Principles of Bioinorganic Chemistry - 2004

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Principles of Bioinorganic Chemistry - 2004
Note The course seminar presentations will be
held on Sunday, October 31, 2004 in the Bush
Room. Please remember that daylight savings time
ends that day.
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Hydrolytic Enzymes, Zinc and other Metal Ions
PRINCIPLES

• M(OH)n centers supply OH- at pH 7 by lowering
  water pKa
• Mn serves as general Lewis acid, activating
  substrates
• Rate acceleration occurs by internal attack
  within coord. sphere
• Protein side chains greatly assist assembly of
  transition state
• Carboxylate shifts can occur, especially at
  dimetallic centers
• Electrostatic interactions predominate
• Non-redox active metal ions often but not
  universally used

Illustrating the Principles

• Carboxypeptidase, carbonic anhydrase - delivering
  hydroxide
• Alcohol dehydrogenase an oxidoreductase
• Dimetallic metallohydrolases are two metals
  better than one?

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Alcohol Dehydrogenase, an Oxidoreductase
Reaction catalyzed
RCH2 OH NAD
RCHO NADH H
Enzyme contains two 40 kDa polypeptides, each
with 2 Zn2centers in separate domains. One zinc
is structural, the other catalytic.
Catalytic zinc is 20 Å from the surface, near
the nicotinamide binding region. This center is
not required for NAD cofactor binding. Alcohol
substate DO require zinc and bind directly to the
metal center, displacing the coordinated water.
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Schematic Diagram NAD binding to the active
site of LADH, with specific, well-positioned
amino acid side chains holding it in place.
Ethanol is shown bound to the zinc, displacing
water. The system is set to undergo catalysis.
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Note hydride transfers from a-C of alcohol to
nicotinamide ring.
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Dimetallics can move the value into the
physiological range near pH 7
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Advantages of Carboxylate-Bridged Dimetallic
Centers in Chemistry and Biology
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Principles illustrated the dimetallic affords
hydroxide the substrate is positioned by
residues in the active site the dimetallic
stabilizes the urea leaving group redox inactive
metal electrostatics
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Alkaline Phosphatase a Dizinc(II) Center
Activates the Substrate
1. The substrate binds to the dizinc center a
nearby Arg also helps activate it. 2. A serine
hydroxyl group attack the phosphoryl group,
cleaving the ester. The phosphate is transferred
to the enzyme, forming a phosphoryl-serine
residue. 3. Hydrolysis of this phosphate ester by
a zinc-bound hydroxide com-pletes the catalytic
cycle. This mechanism is supported by studies
with chiral phosphate esters (ROP18O17O16O)2-
there is no net change in chirality at phoshorus.
1.
3.

2.
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Enzymatic Catalysis of Urea Decomposition
Elimination or Hydrolysis? Guillermina Estiu
and Kenneth M. Merz, Jr. pp 11832 - 11842
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Summary - Points to Remember

• Both mono- and dimetallic centers lower the pKa
  value of bound water, allowing hydroxide to be
  delivered at pH 7.
• Coordination of the leaving group portion of the
  substrate to a metal ion activates the substrate
  for nucleophilic attack.
• Residues not coordinated but in the second
  coordination sphere can participate directly
  (serine in phophatases) or indirectly (arginine
  in alcohol dehydrogenase) in substrate attack,
  orientation, and/or activation.
• Carboxylate shifts facilitate substrate binding,
  activation.
• Redox inactive metal ions (Zn2, Ni2, Mn 2,
  Co2) preferred.

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Dioxygen Carriers Hb, Mb, Hc, Hr Examples of
Atom- and Group-Transfer Chemistry
PRINCIPLES

• Both substrate binding and redox changes occur
• Coupled proton-electron transfer steps set the
  redox potentials
• Closely positioned redox/acid-base units work in
  concert
• Interactions with substrates/other proteins gate
  electron transfer
• Two-electron transfer strategies include 2
  metals, M-porphyrins
• Metal centers used to create or destroy radical
  species
• Changes in metal coordination spheres can
  facilitate allostery
• Bioinorganic chemistry of dioxygen paramount
  example

ILLUSTRATIONS

• O2 Binding and Transport hemoglobin (Hb),
  myoglobin (Mb), hemocyanin (Hc), and hemerythrin
  (Hr)
• O2 Activation cytochrome P-450, tyrosinase,
  methane monooxygenase dioxygenases

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Properties of Protein Dioxygen Carriers