Serine-53 at the tip of the glycine-rich loop of cAMP-dependent protein kinase: Role in catalysis, P-site specificity, and interaction with inhibitors

The glycine-rich loop, one of the most important motifs in the conserved protein kinase catalytic core, embraces the entire nucleotide, is very mobile, and is exquisitely sensitive to what occupies the active site deft. Of the three conserved glycines [G(50)TG(52)SFG(55) in cAMP-dependent protein kinase (cAPK)], Gly(52) is the most important for catalysis because it allows the backbone amide of Ser(53) at the tip of the loop to hydrogen bond to the gamma-phosphate of ATP [Grant, B. D. et al. (1998) Biochemistry 37, 7708]. The structural model of the catalytic subunit:ATP:PKI(5-24) (heat-stable protein kinase inhibitor) ternary complex in the closed conformation suggests that Ser53 also might be essential for stabilization of the peptide substrate-enzyme complex via a hydrogen bond between the P-site carbonyl in PKI and the Ser53 side-chain hydroxyl [Bossemeyer, D. et al. (1993) EMBO J. 12, 849]. To address the importance of the Ser53 side chain in catalysis, inhibition, and P-site specificity, Ser53 was replaced with threonine, glycine, and proline. Removal of the side chain (i.e., mutation to glycine) had no effect on the steady-state phosphorylation of a peptide substrate (LRRASLG) or on the interaction with physiological inhibitors, including the type-I and -II regulatory subunits and PKI. However, this mutation did affect the P-sire specificity; the glycine mutant can more readily phosphorylate a P-site threonine in a peptide substrate (5-6-fold better than wild-type). The proline mutant is compromised catalytically with altered k(cat) and K-m for both peptide and ATP and with altered sensitivity to both regulatory subunits and PKI. Steric constraints as well as restricted flexibility could account for these effects. These combined results demonstrate that while the backbone amide of Sers' may be required for efficient catalysis, the side chain is not.