The glycosidic bond in a disaccharide composed of N-acetylmuramic acid (Mur2Ac) and N-acetylglucosamine (GlcNAc) is cleaved by lysozyme. The first step of the mechanism is shown below. In the second step, Glu35 creates a good nucleophile for the cleavage of the glycosidic bond between Mur2Ac and Asp52 by removing a proton from which of the following molecules?

Question

The glycosidic bond in a disaccharide composed of N-acetylmuramic acid (Mur2Ac) and N-acetylglucosamine (GlcNAc) is cleaved by lysozyme.  The first step of the mechanism is shown below.  In the second step, Glu35 creates a good nucleophile for the cleavage of the glycosidic bond between Mur2Ac and Asp52 by removing a proton from which of the following molecules?

Quizplus · Accepted Answer

11ecba2d_12bb_1e01_a3bf_8718c69aca9e_MD0008_00 A glycosidic bond is the α- or β-linkage between the hemiacetal or hemiketal group of a sugar molecule and a hydroxyl (-OH) group of another molecule. Glycosidic bonds are broken via hydrolysis reactions (addition of H₂O), frequently with the aid of an enzyme, resulting in two smaller components. The question shows that in the cleavage of the glycosidic bond between N-acetylmuramic acid (Mur2Ac) and N-acetylglucosamine (GlcNAc), glutamate at position 35 (Glu35) protonates the GlcNAc portion of the disaccharide, yielding an intermediate of Mur2Ac bonded to Asp52. To complete the reaction, Glu35 must return to its protonated form and a hydroxyl group must be added to Mur2Ac to release Asp52. This can be accomplished by the addition of H₂O, which is deprotonated by Glu35. This deprotonation creates a good nucleophile and facilitates a nucleophilic attack on the Mur2Ac anomeric carbon with Asp52 acting as the leaving group. Therefore, Glu35 removes a proton from H₂O to create a good nucleophile for the cleavage of the glycosidic bond between Mur2Ac and Asp52. (Choice A) NH₃ is not involved in the hydrolysis of glycosidic linkages. (Choice B) Glu35 can deprotonate H₃O⁺ to give H₂O, which is not a good nucleophile. Because cleavage of the glycosidic bond requires a good nucleophile, H₂O would not be a suitable nucleophile. (Choice D) Glu35 cannot deprotonate hydroxide (OH⁻) in the second step because Glu35 and OH⁻ are both negatively charged, so neither can act as an electrophile. Educational objective: A glycosidic bond is the α- or β-linkage between a sugar and an -OH of another molecule. Hydrolysis of a glycosidic bond is cleavage of the linkage by addition of H₂O, breaking the molecule into two smaller units.

The Glycosidic Bond in a Disaccharide Composed of N-Acetylmuramic Acid