A single five-step desaturase is involved in the carotenoid biosynthesis pathway to beta-carotene and torulene in Neurospora crassa.
Phytoene desaturase Al-1 from Neurospora crassa was expressed in Escherichia coli and an active enzyme was isolated which catalyzed the stepwise introduction of up to five double bonds into the substrate phytoene. The major reaction products were 3, 4-didehydrolycopene and lycopene. Several of the desaturation intermediates, zeta-carotene, neurosporene, and lycopene, were also accepted as a substrate by Al-1. In contrast to the structurally related bacterial enzymes, the cofactor involved in the dehydrogenation reaction was NAD for Al-1. In situ competition with a neurosporene- and lycopene-converting hydratase and cyclase indicated that these enzymes can divert intermediates of the desaturation sequence. Based on the in vitro and in vivo results, the organization of the phytoene desaturase from N. crassa was proposed as an assembly of identical protein units which are responsible for the multistep reaction. However, the spatial arrangement should be loose enough to allow an exchange of individual intermediates in both directions in and out of this complex. Since gamma-carotene is not accepted as a substrate by Al-1, the formation of torulene must proceed exclusively by the cyclization of 3,4-didehydrolycopene.
Hausmann
A
A
Biosynthesis Group, Botanical Institute, Goethe University, D-60054 Frankfurt, Germany.
Sandmann
G
G
D002338:Carotenoids / Q000031:analogs & derivatives / Q000096:biosynthesis*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D009492:Neurospora crassa / Q000201:enzymology*; D010088:Oxidoreductases / Q000235:genetics* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014170:Transformation, Genetic; D019207:beta Carotene / Q000096:biosynthesis
D011994:Recombinant Proteins; D019207:beta Carotene; D002338:Carotenoids; C078922:torulene; D010088:Oxidoreductases; C022398:phytoene dehydrogenase