Chalcone isomerase

Chalcone isomerase (EC 5.5.1.6) is an enzyme that catalyzes the general chemical reaction

a chalcone ${\displaystyle \rightleftharpoons }$ a flavanone

In the biosynthesis of anthocyanins in plants, an important example of this reaction converts the chalcone produced by chalcone synthase to naringenin:

This reaction can occur spontaneously but provides racemic material. The enzyme constrains the reaction to give only the (S) isomer of the flavanone.

This enzyme belongs to the family of isomerases, specifically the class of intramolecular lyases. The systematic name of this enzyme class is flavanone lyase (decyclizing). This enzyme is also called chalcone-flavanone isomerase. This enzyme participates in flavonoid biosynthesis.

The Petunia hybrida (Petunia) genome contains two genes coding for very similar enzymes, ChiA and ChiB, but only the first seems to encode a functional chalcone isomerase.