Glycobiology Advance Access originally published online on September 9, 2008
Glycobiology 2008 18(12):1036-1043; doi:10.1093/glycob/cwn083
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Screening a limited structure-based library identifies UDP-GalNAc-specific mutants of 
-1,3-galactosyltransferase
3 Department of Biomedical Science, College of Biomedical Science, Florida Atlantic University, Glades Road, Boca Raton, FL 33431, USA
4 Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
1 To whom correspondence should be addressed: Tel: +1-561-297-0407; Fax: +1-561-297-2221; e-mail: kbrew{at}ad.fau.edu
Received on July 7, 2008; revised on August 21, 2008; accepted on September 2, 2008
Complex glycans have important roles in biological recognition processes and considerable pharmaceutical potential. The synthesis of novel glycans can be facilitated by engineering glycosyltransferases to modify their substrate specificities. The choice of sites to modify requires the knowledge of the structures of enzyme–substrate complexes while the complexity of protein structures necessitates the exploration of a large array of multisite mutations. The retaining glycosyltransferase, 
-1,3-galactosyltransferase (3GT), which catalyzes the synthesis of the -Gal epitope, has strict specificity for UDP-galactose as a donor substrate. Based on the structure of a complex of UDP-galactose with 3GT, the specificity for the galactose moiety can be partly attributed to residues that interact with the galactose 2-OH group, particularly His280 and Ala282. With the goal of engineering a variant of bovine 3GT with GalNAc transferase activity, we constructed a limited library of 456 3GT mutants containing 19 alternative amino acids at position 280, two each at 281 and 282 and six at position 283. Clones (1500) were screened by assaying partially purified bacterially expressed variants for GalNAc transferase activity. Mutants with the highest levels of GalNAc transferase activity, AGGL or GGGL, had substitutions at all four sites. The AGGL mutant had slightly superior GalNAc transferase activity amounting to about 3% of the activity of the wild-type enzyme with UDP-Gal. This mutant had a low activity with UDP-Gal; its crystallographic structure suggests that the smaller side chains at residues 280–282 form a pocket to accommodate the larger acetamido group of GalNAc. Mutational studies indicate that Leu283 is important for stability in this mutant.
Key words: combinatorial library / crystallographic structureglycosyltransferase / mutagenesis / specificity
2 Present address: Faculty of Biosciences and Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia.