Subdomain I of the Kinase Domain of Protein Kinase A

Subdomain I starts at the N-terminus of the kinase domain of the protein. The residues fold up into a beta strand-turn beta strand secondary structure. The maroon ribbon structure in the above picture represents the first subdomain. Beta strands 1 and 2 are included in this subdomain, which acts as a flap that covers the nontransferrable alpha and beta phosphates of ATP.

The glycine rich loop
Subdomain I contains the consensus motif of Gly-X-Gly-X-X-Gly-X-Val that is contained in nearly all protein kinases. This motif is known as the glycine-rich loop,and it helps to anchor ATP to the protein. In the case of proteinkinase A, this motif begins at Gly50, and is Gly-Thr-Gly-Ser-Phe-Gly-Arg-Val. The amides in the backbones of Ser53, Phe54, and Gly55 form hydrogen bonds with the oxygens of ATP's beta phosphate. Leu49, which is not officially within the sequence, acts along with Val 57 to help form a hydrophobic pockets that sequesters the adenine ring in ATP. The consensus motif in subdomain I also contains the invariant residue of Gly52 and the nearly invariant Gly 50. Since they are present almost ubiquitously, they are definite participants in the binding of MgATP. The two glycines, Gly50 and Gly52 are labeled in the above picture.