lecture 15 - 15-1 MCB 354 Lecture 15 More about...

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15-1 MCB 354 Lecture 15 More about Chymotrypsin Lysozyme Mechanism Hexokinase Metal Ions in Enzyme Catalysis Enzyme Regulation: Allostery Lehninger, Chapter 6 : Enzymes
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kcat/Km Chymotrypsin Exhibits a Bell Shaped pH-Rate Profile His57 Ile16 ( β -N-terminus) 15-2
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Mechanism Based Inhibitors can Exploit the Nucleophilicity of Ser195 15-3
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15-4 Phenylmethanesulfonyl fluoride: Inhibits serine proteases such as trypsin and chymotrypsin. Also inhibits cysteine proteases
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- with molecule that….. i) binds to active site because it resembles a substrate ii) forms stable covalent bond with group on enzyme close to active site - tosyl- L -phenylalanine chloromethyl ketone ( TPCK ) i) has L -Phe side chain, so binds to chymotrypsin’s active site ii) chloromethyl ketone group alkylates a ring N of His 57 iii) TPCK -derivative of chymotrypsin is enzymatically dead iv) D -Phe-isomer of TPCK ineffective v) TPCK reaction inhibited by a competitive inhibitor of chymotrypsin Affinity labeling of His57 in chymotrypsin’s active site (= example of a way of identifying αα in an enzyme’s active site) 15-5
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Substrate specificity of serine proteases is determined by binding pocket for R-group of the αα that the protease cleaves after Chymotrypsin: pocket (“lined) with non-polar αα binds side chains of aromatic αα Trypsin: non-polar αα at base of pocket changed to Asp, which can form ion pair with +vely charged side chains of Lys and Arg 15-6
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Lysozyme Catalyzes Glycosidic C- O Cleavage 15-7
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Two Possible Mechanisms 15-8 Dissociative Double Displacement
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High energy carbocation vs. covalent intermediate 15-9
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Glu35 acts as general base for hydrolysis of either carbocation or covalent intermediate 15-10
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15-11
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Which Mechanism is Operative? 15-12 2-deoxy-2-fluoro sugar Trapped Intermediate
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Example of a reaction mechanism that requires metal ions: enolase Glycolytic enzyme that catalyzes reversible dehydration of 2-phosphoglycerate to phosphoenolpyruvate Also illustrates ....
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