cAMP dependent Protein Kinase A [PKA]

adds a P from ATP to SER, THR, or TYR residues of proteins of signal transduction.

PKA's glycine residue (blue)
     traps ATP (green) in active site cleft between 2 small domains,
 as well as target protein (red)

Domains have 2 conformations:
without substrate = open
with substrate = closed
closed = rotates ATP & peptide together