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Conformation gating as a mechanism for enzyme specificity
H X Zhou1, S T Wlodek, J A McCammon
1Department of Physics and Atmospheric Science, Drexel University, Philadelphia, PA 19104, USA. hxzhou@einstein.drexel.edu
Summary
Enzymes like acetylcholinesterase use a gate to control access to their active sites. Rapid gate dynamics allow substrates to bind efficiently, even with a small opening probability, enabling enzyme specificity.
Area of Science:
- Biochemistry
- Enzymology
- Computational Biology
Background:
- Enzymes often feature buried active sites, posing challenges for substrate access.
- Acetylcholinesterase (AChE) has a narrow gorge leading to its active site, controlled by a dynamic gate.
- Molecular dynamics simulations revealed rapid gate opening and closing via aromatic ring reorientation.
Purpose of the Study:
- To quantitatively analyze the impact of the AChE gate's dynamics on substrate binding rates.
- To understand how gate dynamics influence enzyme efficiency and specificity.
Main Methods:
- Utilized molecular dynamics simulation trajectories of AChE.
- Performed quantitative analysis of the gate's effect on substrate binding rate constants.
- Modeled acetylcholine as a 2.4-Å probe.
Main Results:
- The AChE gate is open only 2.4% of the time for a 2.4-Å probe.
- Despite limited opening, substrate binding rate is reduced by only a factor of 2.
- Larger ligands (0.4 Å bulkier) experience a three-orders-of-magnitude reduction in binding rate.
- Brownian motion and repeated entry attempts by the substrate compensate for brief gate openings.
Conclusions:
- Rapid gating dynamics allow enzymes to maintain high substrate binding efficiency.
- Enzyme specificity can be achieved by steric hindrance of the gate for larger molecules.
- This mechanism balances enzyme efficiency with the need for substrate selectivity.