Retinol as electron carrier in redox signaling, a new frontier in vitamin A research

Carotenoids and retinoids are a large family of biomolecules with diverse functions that occur in all three domains of life (1). By far the largest within this group of compounds are the pigments, counting by the hundreds and familiar to us as delightful colors (2). Plants use these as visual signals to advertise fruit ripening, to attract pollinators and seed dispersers (3), among endless other tasks. Insects employ them for camouflage (4), birds display them in their plumage for mate selection (5), fishes decorate their flanks with them to facilitate swimming in formation (6). While these pigments are the most diverse in numbers, a more circumscribed group of carotenes associated with photosynthesis (7) supersede them by sheer biomass. Chlorophyll-based photosynthetic systems employ carotenes as light-harvesting antennas to broaden the range of usable photons (8,9), but also as shields against excessive illumination (10,11). And a single retinoid, retinaldehyde, is the chemical engine of bacteriorhodopsins (12) and animal rhodopsins (13) alike. The former drive proton pumps for ATP generation (14), ion pumps for reverse osmosis (15), or sensors for phototactic responses (16), whereas rhodopsins are the universal chromophores of vision (17,18)