diphospho sugar, but by transglyeosylating enzymes of GH 32 and GH 68 acting on sucrose. Only two enzymes are involved, a sucrose-sucrose 1-fructosyl transferase (1-SST), whieh produees the primer trisaccharide 1-kestose, and fructan 1-fruetosyltransferase (1-FFT), whieh exchanges... 

Recondo and Leloir " have found that adenosine 5-(i>-glucosyl pyrophosphate) will also act as a glucosyl donor for starch-UDPglucosyl transferase and that the rate of transfer to starch is about 10 times that with uridine 5-(n-glucosyl pyrophosphate). The transferred glucose is present... 

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