Kinetic mechanism of fully activated S6K1 protein kinase

Malik M. Keshwani, Thomas K. Harris

Research output: Contribution to journalArticle

18 Scopus citations

Abstract

S6K1 is a member of the AGC subfamily of serine-threonine protein kinases, whereby catalytic activation requires dual phosphorylation of critical residues in the conserved T-loop (Thr-229) and hydrophobic motif (Thr-389). Previously, we described production of the fully activated catalytic kinase domain construct, His6-S6K1αII(ΔAID)-T389E. Now, we report its kinetic mechanism for catalyzing phosphorylation of a model peptide substrate (Tide, RRRLSSLRA). First, two-substrate steady-state kinetics and product inhibition patterns indicated a Steady-State Ordered Bi Bi mechanism, whereby initial high affinity binding of ATP (KdATP = 5-6 μM) was followed by low affinity binding of Tide (KdTide = 180 μM), and values of KmATP = 5-6 μM and K mTide = 4-5 μM were expressed in the active ternary complex. Global curve-fitting analysis of ATP, Tide, and ADP titrations of pre-steady-state burst kinetics yielded microscopic rate constants for substrate binding, rapid chemical phosphorylation, and rate-limiting product release. Catalytic trapping experiments confirmed rate-limiting steps involving release of ADP. Pre-steady-state kinetic and catalytic trapping experiments showed osmotic pressure to increase the rate of ADP release; and direct binding experiments showed osmotic pressure to correspondingly weaken the affinity of the enzyme for both ADP and ATP, indicating a less hydrated conformational form of the free enzyme.

Original languageEnglish (US)
Pages (from-to)11972-11980
Number of pages9
JournalJournal of Biological Chemistry
Volume283
Issue number18
DOIs
StatePublished - May 2 2008

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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