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Phosphorylation modulates catalytic function and regulation in the cAMP-dependent protein kinase
J A Adams1, M L McGlone, R Gibson
1Department of Chemistry, San Diego State University, California 92182.
Biochemistry
|February 28, 1995
Summary
Phosphorylation of Thr-197 in cAMP-dependent protein kinase is critical for catalytic activity. Mutating this site impairs enzyme function and alters regulatory subunit interactions, suggesting its role in holoenzyme complex formation.
Area of Science:
- Enzymology
- Molecular Biology
- Biochemistry
Background:
- cAMP-dependent protein kinase (PKA) activity is regulated by phosphorylation.
- A conserved threonine residue (Thr-197) near the active site is a key phosphorylation site.
Purpose of the Study:
- To investigate the role of Thr-197 phosphorylation in PKA catalytic activity and regulation.
- To elucidate the kinetic mechanism and inhibitor affinities of PKA mutants lacking Thr-197 phosphorylation.
Main Methods:
- Site-directed mutagenesis to replace Thr-197 with aspartic acid and alanine.
- Expression of wild-type and mutant catalytic subunits as free or fusion proteins.
- Kinetic assays (KATP, Kpeptide) and viscosometric measurements.
- Analysis of interactions with regulatory subunits.
Main Results:
- Mutants showed a ~100-fold increase in KATP and Kpeptide, indicating impaired catalytic activity.
- Viscosometry suggested reduced phosphoryl transfer rates, not substrate affinity, due to mutations.
- Mutants exhibited altered interactions with the regulatory subunit, including slower association and altered inhibition.
Conclusions:
- Phosphorylation of Thr-197 is essential for efficient catalytic activity of PKA.
- The phosphothreonine region likely acts as a docking site for the regulatory subunit in the holoenzyme.
- The activation loop does not impede substrate access in inactive PKA forms.