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Protein Kinase A (PKA) is a cytosolic protein that catalyses the O-phosphorylation of protein and peptide substrates.  The structure above is the catalytic subunit, with the peptide inhibitor PKI (5-24) bound in the active site.  The phosphate group is transferred to the substrate from ATP, which is bound to the catlalytic subunit with Mn(II) in a pocket formed by a glycine-rich loop and thus shielded from the bulk solvent.  The glycine-rich loop also contributes to catalysis.  The backbone amides of Phe54 and Gly55 bind the b-phosphate group of ATP, and the backbone amide of Ser53 forms a hydrogen bond with the g-phosphate.  The side chains of Ser53 and Thr51 are involved in hydrogen bonding interactions with PKI (5-24).  The active site of the enzyme contains several ordered water molecules that perform various roles - some are associated with metal binding sites, some with the active site and some with ligand binding.  The catalytic subunit is a flexible molecule, displaying 'open' and 'closed' confomations and marked flexibility within the glycine-rich loop.  Asp166 and Lys168 perform the roles of base and transition state stabilisation respectively.