Metal-Free cAMP-Dependent Protein Kinase Can Catalyze
Phosphoryl Transfer
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Abstract
X-ray structures of several ternary
product complexes of the catalytic
subunit of cAMP-dependent protein kinase (PKAc) have been determined
with no bound metal ions and with Na<sup>+</sup> or K<sup>+</sup> coordinated
at two metal-binding sites. The metal-free PKAc and the enzyme with
alkali metals were able to facilitate the phosphoryl transfer reaction.
In all studied complexes, the ATP and the substrate peptide (SP20)
were modified into the products ADP and the phosphorylated peptide.
The products of the phosphotransfer reaction were also found when
ATP-γS, a nonhydrolyzable ATP analogue, reacted with SP20 in
the PKAc active site containing no metals. Single turnover enzyme
kinetics measurements utilizing <sup>32</sup>P-labeled ATP confirmed
the phosphotransferase activity of the enzyme in the absence of metal
ions and in the presence of alkali metals. In addition, the structure
of the <i>apo</i>-PKAc binary complex with SP20 suggests
that the sequence of binding events may become ordered in a metal-free
environment, with SP20 binding first to prime the enzyme for subsequent
ATP binding. Comparison of these structures reveals conformational
and hydrogen bonding changes that might be important for the mechanism
of catalysis