Superimposition with the B and H forms of the PKA RIa CNBD-A reveals that the PKG Ib CNBD-A:cGMP complex is in a conformation that more closely resembles the H-form of RIa, not the B-form. As seen in Fig. 3B, the helical subdomain of the PKG Ib CNBD-A aligns better with the H-form of CNBD-A, which represents the Csubunit bound state. Like the H-form of RIa, the N-terminal helical bundle, consisting of the aX:N-a310loop-aA helices, interacts with the PBC while the aB helix tilts up MK-1775 approximately 7u without engaging either motif. In particular, the tip of a310 loop reaches across the rigid b barrel SU5416 204005-46-9 making multiple contacts with PBC. The side chain of Asn116 forms a hydrogen bond with Glu183 which anchors the 29 OH of the ribose. As in PKG Ib CNBD-A, the H-form of PKA RIa shows a hydrogen bond between the corresponding asparagine and glutamate residues. In the B-form of RIa, Glu200 forms a salt bridge with Arg241 on the aC helix, which plays a major role in mediating PKA activation. Additional interactions that mediate the 310-helix-PBC interaction include the carboxyl oxygen of Asn116 hydrogen bonding to the backbone amide of Phe118, whose side chain, in turn, makes a hydrophobic contact with Leu184, Tyr188 and Leu187. Each cGMP binding site in the PKG Ib:cGMP crystal shows a clear electron density for cGMP bound in a syn configuration, as previously predicted by mutation and other studies. Contacts between cGMP:A and PBC-B do not influence the overall interaction pattern of cGMP:A with the protein; the amino acid contacts with each cGMP are essentially the same. While the guanine rings are partially exposed to solvent for both molecules, the sugar-phosphates are buried in the pockets formed at the PBCs. The cGMP-binding site is comprised of three parts: the short P-helix together with conserved glutamate and arginine residues at the PBC which captures the sugar phosphate ; a key residue, Thr193 at the end of PBC that bridges the cyclic phosphate to the guanine ring ; and the b5-strand that provides a unique docking site for the guanine ring. While the first site is shared with PKA, the other two sites are unique to PKG. The first binding site consists of a positively charged pocket created by a cluster of unpaired backbone amides at the Nterminus of the P-helix and the side chain of Arg192. The exposed backbone amides of Gly182, Glu183, Leu184 and Ala185 of the P-helix together with the guanidinium group of Arg192, captures the cyclic phosphate through several hydrogen bonds and electrostatic interactions. In addition, the side chain of Glu183 interacts with the 29 OH of the ribose through a strong hydrogen bond.