| Abstract
| - Cofactor requirement was determined for the heterologous expressed phytoene desaturases fromthe cyanobacterium Synechococcus and the higher plant Gentiana lutea. The cyanobacterial enzymeis dependent on either NAD(P) or plastoquinone, whereas only quinones such as plastoquinone canfunction as a cofactor for the phytoene desaturase from G. lutea. Enzyme kinetic studies were carriedout to determine a possible competition between the cofactors and the bleaching herbicidenorflurazon. For the Synechococcus enzyme, competition between norflurazon and NADP, as wellas plastoquinone, could be demonstrated. The Km values for these cofactors were 6.6 mM and 0.23μM, respectively. Inhibition of the phytoene desaturase from G. lutea by norflurazon was alsocompetitive with respect to plastoquinone. The Km values of both enzymes for plastoquinone werevery close. Keywords: Bleaching herbicides; competitive inhibitor; enzyme kinetics; norflurazon; phytoenedesaturase
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