Chlorophyll-mediated in vivo and in vitro photoisomerization of ζ-carotene and lycopene in Synechocystis PCC 6803

Carotenoid biosynthesis in photosynthetic organisms involves converting cis-isomers to trans forms through enzymatic and light-induced reactions. While enzymatic pathways are well-documented, the role of light, particularly chlorophyll-mediated sensitization, remains unclear. This gap in understanding complicates efforts to optimize carotenoid production and fully grasp the evolutionary interplay between enzymatic and light-driven processes. We investigated light-driven photoisomerization of carotenoid precursors 9,15,9'-tri-cis-ζ-carotene and prolycopene in Synechocystis sp. PCC 6803 using mutants deficient in isomerase enzymes Z-ISO and CrtH. The accumulation and transformation of poly-cis carotenoid isomers were examined under red and blue light in vitro and in vivo using HPLC and real-time kinetic analyses. Chlorophyll-a was demonstrated to act as a triplet-state sensitizer, facilitating efficient red-light-driven isomerization of prolycopene and 9,15,9'-tri-cis-ζ-carotene in vitro. In vivo experiments confirmed that light-induced isomerization partially compensates for enzymatic deficiencies, albeit at reduced efficiency due to cellular factors restricting chlorophyll excitation. This study reveals the critical interplay between light-driven and enzymatic processes in carotenoid biosynthesis. Our findings provide new insights into the regulation of carotenoid biosynthesis, emphasizing the evolutionary importance of enzyme specialization alongside light-mediated reactions.