Neophaseic acid biosynthesis is a pathway that begins in the chloroplast and ends in the cytosol by which violaxanthin becomes neophaseate, synthesizing abscisic acid in the process. Neophaseate is an abscisic acid derivative whose synthesis provides a mechanism for controlling abscisic acid concentration. First, neoxanthin synthase catalyzes the opening of the violaxanthin epoxide ring to form neoxanthin. Second, a yet unidentified neoxanthin isomerase is theorized to isomerize neoxanthin to 9'-cis-neoxanthin. Third, 9-cis-epoxycarotenoid dioxygenase (NCED) uses oxygen to cleave 9'-cis-neoxanthin to form xanthoxin and C25-allenic-apo-aldehyde. This enzyme requires Fe2+ as a cofactor. Next, a xanthoxin transporter is theorized to export xanthoxin from the chloroplast into the cytosol to continue abscisic acid biosynthesis, but it has yet to be discovered. Fourth, xanthoxin dehydrogenase, located in the cytosol, catalyzes the conversion of xanthoxin and NAD to abscisic aldehyde, NADH, and a proton with the help of a molybdenum cofactor (MoCo). Fifth, abscisic-aldehyde oxidase converts abscisic aldehyde, water, and oxygen into hydrogen peroxide, hydrogen ion, and abscisic acid. Sixth, abscisic acid 8'-hydroxylase / abscisic acid 9'-hydroxylase uses NADPH, oxygen, and a proton to convert abscisic acid into 9'-hydroxyabscisate and water. Seventh, 9'-hydroxyabscisate spontaneously becomes neophaseate.

Neophaseic Acid Biosynthesis References