Glycosyltransferase

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Glycosyltransferases are an abundant, ubiquitous group of </span>enzymes involved in the catalysis of glycosidic bond formation. There are many different types of glycosyltransferases, explaining the fact that 1-2% of gene products are glycosyltransferases. The variety of glycosyltransferase is necessary because it must be specific to both the donor and acceptor molecules involved in the reaction and the type of glycosidic bond produced [1]

These enzymes are classified into 97 different families [2] according to amino acid sequence similarities. Despite the proteins’ differences in primary structure their tertiary structures are more conserved with common three-dimensional folds [3].

Glycosidic bonds form between simple sugars to form more complex carbohydrate structures called polysaccharides or oligosaccharides. The sugar to be added to the growing carbohydrate structure is in the form of an activated sugar nucleotide. This is where the sugar is bonded to a diphosphate nucleotide and it is called the glycosyl donor. The glycosyl acceptor can be a carbohydrate, a Serine, Threonine or Asparagine residue of a protein to produce a glycoprotein, a lipid to produce a glycolipid or a nucleic acid [4].

Glycosyltransferase involvement in determining blood groups

There are four human blood groups: A, B, O and AB. The antigen, an oligosaccharide, carried on the surface of erythrocytes, determines this. These antigens are part of the immune system’s recognition mechanism. Cells carrying the wrong antigen will be recognised as foreign and destroyed.

Everyone possesses the core O antigen but some inherit alleles, which encode different glycosyltransferases. These glycosyltransferases add specific sugars to the core O antigen, giving the blood groups A (N-acetylgalactosamine is added through an α-1, 3-glycosidic bond) or B (galactose is added through an α-1, 3-glycosidic bond).

Blood group O is caused by a recessive allele, which carries a mutation, resulting in the premature termination of translation. This non-functional gene product explains why homozygous recessive individuals possess only the core O antigen. The alleles encoding the glycosyltransferases giving rise to blood groups A and B are dominant to the O allele but co-dominant to one another. This means inheritance of both alleles encoding the glycosyltransferases for A and B antigens results in the blood type AB. A and B antigens are both present on the surface of erythrocytes [5].

References

  1. Berg J.M., Tymoczko and Stryer L. (2012). Biochemistry 7th Edition, p. 345. Basingstoke: W. H. Freeman and Company.
  2. CAZY, Carbohydrate Active Enzyme. 'GlycosylTransferase family classification' (2014). www.cazy.org/GlycosylTransferase.html [Last accessed 24/11/14]
  3. Breton C., Lanka s., Jeanneau C., Koca J. and Imberty A. (2005). 'REVIEW Structures and mechanisms of glycosyltransferases' Glycobiology vol. 16, no. 2, p. 29R.
  4. Berg J.M., Tymoczko and Stryer L. (2012). Biochemistry 7th Edition, p. 345. Basingstoke: W. H. Freeman and Company.
  5. Berg J.M., Tymoczko and Stryer L. (2012). Biochemistry 7th Edition, p. 345. Basingstoke: W. H. Freeman and Company.
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