Protein Kinase CK2: Biphasic Kinetics with Peptide Substrates

The regulatory β-subunit of the protein kinase CK2 modulates the salt optimum for α-subunit activity. In the presence of salt the β-subunit is stimulatory while in the absence of salt it is inhibitory. In the presence of 150 mMNaCl CK2 has linear kinetics (Lineweaver–Burk) for the synthetic substrate RRRDDDSDDD with an apparentKmof 60 μM. In contrast, CK2 displayed biphasic kinetics for the peptide substrate when assayed in the absence of added NaCl. Biphasic kinetics were also obtained for other peptides but not for calsequestrin or casein. Recombinant α-subunit had strictly linear kinetics in the absence of added NaCl with an apparentKmof 104 μM. Preincubation of CK2 with ATP/Mg2+or GTP/Mg2+, but not adenosine/Mg2+or Mg2+alone, resulted in kinetics that were near linear. This change in kinetics was dependent on enzyme concentration but not autophosphorylation. Under conditions of low salt CK2 displays biphasic kinetics for peptide substrates, the biphasic kinetics require the presence of the β-subunit, and ATP/Mg2+binding reverses the effect.