Adenylyl Cyclase

Adenylyl cyclase is the enzyme that converts ATP to cAMP. Cells can customize their cAMP signaling system by synthesizing up to nine different types of adenylyl cyclase. The adenylyl cyclase enzyme is made up of a cytosolic N-terminal domain (C1) followed by two transmembrane clusters (each made up of 6 helices) followed finally by a cytosolic C-terminal domain (C2). Shown here are the two cytosolic domains

C1 and C2 embody the regulatory capabilities of adenylyl cyclase. Their structures are very similar. Both consist of 6 alpha helices and 9 beta strands.

C1 has three beta sheets (one with 2 strands, one with 3 strands, and one with 4 strands) . C2 has two beta sheets (one with 7 strands and one with 2 strands).

P-SITE INHIBITORS

Adenylyl cyclase is inhibited by adenosine analogs known as P-site inhibitors. These noncompetitive inhibitors only bind when pyrophosphate and either Mg or Mn are present.

Pyrophosphate is bound by:
Arg484,
Arg1029, and
Lys1065

The P-site and the substrate (ATP) binding site are the same. This site is formed by:
Asp396,
Ile397, and
Asp440

The P-site inhibitors form a hydrogen bond with the pyrophosphate and Asp440.

The first C1 alpha helix serves as a lid for the active site and closes upon substrate binding.

Arg1029 and Asn1025 are two residues essential for catalytic activation. Arg1029 stabilizes the reaction intermediate. Asn1025 is important for the conformational change that occurs upon ligand binding.

Effects of Forskolin

Adenylyl cyclase is stimulated by the forskolin (a diterpene) and by the heterotrimeric G-protein.

Forskolin stabilizes the association between the two cytosolic domains resulting in the formation of an active site.

The forskolin-binding site is adjacent to the G-protein (alpha subunit) binding site. The residues that make up the forskolin binding site are:

From C1:
Phe394,
Trp507,
Val511, and
Tyr443

From C2:
Lys896,
Ile940,
Gly941, and
Ser942.

G-PROTEIN

The stimulatory G-protein acts as an allosteric activator of adenylyl cyclase. Binding of the alpha subunit to adenylyl cyclase induces a conformational change which primes the active site for catalysis.

The G-protein inserts its switch II helix into the groove formed by the third C2 helix and the loop between the first two C2 helices.

By inserting this helix, it widens the cleft, causing the loop between the first two C2 helices to rotate away from the C1 core. This forces the C1 core to rotate 7 degrees.

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  • For more information on this adenylyl cyclase, consult:
    Tesmer, J. et al. (1997) Science 278: 1907-1916.