Adenylyl Cyclase

Adenylyl cyclase creates second messengers to amplify signals from G-protein coupled receptors

Structure of adenylyl cyclase determined by cryoelectron microscopy. The membrane is shown schematically.
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Adenylyl cyclase is a signal amplifier. It acts at the center of the signaling cascade that transduces binding of hormones into cellular responses. When hormones such as adrenaline or glucagon bind to G-protein coupled receptors, they activate G-proteins, which in turn activate adenylyl cyclase. Adenylyl cyclase then performs its catalytic reaction, clipping off two phosphates from ATP and forming an additional bond to the remaining phosphate. The resultant molecule, cyclic AMP or cAMP, is released and travels quickly throughout the cell, regulating the function of multiple proteins. In this role, cAMP is often called a "second messenger", delivering the original message of the hormone. As an added benefit, the signal is amplified in the process, because adenylyl cyclase is an enzyme that can create many molecules of cAMP when activated.

Signaling Complex

Signaling by cAMP plays critical roles throughout our body, balancing the breakdown or formation of sugar for energy and controlling diverse processes in cell growth, development, learning, and memory. To regulate these many processes, our cells have ten different subtypes of adenylyl cyclase. Nine of these have a similar form, with a membrane-spanning portion and a catalytic domain on the cytoplasmic side of the membrane. PDB ID 6r3q shows subtype AC9, which is found in lung, brain, heart, and other tissues. In the structure, it is bound to an G-protein, giving us a glimpse of the signaling complex in action.

Second Messengers

Second messengers are so important that adenylyl cyclases are found in most organisms. In bacteria, cAMP made by adenylyl cyclases is sensed by the catabolite activator protein to help control the overall energy balance of the cell. Our own cells have a soluble adenylyl cyclase (see, for example, PDB ID 4clk, not shown) that is quite different from the nine membrane-bound forms, and is involved in sensing levels of bicarbonate and carbon dioxide. Sometimes these enzymes are used for nefarious purposes. For example, anthrax bacteria make a edema factor protein that acts as an adenylyl cyclase that disrupts signaling in infected individuals.

Structure of the hyperpolarization-activated ion channel HCN1 bound to cAMP. The six-membered cyclized phosphate is indicated with an asterisk in the close-up of cAMP at bottom.
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Activating Ion Channels

Cyclic AMP regulates many different proteins. For example PKA (cAMP-dependent protein kinase) is activated by cAMP, and then goes on to phosphorylate many other proteins that control energy utilization. HCN1 (hyperpolarization-activated ion channel) is a voltage-gated channel involved in the flow of ions in and out of pacemaker heart and nerve cells. The structure shown here, from PDB ID 5u6p, has cAMP bound in each of the four identical subunits that form the ion channel. Binding of cAMP increases the activity of the channel and ultimately accelerates the heart rate.

Exploring the Structure

Adenylyl Cyclase and G-protein

Many of the early structures of adenylyl cyclase were determined using only the catalytic domain, given that membrane proteins are so difficult to crystallize. This structure (PDB ID 1cjk) shows how a key alpha helix in the activated G-protein binds to a pocket on the side of adenylyl cyclase. The catalytic domain is composed of two similar halves, with two similar neighboring pockets. One pocket binds to ATP and performs the cyclization reaction--in this structure, a modified, uncleavable form of ATP is bound in this pocket. The other pocket is thought to be regulatory, and has the plant stimulatory protein forskolin bound in it. To explore this structure in more detail, click on the image for an interactive Jsmol.

Topics for Further Discussion

  1. Try searching for "cAMP" at the main RCSB PDB site to see structures of many of the proteins that are regulated by cAMP.
  2. You can explore the structure of cyclic AMP in the Chemical Component Library for CMP.

References

  1. 6r3q: Qi, C., Sorrentino, S., Medalia, O., Korkhov, V.M. (2019) The structure of a membrane adenylyl cyclase bound to an activated stimulatory G protein. Science 364: 389-394
  2. Halls, M. L., Cooper, D. M. F. (2017) Adenylyl cyclase signalling complexes - pharmacological challenges and opportunities. Pharmacology & Therapeutics 172, 171-180.
  3. 5u6p: Lee, C.H., MacKinnon, R. (2017) Structures of the Human HCN1 Hyperpolarization-Activated Channel. Cell 168: 111-120.e11
  4. 4clk: Kleinboelting, S., Diaz, A., Moniot, S., Van Den Heuvel, J., Weyand, M., Levin, L.R., Buck, J., Steegborn, C. (2014) Crystal Structures of Human Soluble Adenylyl Cyclase Reveal Mechanisms of Catalysis and of its Activation Through Bicarbonate. Proc Natl Acad Sci U S A 111: 3727
  5. 1cjk: Tesmer, J.J., Sunahara, R.K., Johnson, R.A., Gosselin, G., Gilman, A.G., Sprang, S.R. (1999) Two-metal-Ion catalysis in adenylyl cyclase. Science 285: 756-760

November 2020, David Goodsell

doi:10.2210/rcsb_pdb/mom_2020_11
About Molecule of the Month
The RCSB PDB Molecule of the Month by David S. Goodsell (The Scripps Research Institute and the RCSB PDB) presents short accounts on selected molecules from the Protein Data Bank. Each installment includes an introduction to the structure and function of the molecule, a discussion of the relevance of the molecule to human health and welfare, and suggestions for how visitors might view these structures and access further details. More