lecture_15b - Mechanism of lysozyme Lysozyme digests...

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Mechanism of lysozyme Lysozyme digests bacterial cell walls by breaking β (1- 4) glycosidic bonds between (N- acetylmuramic acid (NAM) and N- acetylglucosamine (NAG)
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The interactions of lysozyme with its substrate View of the binding cleft with the substrate
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The Phillips mechanism 1. Lysozyme attaches to a bacterial cell wall by binding to a hexasaccharide unit. The D residue is distorted towards the half-chair. 2. Glu 35 transfers its proton to the O1 of the D ring (general acid catalysis) C1-O1 bond is cleaved generating an oxonium ion at C1. 3. Asp 52 stabilizes the oxonium ion through charge-charge interactions. The carboxylate can not form a covalent bond because distances are too great. Reaction via a S N 2 mechanism with transient formation of a C --O bond to the enzyme. 4. E ring group is released from the enzyme yielding a glycosyl-enzyme intermediate which adds water to reverse the chemistry and reprotonate Glu 35.
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Serine proteases P-Nitrophenolate is very yellow while the acetate is colorless. This is an example of an artificial substrate!
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