adenylyl cyclase

adenylyl cyclase
Name	Adenylyl cyclase
EC number	4.6.1.1
CAS number	9012-42-4

adenylyl cyclase. This pivotal enzyme catalyzes the conversion of adenosine triphosphate into the ubiquitous second messenger cyclic adenosine monophosphate. Its activity is primarily regulated by heterotrimeric G proteins following activation of G protein-coupled receptors. Found across all kingdoms of life, from bacteria to humans, it serves as a fundamental signaling node, translating extracellular signals into intracellular responses that govern a vast array of physiological processes.

Structure and isoforms

In mammals, adenylyl cyclases are integral membrane proteins with a complex topology, typically featuring two sets of six transmembrane helices and two cytoplasmic catalytic domains. The discovery of multiple isoforms, designated AC1 through AC9, revealed significant diversity in their tissue distribution and regulatory properties. These isoforms are differentially expressed; for instance, AC1 is prominent in the brain, while AC5 and AC6 are critical in cardiac muscle. Structural studies, including those using X-ray crystallography, have elucidated details of their active site and interactions with regulators like forskolin. The soluble adenylyl cyclase found in spermatozoa and other tissues represents a distinct, evolutionarily conserved class sensitive to bicarbonate.

Mechanism of action

The core catalytic mechanism involves a two-metal-ion-assisted phosphoryl transfer reaction. The enzyme binds Mg2+ or Mn2+ ions, which coordinate the substrate adenosine triphosphate, facilitating nucleophilic attack and cyclization to form cyclic adenosine monophosphate and pyrophosphate. This reaction occurs at the interface of the two cytoplasmic C1 and C2 domains, which come together to form the active site. The transition state is stabilized by key amino acid residues, a mechanism informed by studies of homologous cyclases from organisms like Bacillus anthracis. The product, cyclic adenosine monophosphate, then diffuses to activate effector proteins such as protein kinase A.

Regulation

Regulation is multifaceted and isoform-specific. The canonical pathway involves stimulation by the Gαs subunit or inhibition by the Gαi subunit of heterotrimeric G proteins downstream of G protein-coupled receptors. Other major regulators include Ca2+, which can stimulate isoforms like AC1 via calmodulin or inhibit others like AC5 and AC6. Protein kinase C and protein kinase A can phosphorylate certain isoforms, providing feedback control. Importantly, many isoforms are directly inhibited by P-site inhibitors, which are adenosine analogs. The soluble adenylyl cyclase is uniquely regulated by bicarbonate, Ca2+, and ATP.

Physiological roles

Adenylyl cyclase is central to signal transduction in numerous systems. In the cardiovascular system, it mediates the positive inotropic and chronotropic effects of catecholamines like epinephrine on the heart. Within the central nervous system, it is crucial for processes underlying memory and learning, particularly in pathways involving the hippocampus. It regulates hormone secretion in the pituitary gland and insulin release from the pancreas. In olfaction, it is part of the signaling cascade in olfactory receptor neurons. Furthermore, it controls water permeability in the kidney via vasopressin signaling.

Clinical significance

Dysregulation of adenylyl cyclase signaling is implicated in various diseases. Mutations in Gαs that constitutively activate the enzyme are found in McCune-Albright syndrome and certain pituitary tumors. Conversely, inactivation mutations in Gαs are seen in Albright's hereditary osteodystrophy. Altered adenylyl cyclase activity is a hallmark of heart failure and cardiomyopathy. In bacterial pathogenesis, a key virulence factor of Bacillus anthracis is edema factor, a bacterial adenylyl cyclase toxin that disrupts host cell signaling. The enzyme is also a target for drug development, with compounds like forskolin used as research tools to directly stimulate activity.

Research history

The discovery of cyclic adenosine monophosphate by Earl Sutherland and his colleagues at Vanderbilt University in the late 1950s, for which Sutherland received the Nobel Prize in Physiology or Medicine in 1971, implied the existence of its synthesizing enzyme. Adenylyl cyclase was first characterized in the 1960s from studies of liver and heart tissues. The pivotal link to G proteins was established by Alfred Gilman and Martin Rodbell, whose work on G protein-coupled receptor signaling earned them the Nobel Prize in Physiology or Medicine in 1994. The subsequent cloning of multiple mammalian isoforms in the late 1980s and 1990s, notably by the laboratory of Ravi Iyengar, revealed the molecular diversity and complex regulation of this enzyme family.

Category:Enzymes Category:Signal transduction