期刊
APPLIED MICROBIOLOGY AND BIOTECHNOLOGY
卷 102, 期 2, 页码 751-761出版社
SPRINGER
DOI: 10.1007/s00253-017-8639-0
关键词
Chondroitin; Escherichia coli; UDP-N-Acetylglucosamine/glucose-4-epimerase; Substrate specificity; Catalytic mechanism
资金
- National Natural Science Foundation of China [31770845]
- Science and Technology Department of Shandong Province [2015GSF121002, 2015ZDJS04002]
Capsule of Escherichia coli O5:K4:H4 is formed of a chondroitin-repeat disaccharide unit of glucuronic acid (GlcA)-N-acetylgalactosamine (GalNAc). This polysaccharide, commonly referred to as K4CP, is a potentially important source of precursors for chemoenzymatic or bioengineering synthesis of chondroitin sulfate. KfoA, encoded by a gene from region 2 of the K4 capsular gene cluster, shows high homology to the UDP-glucose-4-epimerase (GalE) from E. coli. KfoA is reputed to be responsible for uridine 5'-diphosphate-N-acetylgalactosamine (UDP-GalNAc) supply for K4CP biosynthesis in vivo, but it has not been biochemically characterized. Here, we probed the substrate specificity of KfoA by a capillary electrophoresis (CE)-based method. KfoA could epimerize both acetylated and non-acetylated substrates, but its k (cat)/K (m) value for UDP-GlcNAc was approximately 1300-fold that for UDP-Glc. Recombinant KfoA showed a strong preference for acetylated substrates in vitro. The conclusion that KfoA is a higher efficiency UDP-GalNAc provider than GalE was supported by a coupled assay developed based on the donor-acceptor combination specificity of E. coli K4 chondroitin polymerase (KfoC). Furthermore, residue Ser-301, located near the UDP-GlcNAc binding pocket, plays an important role in the determination of the conversion ratio of UDP-GlcNAc to UDP-GalNAc by KfoA. Our results deepen the understanding of the mechanism of KfoA and will assist in the research into the metabolic engineering for chondroitin sulfate production.
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