Molecule of the Month: Adenylyl Cyclase

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 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 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.

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.