One ring or two? Determination of ring number in carotenoids by lycopene «-cyclases

Carotenoids in the photosynthetic membranes of plants typically contain two b-rings (e.g., b-carotene and zeaxanthin) or one «and one b-ring (e.g., lutein). Carotenoids with two «-rings are uncommon. We reported earlier that the Arabidopsis thaliana lycopene «-cyclase (LCYe) adds one «-ring to the symmetrical linear substrate lycopene, whereas the structurally related lycopene b-cyclase (LCYb) adds two b-rings. Here we describe a cDNA encoding LCYe in romaine lettuce (Lactuca sativa var. romaine), one of the few plant species known to accumulate substantial quantities of a carotenoid with two «-rings: lactucaxanthin. The product of the lettuce cDNA, similar in sequence to the Arabidopsis LCYe (77% amino acid identity), efficiently converted lycopene into the bicyclic «-carotene in a heterologous Escherichia coli system. Regions of the lettuce and Arabidopsis «-cyclases involved in the determination of ring number were mapped by analysis of chimeric «-cyclases constructed by using an inverse PCR approach. A single amino acid was found to act as a molecular switch: lettuce LCYe mutant H457L added only one «-ring to lycopene, whereas the complementary Arabidopsis LCYe mutant, L448H, added two «-rings. An R residue in this position also yields a bi-«-cyclase for both the lettuce and Arabidopsis enzymes. Construction and analysis of chimera of related enzymes with differing catalytic activities provide an informative approach that may be of particular utility for studying membrane-associated enzymes that cannot easily be crystallized or modeled to existing crystal structures.