The adenylyl cyclases contains two transmemebrane domains and two cytosolic domains. The cytosolic domains are highly consevered and can be divided into two regions, part a and part b. This text will focus on the lyase activity of the C2 domain. This lyase is in the Alpha beta class and has a 3-layer (bba) sandwich architecture. It has an adenylyl cyclase topology and belongs to the adenylyl cyclase homologous superfamily.(1)
The outer-layer of the sandwich is primarily composed of the second and third a-helicies. The central layer is composed of the 2,3,6,7,8, and portions of the 1st and 4th beta strands. This sheet is predominantly antiparallel except for strands 4 and 6. The inner-layer consists of the portions of strands 1 and 4 that are doubled back on themselves, strand 5, and the 4th helix.(9)
Two C2 monomers join together through the interaction between the opposite ends of each sandwich. The “ventral” side forms a cleft and the “dorsal” side reveals a groove.
These two configurations virtually form a tunnel. Isoleucine 940 from each monomer is the only residue preventing the formation of the tunnel.
The cleft on the ventral side is very polar/charged and is the ATP binding site.
Forskolin, a regulation substrate, is bound in the ventral cleft. Forskolin or the G-protein stimulate the enzyme. Forskolin activates the enzyme by essentially gluing the two domains together through hydrophobic and hydrogen bonding interactions. The forskolin interacts with Gly 1021, Ser 942, and Lys 896.
After binding the ATP, the 3'hydroxyl group is deprotonated. This leaves it open for nucleophilic attack by the alpha phosphate. Pyrophosphate acts as a leaving group. The attack results in the formation of the cyclic AMP. The enzyme stabilizes the transition state, and neutralizes the negative charge of the pyrophosphate.