Catalytic Enzymes: Mechanisms
Feb. 6, 2008
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Overview
Double-Reciprocal Plots of Enzyme Inhibition Enzyme Catalytic Mechanisms
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Double-Reciprocal Plot of Competitive Inhibition
= 1 + [I]/KI
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Double-Reciprocal Plot of Uncompetitive Inhibition
' = 1 + [I]/K'I
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Double-Reciprocal Plot of Mixed Inhibition
= 1 + [I]/KI ' = 1 + [I]/K'I
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Irreversible Inhibition
Competitive, uncompetitive, and mixed inhibition are reversible Irreversible inhibitors permanently inhibit enzyme activity
May bind an essential functional group May damage an essential functional group May form a stable noncovalent complex
Suicide inhibitors are a special type of irreversible inhibitor
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Specific Catalytic Groups Contribute to Catalysis
Catalysis provides a new, lower-energy rxn pathway Catalytic functional groups aid in the cleavage and formation of bonds by several mechanisms:
Acid-base cat. Covalent cat. Metal ion cat.
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General Acid-Base Catalysis
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Covalent Catalysis
Transient covalent bond E between and S is involved Transient covalent complex undergoes further rxn to generate P and free E
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Amino acids in general acid-base catalysis and covalent catalysis
Listed aa can act as proton donors and acceptors Listed aa can also act as nucleophiles Enzyme cofactors (not shown) may also have functional groups that serve similar purposes
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Metal Ion Catalysis
Metal ion may be tightly associated with E or may be acquired from solution Oxidation-reduction rxn may be mediated by metal ion Ionic interactions between S and metal ion may contribute to binding energy by
positioning S or stabilizing charged transition state
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Examples of Enzymatic Reactions
Hexokinase
Induced fit mechanism (we've already discussed this) Also uses general acid-base catalysis and transition state stabilization but we won't go into this
Enolase
Metal ion catalysis
Lysozyme
Covalent catalysis and general acid catalysis
Chymotrypsin
Serine protease
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Enolase (Lyase)
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Hen egg white lysozyme (hydrolase)
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