Phytoene desaturase (lycopene-forming)

Phytoene desaturase (lycopene-forming) (CrtI, four-step phytoene desaturase) (, 15-cis-phytoene:acceptor oxidoreductase (lycopene-forming)) are enzymes found in archaea, bacteria and fungi that are involved in carotenoid biosynthesis. They catalyze the conversion of colorless 15-cis-phytoene into a bright red lycopene in a biochemical pathway called the poly-trans pathway. The same process in plants and cyanobacteria utilizes four separate enzymes in a poly-cis pathway.

Biochemistry
Bacterial phytoene desaturases were shown to require FAD as a cofactor for their function. During the chemical reaction in total four additional double bonds are introduced into phytoene:


 * 15-cis-phytoene + 4 acceptor $$\rightleftharpoons$$ all-trans-lycopene + 4 reduced acceptor (overall reaction)
 * (1a) 15-cis-phytoene + acceptor $$\rightleftharpoons$$ all-trans-phytofluene + reduced acceptor
 * (1b) all-trans-phytofluene + acceptor $$\rightleftharpoons$$ all-trans-zeta-carotene + reduced acceptor
 * (1c) all-trans-zeta-carotene + acceptor $$\rightleftharpoons$$ all-trans-neurosporene + reduced acceptor:
 * (1d) all-trans-neurosporene + acceptor $$\rightleftharpoons$$ all-trans-lycopene + reduced acceptor

Applications
In 2000 it was discovered that the gene insertion of a bacterial phytoene desaturase into transgenic tomatoes increased the lycopene content without the need to alter several of the plants enzymes. This approach was later used in rice to increase its β-carotene content resulting in the Golden Rice project.