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A classical enzyme active center motif lacks catalytic competence until modulated electrostatically
S Pinitglang1, A B Watts, M Patel
1Laboratory of Structural and Mechanistic Enzymology, Department of Biochemistry, Queen Mary and Westfield College, University of London, London E1 4NS, U.K.
Biochemistry
|August 19, 1997
Summary
Cysteine proteinases achieve catalytic activity not just from their ion pair state, but require an additional protonic dissociation. This finding challenges the long-held assumption about enzyme mechanism and catalytic competence.
Area of Science:
- Biochemistry
- Enzymology
- Structural Biology
Background:
- Cysteine proteinases are crucial enzymes with diverse natural structural variants.
- The catalytic site's imidazolium-thiolate ion pair has been considered essential for enzyme activity.
- Previous understanding assumed catalytic competence directly mirrored ion pair formation.
Purpose of the Study:
- To investigate the precise mechanism of cysteine proteinase catalytic activity.
- To challenge the axiomatic view of ion pair formation as the sole determinant of enzyme competence.
- To elucidate the role of pH-dependent ionizations in enzyme function.
Main Methods:
- Kinetic studies using 4,4'-dipyrimidyl disulfide and 2,2'-dipyridyl disulfide.
- Electrostatic potential calculations.
- Analysis of pH dependence of rate constants (k) and catalytic efficiency (kcat/Km).
Main Results:
- The nucleophilic/acid-base chemistry ion pair state is formed at low pH where enzymes are inactive.
- An additional protonic dissociation (pKa ~4) is necessary for catalytic competence.
- Discrepancies in pKa values for ion pair formation and catalytic activity were observed for ficin, caricain, and papain.
Conclusions:
- Catalytic competence in cysteine proteinases requires more than just the ion pair state.
- A secondary protonic dissociation event is critical for controlling ion pair geometry and enabling catalysis.
- This research refines our understanding of cysteine proteinase mechanisms and pH-dependent activity.