1994 · Chakravarty — Drug–Protein Interactions: Refined Structures of Three Sulfonamide Drug Complexes of Human Carbonic Anhydrase I Enzyme
Super-Abstract
This in-vitro crystallography study resolved the atomic structures of three sulfonamide drugs bound to human carbonic anhydrase I (HCAI) at 2 Å resolution, revealing how hydrogen bonds within the enzyme's active site determine inhibitor orientation and potency. The connection to hydrogen chemistry lies in the central role of hydrogen bonding networks — not in molecular H₂ therapy. (Journal of Molecular Biology, 1994.)
Commentary
This is a structural biochemistry study using X-ray crystallography to map drug–enzyme interactions in human carbonic anhydrase I. The clinical context is ophthalmic disorders treated with sulfonamide inhibitors of carbonic anhydrase. Hydrogen appears here not as molecular H₂, but in the classical biochemical sense: the hydrogen bonds between the sulfonamide group and the active-site zinc ion and surrounding amino acid residues (Thr199, His200, His67). The paper provides first crystallographic evidence for the role of His200 in enzyme inhibition. This is fundamental structural pharmacology with no direct relevance to therapeutic molecular hydrogen.
Key quotes
- „The active site loop of Leu198, Thr199 and His200 has been identified to be important for binding of the drug molecules due to their appreciable atomic displacements and intra-molecular hydrogen bonds arising out of their interactions with the sulfonamides.“ — hydrogen bonds as the structural determinant of drug–enzyme interaction
- „This is the first crystallographic evidence of the possible involvement of His200 in the inhibition of HCAI.“ — a novel mechanistic finding in carbonic anhydrase pharmacology
- „An important role of Thr199 in distinguishing between the substrate and inhibitor binding modes of HCO3- to the enzyme at high pH is also inferred.“ — mechanistic insight into selectivity of substrate vs. inhibitor binding
Our assessment
This is a structural biochemistry study in drug design — not a hydrogen therapy study. Hydrogen in this paper refers entirely to hydrogen bonds in protein–drug interactions, not to molecular H₂ or hydrogen-rich water. No implications for therapeutic H₂ use exist in this work. It is valuable for medicinal chemists and crystallographers studying carbonic anhydrase inhibitors, but has no connection to the molecular hydrogen medicine field.
Study design
- Type: in-vitro crystallography study · Model: human carbonic anhydrase I (HCAI) enzyme, crystal structure at 2 Å resolution · H₂ delivery: not applicable — hydrogen refers to hydrogen bonds in structural biochemistry
- Result: three sulfonamide drugs (including methazolamide) bind in the HCAI active site displacing the zinc-coordinated water molecule; orientational differences of sulfamido groups explain differences in inhibitory potency; His200 first identified crystallographically as involved in HCAI inhibition
Abstract
N-unsubstituted sulfonamide drugs are widely used for opthalmic disorders. Inhibition of carbonic anhydrase enzyme is believed to be the chief reason for their therapeutic effects. Structures of three such sulfonamide drugs complexed to human carbonic anhydrase I enzyme (HCAI) refined crystallographically at 2 A resolution are reported here. The drug molecules are all bound in the active site of the enzyme, but among themselves show differences in the orientations of the sulfamido groups interacting with the essential zinc ion in the active site. The activity linked solvent molecule coordinated to zinc in the native enzyme is displaced by all the three sulfonamides. The active site loop of Leu198, Thr199 and His200 has been identified to be important for binding of the drug molecules due to their appreciable atomic displacements and intra-molecular hydrogen bonds arising out of their interactions with the sulfonamides. These interactions along with active site charge requirements are proposed to be responsible for the orientational differences of the sulfamido groups and also for differences in the inhibitory powers of the drugs. A hydrogen bond network involving solvent molecules and active site residues His200 and His67 amongst others in the native enzyme, is disrupted upon binding of methazolamide but not in the other two sulfonamides. This is the first crystallographic evidence of the possible involvement of His200 in the inhibition of HCAI. An important role of Thr199 in distinguishing between the substrate and inhibitor binding modes of HCO3- to the enzyme at high pH is also inferred.
Source & links
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