The catalytic core of adenylyl cyclase undergoes dramatic conformational changes upon binding of activated Gsalpha . We have generated models of the low activity structure of the catalytic core in comparison to the high activity X-ray crystal structure of Gsalpha-and forskolin-bound form. Gsalpha contacts both the C1 (lime) and C2 domains (ice blue) resulting in a 7 degree rotation of the C1 domain around the C2 domain. The conformational change orients the active site to receive a nucleotide substrate.

Nucleotide Binding

Upon binding nucleotide (ATP) the active site of adenylyl cyclase undergoes a dramatic collapse. The position of the residues that coordinate the binding of the adenine ring in adenylyl cyclases are also conserved in guanylyl cyclase. The differences in the charge on the guanylyl cyclases would presumably accomodate the structure of the guanine ring of GTP.

Also found in the active site are two metal ions (Mg2+) that are stablized by the phosphate moieties of ATP and by two conserved aspartate residues on the C1 domain. As well as being conserved in guanylyl cyclases the geometry and coordination are also conserved in DNA and RNA polymerases.

Catalysis

As was proposed for DNA and RNA polymerases adenylyl and guanylyl cyclases utilizes the two Mg2+ ions to catalyse the formation of cAMP or cGMP, respectively. As illustrated in the movie (middle), Mg1 participates in the deprotonation of the 3'OH on the ribose ring. The newly formed oxyanion performs a nucleophilic attack on the alpha phosphate. The inversion results in the release of pyrophosphate and the formation of cAMP. Note the disappearance of one metal ion (Mg2+). Models were generated based on the crystal structures the ATPalphaS(RP), a competitive inhibitor, and 2'deoxy-3'-AMP, an uncompetitive P-site inhibitor, bound to Gsalpha-bound AC. These structures serve as models for the ATP-and cAMP-bound forms, repectively.

All models were generated from PDB files described in the following reports:

Tesmer JJ, Sunahara RK, Johnson RA, Gosselin G, Gilman AG, Sprang SR. Two-metal-Ion catalysis in adenylyl cyclase. Science. 1999 285(5428):756-60.

Tesmer JJ, Sunahara RK, Gilman AG, Sprang SR. Crystal structure of the catalytic domains of adenylyl cyclase in a complex with Gsalpha.GTPgammaS. Science. 1997 278(5345):1907-16

Sunahara RK, Tesmer JJ, Gilman AG, Sprang SR. Crystal structure of the adenylyl cyclase activator Gsalpha. Science. 1997 278(5345):1943-7.

Zhang G, Liu Y, Ruoho AE, Hurley JH. Structure of the adenylyl cyclase catalytic core. Nature 1997 386:247-253

Sunaharal 2006®