On the Structure and Function of the Phytoene Desaturase CRTI from Pantoea ananatis, a Membrane-Peripheral and FAD-Dependent Oxidase/Isomerase
Figure 1
Phytoene desaturation – “complex” vs. “simple”.
Left, the plant/cyanobacterial system consisting of the two desaturases, phytoene desaturase (PDS) and ζ-carotene desaturase (ZDS). The pathway involves specific poly-cis-intermediates and results in the formation of 7,9,9′7′-tetra-cis-lycopene ( = prolycopene). Cis-trans isomerases act at the 9,15,9′-tri-cis-ζ-carotene (Z-ISO) and prolycopene (CRTISO) stage, the latter forming all-trans-lycopene, the substrate for lycopene cyclases. The electron acceptors identified so far for PDS (assumed here to be the same for the related ZDS) are plastoquinone and the plastoquinone:oxygen oxidoreductase PTOX. The necessity for an electron donating branch, resulting in redox chains into which PDS integrates has been suggested. Right, CRTI-mediated phytoene desaturation encompassing all four desaturation steps and one cis-trans isomerization step to form all-trans-lycopene. The desaturase CRTI and the isomerase CRTISO share sequential similarity.