Although many organisms capture or respond to sunlight, few enzymes are known to be driven by light. Among these are DNA photolyases and the photosynthetic reaction centers. Here, we show that the microalga NC64A harbors a photoenzyme that acts in lipid metabolism. This enzyme belongs to an algae-specific clade of the glucose-methanol-choline oxidoreductase family and catalyzes the decarboxylation of free fatty acids to n-alkanes or -alkenes in response to blue light. Crystal structure of the protein reveals a fatty acid-binding site in a hydrophobic tunnel leading to the light-capturing flavin adenine dinucleotide (FAD) cofactor. The decarboxylation is initiated through electron abstraction from the fatty acid by the photoexcited FAD with a quantum yield>80%. This photoenzyme, which we name fatty acid photodecarboxylase, may be useful in light-driven, bio-based production of hydrocarbons.

Cryptochromes are flavin-based photoreceptors occurring throughout the biological kingdom, which regulate growth and development in plants and are involved in the entrainment of circadian rhythms of both plants and animals. A number of recent theoretical works suggest that cryptochromes might also be the receptors responsible for the sensing of the magnetic field of the earth (e.g. in insects, migratory birds, or migratory fish). Cryptochromes undergo forward light-induced reactions involving electron transfer to excited state flavin to generate radical intermediates, which correlate with biological activity. Here, we give evidence of a mechanism for the reverse reaction, namely dark reoxidation of protein-bound flavin in Arabidopsis thaliana cryptochrome (AtCRY1) by molecular oxygen that involves formation of a spin-correlated FADH ⅐ -superoxide radical pair. Formation of analogous radical pairs in animal cryptochromes might enable them to function as magnetoreceptors.Cryptochromes are blue light-absorbing photoreceptors found throughout the biological kingdom, ranging from microbes to plants, animals, and humans (1-3). They are evolutionarily derived from photolyases, DNA repair enzymes using violet/blue light to repair damage (cyclobutane pyrimidine dimers) caused by exposure of DNA to UV light. Cryptochromes and photolyases exhibit a high degree of structural homology and bind the same flavin adenine dinucleotide (FAD) and folate light-absorbing cofactors; however, they differ very much in their function in biological systems. Cryptochromes have lost the ability to repair DNA, but they play a key role in controlling the growth and development in plants and in the circadian clock in plants and animals (4). Furthermore, recent evidence suggests that cryptochromes are likely to function as light-dependent magnetic field sensors, used for example by insects or migratory birds and fish for directional responses (5-7).Although photolyases and the mechanism of their action have been subject to extensive studies in the last five decades (3, 9), the photochemistry and the signaling pathways of cryptochromes are just beginning to become known. Recent attention has focused primarily on the pathway of flavin photoreduction as a possible mechanism of photoreceptor activation (10 -14). In the case of both plant and insect cryptochromes, studies with isolated proteins have shown that these can be photoreduced from flavin in the oxidized state to the radical state by light in the presence of reducing agent. This is in marked contrast to photolyases, where photoreduction results in formation of the fully reduced (FADH Ϫ ) form, which is active in DNA repair (11). Photoreduction of flavin correlates with biological activity, as shown by action spectra that demonstrate typical 450-nm peak and shoulders indicative of oxidized flavin with almost no activity above 500 nm (13, 15). Furthermore, both plant and animal cryptochromes show reduced biological activity upon co-illumination with green light, which selectively depletes the...