Glycosyltransferase



Glycosyltransferases are enzymes (EC 2.4) that act as a catalyst for the transfer of a monosaccharide unit from an activated nucleotide sugar (also known as the "glycosyl donor") to an Glycosyl acceptor molecule, usually an alcohol.

The result of glycosyl transfer can be a carbohydrate, glycoside, oligosaccharide, or a polysaccharide. Some glycosyltransferases catalyse transfer to inorganic phosphate or water. Glycosyl transfer can also occur to protein residues, usually to tyrosine, serine, or threonine to give O-linked glycoproteins, or to asparagine to give N-linked glycoproteins. Mannosyl groups may be transferred to tryptophan to generate C-mannosyl tryptophan, which is relatively abundant in eukaryotes. Transferases may also use lipids as an acceptor, forming glycolipids, or even lipid-linked sugar phosphate donors, such as dolichol phosphates.

It is common that sugar nucleotide derivatives are used as glycosyl donors. Glycosyltransferases that use sugar nucleotides are called Leloir enzymes, after Luis F. Leloir, the scientist who discovered the first sugar nucleotide and who received the 1970 Nobel Prize in Chemistry for his work on carbohydrate metabolism.

Glycosyltransferases that utilize non-nucleotide donors, which may be polyprenol pyrophosphates, polyprenol phosphates, sugar-1-phosphates, or sugar-1-pyrophosphates, are termed non-Leloir glycosyltransferases. Such non-Leloir enzymes occur in a variety of organisms.

Mechanism
Glycosyltransferases, by analogy with glycoside hydrolases, can catalyze the transfer of a glycosyl moiety with either retention or inversion of anomer. Glycosyltransferases are usually metal ion-dependent, with metals such as magnesium or manganese being found in the active site and acting as a Lewis acid by binding to the (di)phosphate leaving group.

Mammals utilize only 9 sugar nucleotide donors for glycosyltransferases: UDP-glucose, UDP-galactose, UDP-GlcNAc, UDP-GalNAc, UDP-xylose, UDP-glucuronic acid, GDP-mannose, GDP-fucose, and CMP-sialic acid. Other organisms have an extensive range of nucleotide sugar donors. Many glycosyltransferases in a number of organisms use lipid-linked glycosyl donors wherein the lipid is frequently a terpenoid such as dolichol or polyprenol.

Classification by sequence
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.

Structure
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.

Inhibitors
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. Moenimycin 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.

Determinant of blood type
The ABO blood group system is determined by what type of glucosyltransferases are expressed in the body.

The ABO gene locus expressing the glucosyltransferases has three main alleleic forms: A, B, and O. The A allele encodes a glycosyltransferase that bonds α-N-acetylgalactosamine to D-galactose end of H antigen, producing the A antigen. The B allele encodes a glycosyltransferase that joins α-D-galactose bonded to D-galactose end of H antigen, creating the B antigen. In case of O allele the exon 6 contains a deletion that results in a loss of enzymatic activity. The O allele differs slightly from the A allele by deletion of a single nucleotide - Guanine at position 261. The deletion causes a frameshift and results in translation of an almost entirely different protein that lacks enzymatic activity. This results in H antigen remaining unchanged in case of O groups.

The combination of glucosyltransferases by both alleles present in each person determines whether there is a AB, A, B or O blood type.

Uses
Glycosyltransferases have been widely used in the synthesis of glycoconjugates. Suitable enzymes can be isolated from natural sources or produced recombinantly. As an alternative, whole cell-based systems utilizing 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.