The General Structure of an Activated Kinase

Another very important conserved domain in kinases is the activation loop. This 20-30 amino acid region is positioned between a highly conserved DFG and APE motif (IDA region, inter DFG-APE region). In its activated state the activation loop is in an open, extended conformation, which allows substrate binding to the kinase. The aspartate residue (Asp-145) of the DFG motif interacts with one of the two magnesium ions in the active site. 

The presence of the two magnesium ions, which chelate the (3- and y-phosphate oxygens, positions the terminal phosphate group and reduces electrostatic repulsion of the incoming nucleophile [4 (i)]. 

Several other highly conserved key residues assist in the phosphotransfer catalysis. Asp-127 is located near the incoming nucleophile and may direct the hydroxyl function for the attack on the terminal phosphate. Asn-132 interacts with the second magnesium ion, and Lys-129 forms an ion pair with the terminal phosphate. 

Although most of the conserved phosphate-binding residues in the ATP-binding site are crucial for the catalytic process, they do not contribute much to the free energy of binding of the ATP kinase, which is nicely reflected in the equipotent affinity of PKA for ATP, ADP, and adenosine [10], 

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