Molecular oxygen and the state of geometric isomerism of intermediates are essential in the carotene desaturation and cyclization reactions in daffodil chromoplasts.
The membrane-bound carotenogenic reaction sequence in daffodil chromoplasts can be subdivided in vitro into three reaction segments by varying the incubation parameters O2 and light. In the first segment, 15-cis-phytoene is desaturated to 15-cis-zeta-carotene (trans elimination of hydrogen) in the dark and in the presence of O2 as an essential cofactor. A photoisomerization of the 15-cis double bond of the accumulated zeta-carotene to trans is the prerequisite for the function of the second segment, the desaturation to 7,9,9',7'-tetra-cis-lycopene (prolycopene, cis elimination of hydrogen). The role of O2 as an electron acceptor is discussed and evidence for an oxidoreductase acting as a redox mediator between the desaturase (forming the polyene chromophore) and O2 is presented. A certain analogy to the desaturation of stearoyl-carrier protein, which also occurs in plastids, is proposed. The third segment, the cyclization of prolycopene, is active only in the absence of O2 and involves additional cis-trans isomerization reactions.
Institut für Biologie II, Zellbiologie, Universität Freiburg, Federal Republic of Germany.
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Beyer
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Mayer
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Kleinig
D002250:Carbon Radioisotopes; D002338:Carotenoids / Q000378:metabolism*; D002458:Cell Fractionation; D007536:Isomerism; D007700:Kinetics; D015388:Organelles / Q000378:metabolism* / Q000648:ultrastructure; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism; D010944:Plants / Q000378:metabolism*
D002250:Carbon Radioisotopes; D002338:Carotenoids; D010088:Oxidoreductases