Problem Set 5 Key

7 points gnt i inverting mechanism involves an

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Unformatted text preview: sugar, which is often the case with glycoside hydrolases. Also, conservation of this residue in family 2 glycosyltranferase may explain it as a catalytic base. 4 Distances should also be labeled to get credit. b. Unligil, U. M., et al. (2000, EMBO, 19, 5269- 5280) elucidated the catalytic mechanism of N- acetylglucosaminyltransferase I (GnT I) based on evidence from its crystal structures. GnT I is a glycosyltransferase that transfers the first N- acetylglucosamine (GlcNAc) residue onto the oligomannose core to build up either a hybrid or complex N- glycans. i. What is the actual sequence of DXD motif in GnT I? What is its role in GnT I? (3 points) The EDD (resi 211- 213) motif in GnT I is involved in the interactions with UDP- GlcNAc and the Mn2+ ion. D213 makes direct interaction with the bound Mn2+ ion and a hydrogen bond with a metal- coordinating water molecule, which makes a hydrogen bond with E211. Octahedral geometry of Mn2+ and the nucleotide- sugar coordination of the Mn2+ ion well define the relative orientation of the nucleotide- sugar to the two residues and well positions sugar moiety for interactions with E211. ii. GnT I uses the same kind of mechanism as BsSpsA to catalyze the transfer of GlcNAc. The authors proposed that GnT I also shares a part of the mechanism with glycosidases. Explain what part of the mechanism is shared between GnT I and glycosidases. (7 points) GnT I inverting mechanism involves an oxocarbenium- ion- like transition state similar to the proposed mechanism for retaining glycosidases. In the overlay of α- linked sugar moiety from three retaining β- glycosidases and GlcNAc moiety of th...
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