Figure 2. Structural features of peptide and lectin domain-binding sites.

(a) View (left panel) of complete sugar nucleotide, peptide and lectin domain-binding sites of the GalNAc-T2-UDP-MUC5AC-13 complex. Close-up view (right panel) of peptide and lectin domain-binding sites. The residues forming sugar-nucleotide, peptide and lectin domain-binding sites are depicted as black, yellow and grey carbon atoms, respectively. UDP and the glycopeptide are shown as brown and green carbon atoms, respectively. Mn⁺² and GalNAc moiety are depicted as a pink sphere and orange carbon atoms, respectively. Hydrogen bond interactions are shown as dotted green lines. (b) Close-up view of the peptide-binding site of the GalNAc-T2-UDP-EA2 complex. Colours are the same as above. (c) Graph that shows the velocity for the wild-type (WT) enzyme and mutants with peptides MUC5AC and MUC5AC-13. Time-course experiments were carried out and the reactions were analysed and evaluated by MALDI-TOF-MS. The specific activities under linear conditions were inferred from the mass spectrometry data (see Methods for details). One unit of enzyme is defined as the amount of enzyme that transfers 1 μmol of GalNAc in 1 min using the standard reaction mixture and conditions. The velocity values were obtained from three independent experiments and errors are <20%.