All-trans-zeta-carotene desaturase (EC 1.3.99.26, Crtlb, phytoene desaturase (ambiguous), 2-step phytoene desaturase (ambiguous), two-step phytoene desaturase (ambiguous), CrtI (ambiguous)) is an enzyme with systematic name all-trans-zeta-carotene:acceptor oxidoreductase. This enzyme catalyses the following chemical reaction all-trans-zeta-carotene + 2 acceptor all-trans-lycopene + 2 reduced acceptor (overall reaction)(1a) all-trans-zeta-carotene + acceptor all-trans-neurosporene + reduced acceptor(1b) all-trans-neurosporene + acceptor all-trans-lycopene + reduced acceptor