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General Comment |
Yada T, et al 2003 reported that Chondroitin Sulfate Synthase ? 2,the Molecular cloning and characterization of a novel human glycosyltransferase homologous to chondroitin sulfate glucuronyltransferase, which has dual enzymatic activities.
Chondroitin sulfate is found in a variety of tissues as proteoglycans and consists of repeating disaccharide units of N-acetylgalactosamine and glucuronic acid residues with sulfate residues at various places. The authors found a novel human gene (GenBank AB086063) which possesses a sequence homologous with the human chondroitin sulfate glucuronyltransferase gene. The full-length open reading frame encodes a typical type II membrane protein comprising 775 amino acids The protein had a domain containing beta3-glycosyltransferase motif, but lacked a typical beta4-glycosyltransferase motif, which is the same as chondroitin sulfate glucuronyltransferase, while chondroitin synthase had both domains. The putative catalytic domain was expressed in COS-7 cells as a soluble enzyme. Surprisingly, both glucuronyltransferase and N-acetylgalactosaminyltransferase activities were observed when chondroitin, chondroitin sulfate, and their oligosaccharides were used as the acceptor substrates. The reaction products were identified to have the linkage of GlcAbeta1-3GalNAc and GalNAcbeta1-4GlcA at the non-reducing terminus of chondroitin for glucuronyltransferase activity and N-acetylgalactosaminyltransferase activity, respectively. Quantitative real-time PCR analysis revealed that the transcripts were ubiquitously expressed in various human tissues but highly expressed in the pancreas, ovary, placenta, small intestine, and stomach. These results indicate that this enzyme could synthesize chondroitin sulfate chains as a chondroitin sulfate synthase which has both glucuronyltransferase and N-acetylgalactosaminyltransferase activities. Sequence analysis based on three-dimensional structure revealed the presence of not typical but significant beta4-glycosyltransferase architecture.
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