A cyclase is an enzyme, almost always a lyase, that catalyzes a chemical reaction to form a cyclic compound. Important cyclase enzymes include:

Soluble Adenylyl Cyclase as an Evolutionarily Conserved Bicarbonate Sensor
Pituitary adenylate cyclase-activating polypeptide and its receptors in amphibians.
Pituitary Adenylate Cyclase-Activating Polypeptide and Its Receptors: From Structure to Functions
Pituitary adenylate cyclase-activating polypeptide and its receptors: 20 years after the discovery
Purification of a soluble isoform of guanylyl cyclase-activating-factor synthase
The glucagon-sensitive adenyl cyclase system in plasma membranes of rat liver. VI. Evidence for a role of membrane lipids.
The Glucagon-sensitive Adenyl Cyclase System in Plasma Membranes of Rat Liver IV. EFFECTS OF GUANYL NUCLEOTIDES ON BINDING OF 125I-G...
Soluble guanylate cyclase from bovine lung: activation with nitric oxide and carbon monoxide and spectral characterization of the fe...
Potent and selective inhibition of nitric oxide-sensitive guanylyl cyclase by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one.
A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor