Introduction to Enzymes

1
Introduction to Enzymes

• Biological Catalysts

2
Life Process Chemical Reactions

• Enzymes

3
Chemical Reactions
Spontaneous and Fast
Spontaneous but Slow
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Types of Reactions

• Spontaneous Reactions thermodynamically or
  energetically favorable
• Kinetically Unfavorable Reactions
• Requirement for Catalysts
• Protein Catalysts Enzymes
• RNA Catalysts Ribozymes

5
General Properties of Enzymes

• Higher reaction rates (catalytic power)
• Milder reaction conditions
• Greater reaction specificity
• Capacity for regulation

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Higher Reaction RatesCarbonic Anhydrase
105 molecules CO2 per enzyme molecule per
second 107 x uncatalyzed reaction
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Catalytic Power of Some Enzymes
Table 11-1
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Enzyme Classes

• Oxidoreductases oxidation-reduction reactions
• Transferases transfer of functional groups
• Hydrolases hydrolysis reactions (cleavage and
  introduction of water)
• Lyases group elimination to form double bond
• Isomerases isomerization (intramolecular
  rearrangements
• Ligases (synthases) bond formation coupled with
  ATP hydrolysis

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Enzyme Nomenclature(Usual usage often use
suffix ase)

• Examples
• Urease
• Arginase
• Exceptions
• Trypsin
• Chymotrypsin

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Enzyme Nomenclature(Common Name versus
Systematic Name)
Aconitase Aconitate Hydratase EC 4.2.1.3
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Enzyme Nomenclature(Common Name versus
Systematic Name)
Lactate Dehydrogenase L-LactateNAD Oxidoreductase
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Enzyme Catalysis
Accelerate interconversion to Equilibrium
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Reaction Pathway (Coordinate)
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Transition State Diagram
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Catalysts
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Pathway of Enzyme Catalysis
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Enzyme-Substrate Complex
Binding Site
Figure 11-1
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Substrate Specificity

• Active Site
• Lock and Key Model
• Induced Fit Model
• Stereospecificity 3-point attachment
• Geometric Specificity e.g. trypsin and
  chymotrypsin

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Principle of Complementarity

• Geometric complementarity
• Electronic complementarity

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Models of Complementarity

• Lock-and-Key Model
• Induced Fit Model

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Enzymes Vary in Geometric Specificity(Alcohol
Dehydrogenase)
Ethanol gt Acetaldehyde Methanol gt
Formaldehyde Isopropanol gt Dimethylketone RATE
Ethanol gt Methanol gt Isopropanol
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Trypsin and Chymotrypsin
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Trypsin
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Chymotrypsin
25
Chymotrypsin
Page 326
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Enzymes are Stereospecific
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Aconitase Reaction
Prochiral Substrate
Chiral Product
Page 325
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Stereospecificity in Substrate Binding
Figure 11-2
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Some Enzymes Require Cofactors
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Coenzymes

• Simple Proteins
• Protein plus Cofactor
• Apoenzyme protein only
• Holoenzyme protein plus cofactor
• Coenzyme organic cofactor
• Prosthetic Group tightly bound cofactor

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Types of Cofactors
Figure 11-3
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Coenzymes or CosubstratesNAD(P) gt NAD(P)H
H
Figure 11-4
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NADP
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NADPH
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Coenzymes and Cosubstrates(Alcohol Dehydrogenase)
Page 327
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Prosthetic Groups(Cytochromes)
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Coenzymes Must be Regenerated
Alcohol Dehydrogenase
Cytochromes
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Control of Enzyme Activity
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Irreversible Covalent Modification

• Zymogen Activation
• Proteolysis
• Lysosomes
• Proteosomes (ubiquitin)

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Zymogen Activation
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Reversible Covalent Modification
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Non-covalent Modification

• Effectors or Ligands

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Negative Effectors
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Positive Effectors
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Allosteric (Regulatory) Enzymes
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Allosteric Proteins