The Glu208 is a nearly invariant residue, and forms an ion pair with Arg280 in sudomain XI, stablizing the large lobe of the kinase.
Subdomain VIII probably is the most important one in the entire protein, because it plays a major in the recognition of the substrate. No crystal structure of a naturally occuring substrate bound to protein kinase A has been elucidated yet. However, the crystal structure of the protein bound to a synthetic pseudosubstrate peptide is well known, and is the model being used for this page. The sequence of the pseudosubstrate inhibitor is T-T-Y-A-D-F-I-A-S-G-R-T-G-R-R-N-A-I-H-D, numbering from 5 on the Nterminal end to 24 on the C-terminal end. More nomenclature of the inhibitor is discussed in the inhibitor section.
Residues Leu198, Cys199, Pro202 and Leu205 in subdomain VIII provide a hydrophobic pocket that protects the side chain of the hydrophobic residue at position +1 in the substrate consensus sequence. In the case of the inhibitor, Ile23 is protected in the pocket. Glu203 in the kinase forms two ion pairs with the Arg in the high affinity binding region (i.e. Arg15 in the inhibitor peptide).
The activation of many protein kinases is known to be done by phosphorlyation of residues in subdomain VIII. While protein kinase A has a different mode of regulation, that of the dimeric cAMP binding regulatory domain, it still requires phosphorylation at Thr197 for maximal activity. The mechanism of this phosphorylation is thought to be an intermolecular autophosphorylation. The activity increase is thought to come from the conformational change induced by the interaction of the Thr-phosphate's oxygens with the polar side chains of Arg165 and Lys189 and the hydroxyl group of Thr195. Other protein kinases have the phosphate-side chain inteactions and different residue positions, and the active conformations induced are very similar to the one of protein kinase A.