cyclases

Cyclases are enzymes that catalyze the formation of cyclic structures in substrates, linking atoms to form rings or rearranging the carbon framework to create cyclic products. They span a wide range of metabolic contexts, from primary signaling pathways to the biosynthesis of secondary metabolites. The unifying feature of cyclases is their promotion of ring formation, rather than simple bond cleavage or oxidation.

Several major families illustrate the diversity of cyclases. Adenylyl cyclase and guanylyl cyclase convert nucleotides to cyclic nucleotides, producing cAMP and cGMP, respectively. These cyclic nucleotides serve as key second messengers in cellular signaling and are regulated by receptors, ligands, or other enzymes, with both membrane-bound and soluble forms described. Terpene cyclases catalyze the cyclization of linear isoprenoid precursors such as geranyl, farnesyl, and geranylgeranyl pyrophosphates to form a variety of cyclic terpenes (for example, limonene, pinene, and caryophyllene). Their mechanisms often involve ionization of diphosphate groups and carbocation-driven cyclizations, with distinct class I and class II modes.

Cyclooxygenases are another well-known family, exemplified by COX-1 and COX-2, which catalyze the cyclization steps in prostaglandin biosynthesis from arachidonic acid. These enzymes are important pharmacological targets, as reflected by nonsteroidal anti-inflammatory drugs. In addition, nucleic and natural product biosynthesis can involve radical-SAM–type cyclases or other specialized cyclases that forge complex rings in diverse molecular architectures.

In summary, cyclases drive the formation of cyclic structures across biology, enabling signaling molecules, pigments, hormones, and countless natural products that rely on ring-containing frameworks.