Molecular oxygen and the state of geometric isomerism of intermediates are essential in the carotene desaturation and cyclization reactions in daffodil chromoplasts

Abstract

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‐ζ‐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 ζ‐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.