Biological context
Essential for CO₂ transport in blood, pH homeostasis, acid secretion in stomach, aqueous humour production in eye. Target of diuretics and glaucoma drugs (acetazolamide, dorzolamide).
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Lyase / metalloenzyme · Carbonic anhydrase α-class
Human carbonic anhydrase II
EC EC 4.2.1.1PDB 3KS3Homo sapiens
One of the fastest enzymes known (kcat ≈ 10⁶ s⁻¹). Catalyses the reversible hydration of CO₂ to bicarbonate via a zinc-hydroxide mechanism.
Mechanistic insight
Rate is limited by proton transfer from the zinc-bound water to bulk solvent, shuttled by His64 via a network of ordered water molecules (the "proton wire"). Water network dynamics IS the rate-limiting step — hard to appreciate without explicit solvent simulation.
metalloenzymezinclyaseproton wiredrug targetfastest
Catalytic mechanism
Type: Zinc-hydroxide mechanism with intramolecular proton shuttle
Rate-limiting: Intramolecular proton transfer from Zn-H2O to His64 via ordered water network (the 'proton wire')
Overall reaction
CO2 + H2O ⇌ HCO3⁻ + H⁺
01
Resting state — Zn-bound hydroxide
At physiological pH the zinc-bound water is mostly deprotonated (pKa ~6.8, drastically lowered from bulk by Zn²⁺ coordination and the Thr199–Glu106 hydrogen-bond network). The tetrahedrally coordinated Zn–OH⁻ is the active nucleophile.
A:94A:96A:119A:199A:106
Step 1 / 5
References (3)
- Silverman DN, Lindskog S. The catalytic mechanism of carbonic anhydrase: implications of a rate-limiting protolysis of water. Acc Chem Res 21:30–36 (1988).
- Krishnamurthy VM et al. Carbonic anhydrase as a model for biophysical and physical-organic studies of proteins and protein-ligand binding. Chem Rev 108:946–1051 (2008).
- Fisher SZ et al. Atomic crystal and molecular dynamics simulation analyses of human carbonic anhydrase II. Biochemistry 46:2930–2937 (2007).
Kinetic parameters
Steady-state kcat, KM and specificity constants from literature.
| Substrate | kcat (s⁻¹) | KM (mM) | kcat/KM (M⁻¹s⁻¹) | pH | T (°C) | Source |
|---|---|---|---|---|---|---|
| CO2 (hydration direction) | 1.40 × 10⁶ | 9.30 | 1.50 × 10⁸ | 9 | 25 | Khalifah, J. Biol. Chem. 1971 |
| HCO3⁻ (dehydration direction) | 4.00 × 10⁵ | 26.00 | 1.50 × 10⁷ | 7.5 | 25 | Khalifah, J. Biol. Chem. 1971 |
| p-Nitrophenyl acetate (esterase activity) | 0.0400 | 1.40 | 2.86 × 10¹ | 7.5 | 25 | Verpoorte et al., J. Biol. Chem. 1967 |
CA II is one of the fastest enzymes known — kcat approaches 10^6 s⁻¹. The proton-transfer step (via His64 water wire) is rate-limiting at [CO2] above ~5 mM. Inhibitors that replace the Zn-bound water (sulfonamides) are clinically used as diuretics and for glaucoma.
Michaelis–Menten · steady-state simulator
Run CA II in silico.
Integrate the reaction v = Vmax·[S] / (Km,app + [S]) in closed form with RK4, across any [S], [E], and an optional reversible inhibitor. Defaults are drawn from the curated kinetic record for this enzyme where available.
Progress curve · [S](t) and [P](t)30 s window
substrate [S]product [P]
Saturation · v vs. [S]Km,app = 9.30 mM · Vmax,app = 1.40e+3 mM/s
Inhibitor
Curated kinetic record
Defaults seeded from CO2 (hydration direction) at pH 9 / 25 °C — source: Khalifah, J. Biol. Chem. 1971.
Why this enzyme is in the catalog
Best example of: metalloenzyme catalysis, proton-transfer networks, drug target enzyme, near-maximum catalytic efficiency.