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A link between protein structure and enzyme catalyzed hydrogen tunneling
B J Bahnson1, T D Colby, J K Chin
1Department of Chemistry, University of California, Berkeley 94720, USA.
Summary
Enzyme active site side chain size influences hydrogen tunneling during reactions. Smaller residues reduce tunneling and catalytic efficiency, impacting hydride transfer mechanisms.
Area of Science:
- Biochemistry
- Enzyme kinetics
- Structural biology
Background:
- Hydrogen tunneling is a quantum mechanical phenomenon observed in some enzyme-catalyzed reactions.
- The active site environment and residue interactions can influence reaction dynamics.
Purpose of the Study:
- To investigate the role of active site side chain size in modulating hydrogen tunneling.
- To correlate catalytic efficiency with the extent of hydrogen tunneling in alcohol dehydrogenase mutants.
Main Methods:
- Measurement of primary and secondary kinetic isotope effects (kH/kT and kD/kT).
- Site-directed mutagenesis of horse liver alcohol dehydrogenase at position 203.
- X-ray crystallography of enzyme-cofactor-substrate ternary complexes.
Main Results:
- Reduced side chain size at position 203 correlated with diminished hydrogen tunneling and catalytic efficiency.
- X-ray structures revealed increased hydrogen transfer distance in a low-tunneling mutant.
- Mutant structures showed altered interdomain geometry, suggesting flexibility in enzyme dynamics.
Conclusions:
- Active site residue size is a key factor in controlling hydrogen tunneling in enzyme catalysis.
- Structural changes, including altered distances and interdomain movements, underpin the observed modulation of tunneling.
- Understanding these mechanisms provides insights into enzyme efficiency and quantum effects in biology.