Ciclasa

Ciclasa, also called cyclase, is a general term in biochemistry for enzymes that catalyze the formation of cyclic structures from linear substrates. The defining reaction is cyclization, often accompanied by rearrangements or dehydration. Cyclases are found in diverse metabolic contexts, including signal transduction, terpenoid biosynthesis, and nucleotide metabolism.

There are two broad classes: nucleotide cyclases such as adenylyl cyclase and guanylyl cyclase that generate cyclic nucleotides used as intracellular second messengers; and a wider group that forms cyclic rings in small molecules or macromolecules, including terpene cyclases and steroid cyclases involved in secondary metabolism.

Mechanistically, cyclases employ different strategies. Many require metal ions such as Mg2+ or Mn2+ and promote cyclization through electrophilic activation, nucleophilic attack, or carbocation rearrangements. Terpene cyclases often trigger carbocation cascades to forge multiple rings, while nucleotide cyclases catalyze condensation and ring closure to yield cyclic nucleotides.

Biological roles are diverse. Adenylyl and guanylyl cyclases regulate signaling pathways by producing cAMP and cGMP, respectively; these cyclic nucleotides activate downstream kinases and ion channels. Other cyclases contribute to the biosynthesis of terpenoids, sterols, and other cyclic natural products essential for membrane structure, defense, and ecology.

In research and industry, cyclases are studied to understand catalytic mechanisms and to enable biocatalytic synthesis of cyclic compounds and therapeutics.