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Rate-determining steps in penicillopepsin-catalysed reactions
A Cunningham1, M I Hofmann, T Hofmann
1Department of Biochemistry, University of Toronto, Canada.
FEBS Letters
|December 10, 1990
Summary
Penicillopepsin enzyme hydrolysis differs between two substrates. Substrate II shows a significant solvent isotope effect, indicating multiple proton involvement in its rate-determining step.
Area of Science:
- Biochemistry
- Enzymology
- Chemical Kinetics
Background:
- Enzyme catalysis mechanisms are often elucidated through kinetic studies, including the use of solvent isotope effects.
- Penicillopepsin, an aspartic protease, catalyzes peptide bond hydrolysis.
- Understanding the specific interactions and transition states involved in enzyme-substrate hydrolysis is crucial for drug design and biochemical research.
Purpose of the Study:
- To investigate the hydrolysis mechanism of two distinct substrates, Ac-Lys-Nph-amide (I) and Ac-(Ala)2-Lys-Nph-(Ala)2-amide (II), by penicillopepsin.
- To determine the role of proton transfer in the rate-determining step of these hydrolysis reactions.
- To differentiate the catalytic mechanisms employed by penicillopepsin for structurally different substrates.
Main Methods:
- Kinetic analysis of substrate hydrolysis by penicillopepsin under varying concentrations of deuterium oxide (D2O) in water.
- Measurement and analysis of solvent isotope effects (SIEs) to probe the involvement of protons in the transition state.
- Comparison of SIEs for different substrates to infer differences in their catalytic pathways.
Main Results:
- Hydrolysis of substrate I exhibited no significant solvent isotope effect, suggesting proton transfer is not rate-limiting.
- Hydrolysis of substrate II displayed a substantial solvent isotope effect (2.11), indicating the involvement of two or more protons in the rate-determining step.
- The non-linear dependence of the SIE on D2O concentration for substrate II further supports complex proton transfer dynamics.
Conclusions:
- The rate-determining step for substrate I hydrolysis likely involves the distortion of the scissile bond to a tetrahedral intermediate.
- For substrate II, the rate-determining step is proposed to be a conformational change within the enzyme, possibly induced by substrate binding to the S3 pocket.
- These findings highlight substrate-dependent variations in penicillopepsin's catalytic mechanism, emphasizing the enzyme's adaptability.