Subdomain VIB of Kinase Domain of Protein Kinase A

Subdomain VIB is comprised of the small, hydrophobic beta strands (6&7) with a loop between them. This subdomain contains the invariant residues Asp166 and Asn171. The invariant residues are members of the larger consensus motif of His-Arg-Asp-Leu-Lys-X-X-Asn.

The loop between the two strands is known as the 'catalytic loop', because Asp166, which is present in the loop, is most likely the catalytic base of the protein. The Asp166 accepts the proton from the substrate's hydroxyl group at the site of phosphorlyation (Ser or Thr in the case of PKA), This leaves an oxyanion, to which the phosphate from MgATP can be transferred.

Lys168, also present in the catalytic loop is thought to assist the phosphotransfer by neutralizing the local negative charge of the gamma phosphate of the MgATP. Asn171 assists in the stabilization of the catalytic loop by hydrogen bonding to the backbone carbonyl of Asp166. It also chelates the Mg2+ ion tha spans the alpha and gamma phosphates of ATP. The carbonyl group of Glu170 hydrogen bonds with a hydroxy group on the ribose ring of ATP, and forms an ion pair with the second Arg og the peptide recoginition consensus. (Arg-*Arg*-X-Ser-Y)