Method for isomerizing double bonds

A method for isomerizing a double bond, the method comprising sequential steps of:

• exposing a carotenoid substrate to an isomerase enzyme, wherein the carotenoid substrate comprises a conjugated double bond system with at least three double bonds including a central double bond, wherein the isomerase enzyme comprises a heme b cofactor, wherein the heme b cofactor comprises an iron having an iron (III) oxidation state and the central double bond in the carotenoid substrate has a cis stereochemistry, wherein the isomerase enzyme has fewer than four hundred residues and comprises SEQ ID NO:
• 11;
• exposing the isomerase enzyme to a reducing agent in vitro such that the isomerase enzyme changes from the iron (III) oxidation state to an iron (II) oxidation state, wherein the isomerase enzyme isomerizes the central double bond when in the iron (II) oxidation state and does not isomerize the central double bond when in the iron (III) oxidation state;
• and
• permitting the central double bond in the carotenoid substrate to undergo isomerization from the cis stereochemistry to a trans stereochemistry, wherein the isomerization is catalyzed by the isomerase enzyme, wherein the carotenoid substrate is 9,15,9′-cis-ζ-carotene and the central double bond is a 15-15′ double bond of the carotenoid substrate.