A high-throughput pH indicator assay for screening glycosyltransferase saturation mutagenesis libraries.

Protein engineering using directed evolution or saturation mutagenesis at hot spots is often used to improve enzyme properties such as their substrate selectivity or stability. This requires access to robust high-throughput assays to facilitate the analysis of enzyme libraries. However, relatively few studies on directed evolution or saturation mutagenesis of glycosyltransferases have been reported in part due to a lack of suitable screening methods. In the present study we report a general screening assay for glycosyltransferases that has been developed using the blood group alpha-(1-->3)-galactosyltransferase (GTB) as a model. GTB utilizes UDP-Gal as a donor substrate and alpha-L-Fucp-(1-->2)-beta-D-Galp-O-R (H antigen) as an acceptor substrate and synthesizes the blood group B antigen alpha-D-Galp-(1-->3)-[alpha-L-Fucp-(1-->2)]-beta-D-Galp-O-R. A closely related alpha-(1-->3)-N-acetylgalactosaminyltransferase (GTA) uses UDP-GalNAc as a donor with the same H acceptor, yielding the A antigen alpha-D-Galp-NAc-(1-->3)-[alpha-L-Fuc(1-->2)]-beta-D-Gal-O-R. GTA and GTB are highly homologous enzymes differing in only 4 of 354 amino acids, Arg/Gly-176, Gly/Ser-235, Leu/Met-266, and Gly/Ala-268. The screening assay is based on the color change of the pH indicator bromothymol blue when a proton is released during the transfer of Gal/GalNAc from UDP-Gal/UDP-GalNAc to the acceptor substrate. Saturation mutagenesis of GTB enzyme at M214, a hot spot adjacent to the (211)DVD(213) metal binding motif, was performed and the resulting library was screened for increases in UDP-GalNAc transfer activity. Two novel mutants, M214G and M214S, identified by pH indicator screening, were purified and kinetically characterized. M214S and M214G both exhibited two-fold higher k(cat) and specific activity than wild-type GTB for UDP-GalNAc. The results confirm the importance of residue M214 for donor enzyme specificity.