Lysozyme is an enzyme that defends your body against bacterial invaders in fluid
ID: 1050895 • Letter: L
Question
Lysozyme is an enzyme that defends your body against bacterial invaders in fluids like tears, mucus, breast milk, and saliva. Lysozyme cleaves the peptidoglycan component of the bacterial cell wall, resulting in the death of the bacterium. Peptidoglycan is made of long polysaccharide chains linked to each other by peptides. The active site of lysozyme binds to six sugar residues, labeled A through F, and breaks the ?1-4 glycosidic bond between the D and E rings. Depicted below are the D (left side) and E (right) sugars of peptidoglycan in the absence of lysozyme. Note that the polysaccharide continues to both the left and right with repeats of the disaccharide shown. The lysozyme peptidoglycan hydrolysis reaction proceeds in two steps. In the first step, the peptidoglycan molecule becomes covalently bound to the enzyme. In the second step, a water molecule is consumed and the catalytic residues of the enzyme are restored. Depicted below are two snapshots of the peptidoglycan hydrolysis reaction. The drawings show the D ring (left) and E ring (right) sugars bound in the active site of lysozyme along with the carboxylic acid groups of two catalytically important amino acids. Aspartate (Asp52, indicated by R1) is positioned below the sugars and glutamic acid (Glu35, indicated by R2) is above. For simplicity, only one pair of electrons is shown on the carboxylate ions. In the first step of the peptidoglycan hydrolysis reaction, the N-acetylglucosamine product (NAG, with attached sugar chain) is released. To emphasize the newly formed hydroxyl group on the product, the product will be referred to as NAG-OH. As the glycosidic CO bond breaks and NAG-OH is being formed, the oxygen atom begins to develop a negative charge making it unstable. However, several features of the lysozyme active site promote the stabilization of the NAG-OH leaving group as it is being formed. Select the best explanations of how the lysozyme active site contributes to the formation and stabilization of the NAG-OH leaving group. Note: If your book or professor has not covered these specifics, then choose answers that are not self-contradictory.
tnat are not self-contradictory. The developin negative charge on the glycosidic oxygen atom of NAG is neutralized when it is protonated by Glu35 Glu35 has an abnormally high pKa of-6 (normal pKa- 4) due to the surronding nonpolar environment. This pKa shift allows Glu35 to donate the proton that results in the stable NAG-OH leaving group. Note: Lysozyme has optimum activity around pH 5. Asp52 acts a general base to form the stable NAG-OH leaving group. Awater molecule in the active site acts as a specific base and donates an OH' to form the NAGH leaving group.Explanation / Answer
lysozyme is an enzyme that breaks peptidoglycan as a way of defense to the body against bacterial infvaders in fluids, tears, etc.
The lysozyme active site contributes to the formation and stabilization of the NAG-OH leaving group. the statement best describing this approach of lysozyme is,
The developing negative charge on the glycosidic oxygen atom of NAG is neutralized when it is protonated by Glu35.
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