Carbonic Anhydrase

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Carbonic Anhydrase
There are at least thirteen carbonic anhydrases
in humans
K1 0.0027 M-1S-1 k-1 50 s-1
Keq k1/k-1 5.4 x 10-5 M-1
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Carbonic Anhydrase II
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Carbonic Anhydrase II
Hydroxide ion
pKa of pure water is about 15.7
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Carbonic Anhydrase II
Hydroxide ion
nucleophilic
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Zinc in Carbonic Anhydrase

• Generates hydroxide ion
• A rate-limiting step
• Stabilizes intermediates

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Histidine is a Proton Shuttle in Carbonic
Anhydrases
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Buffer Provides Receptors of Protons from
Zinc-Bound Water
Buffers are ionic compounds that resist changes
of pH. Often, buffers are weak acids.
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Example 1,2-Dimethyl Benzimidazole
H
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1,2-Dimethyl Benzimidazole
pKa 7.0
DB H
DBH
pH pKa log (DB/DBH) DBH DB 10
mM At pH7.0, DB DBH 5 mM If 1 mM HCl
is added to this buffer, DB 5-1 4 mM
DBH 51 6 mM
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1,2-Dimethyl Benzimidazole
pKa 7.0
DB H
DBH
pH pKa log (DB/DBH) DB 5-1 4 mM
DBH 51 6 mM pH pKa log (DB/DBH) 7.0
log (4/6) 7-0.176 6.83 Without
buffer, Add 1 mM HCl to water, pH 3
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The Proton from the Zinc-Bound Water is
Continuously Removed by Buffer
H
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The Cycle of Carbonic Anhydrase
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Convergent Evolution of Zinc-Based Sites

• a-carbonic anhydrases (in mammals)
• Three histidine as ligands to Zn
• b-carbonic anhydrases (prokaryotic and
  chloroplasts)
• One histidine, two cysteine as ligands to Zn
• g-carbonic anhydrases (methane-producing bacteria
  or )
• Trimeric protein, with three Zn centers between
  monomers

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g-Carbonic Anhydrase
Cadmium may replace zinc in the active center.
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Convergent Evolution and Divergent Evolution

• Convergent Evolution
• Two proteins acquire common structural and
  functional features from unrelated ancestors
• e.g. a-carbonic anhydrases
• Divergent Evolution
• Two proteins are related by divergence from a
  common ancestor
• e.g. DNA polymerases