Introduction to Enzymes

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Chapter 13
Introduction to Enzymes
Read pages 459 466 470
Chymotrypsin with bound substrate
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Enzymes

• Enzymes are the agents of metabolic function
• Enzymes endow cells with the remarkable capacity
  to exert kinetic control over thermodynamic
  potentiality

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Properties of Enzymes
(1) High reaction rates (2) Catalyze reactions
at physiological conditions (milder reaction
conditions) (3) Have a high degree of
specificity (e.g. only A is converted to B) (4)
Can be regulated (e.g., A is only converted to B
under certain conditions)
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Catalytic Power

• Enzymes can accelerate reactions as much as 1016
  over uncatalyzed rates!
• Urease is a good example
• Catalyzed rate 3x104/sec
• Uncatalyzed rate 3x10 -10/sec
• Ratio is 1x1014 !

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Some enzymes achieve remarkable catalytic
efficiency
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Glycolysis
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Enzyme catalyzed reactions are much faster than
the corresponding uncatalyzed reaction
E S
ES
EP
E P
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Activation energies
Biochemical free energy change at pH 7.0
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Function of DNA ligase
Enzyme
Substrate(s) Product(s)
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Coenzymes
Cofactors may be metal ions (such as Zn2
required for the catalytic activity of
carboxypeptidase A) Coenzymes are organic
molecules (such as the NAD in YADH)
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Metal Cofactors
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Organic Cofactors (also called coenzymes)
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• Other cofactors
• Known as prosthetic groups
• Permanently attached with their protein
• Often by covalent bonds
• Example Heme in hemoglobin

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Specificity

• Enzymes selectively recognize proper substrates
  over other molecules
• Enzymes produce products in very high yields -
  often much greater than 95
• Specificity is controlled by structure - the
  unique fit of substrate with enzyme controls the
  selectivity for substrate and the product yield

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An enzyme-substrate complex
Illustrates both the geometrical physical
complementarity between enzymes and substrates
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Stereospecificity
Enzymes are highly specific in both in binding
chiral substrates in catalyzing their
reactions. Stereospecificity arises because
enzymes virtue of their inherent
chirality. Proteins consists of only L-amino
acids.
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The enzyme binding site
Induced fit mechanism is most prevalent in
enzymes. The enzyme active site adapts its
structure to interact with the substrate
transition states.
The same set of non-covalent interactions that
enable a protein to fold are involved in
stabilizing the interaction between the
substrate and enzyme.
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• How do enzymes accelerate reactions?
• Chemical reactions between the substrate and
  functional groups on the enzyme can provide
  alternative, lower-energy reaction pathways.
• (Examplegroup transfer through an intermediate
  with the group transiently covalently attached to
  the enzyme)
• 2. Binding energy, DGB, is a major source of
  free energy used by enzymes to lower the
  activation energies of reactions.