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Mechanism and dynamics of fatty acid photodecarboxylase

The microalga Chlorella variabilis NC64A is able to capture light to initiate enzymatic photodecarboxylation of fatty acids. X-ray crystallography revealed the crystal structure of the fatty acid photodecarboxylase (CvFAP). The results, along with a range of computational and spectroscopic techniques, provide insight into the mechanism of hydrocarbon production using light energy.

Photoenzymes offer the possibility to trigger and monitor catalytic steps and associated structural changes on very short time scales. The light-driven fatty acid photodecarboxylase (FAP) photoenzyme is interesting for practical applications such as the production of cosmetics emollients, chemical synthons, solvents, and fuels. Increasing our understanding and the repertoire of photoenzymatic mechanisms may help in the design of

catalysts to perform new reactions or in the development of new light-controlled proteins for optogenetics.

The crystal structure of CvFAP was solved at 1.8Å resolution, providing a detailed view of the active-site architecture and revealing a fatty acid-binding site in a hydrophobic tunnel leading to the light-capturing flavin adenine dinucleotide (FAD) cofactor [1]. Two C18 fatty acids were visible, one on the surface of the protein and the other in the active site (Figure 37a-d).

The oxidised dark-state crystal structure obtained at beamline ID29 shows the FAD isoalloxazine ring with a butterfly bending shape deviating by 17.4° from planarity (Figure 37e). Photoreduction could explain the conversion of the supposedly planar oxidised flavin to the bent reduced form. However, in crystallo ultraviolet- visible absorption microspectrophotometry and Raman microspectrophotometry performed with icOS on ID29 and FIP, low-dose crystallography and molecular dynamics simulations using a recent flavin force field

Fig. 37: a) Structure of CvFAP determined from synchrotron data at 100 K. b) Binding of the two substrate molecules. c) Position of the peripheral substrate (FA2) partly obstructing the channel leading to the active site tunnel. d) Close-up

view of the catalytic site. e) Superposition of the FAD isoalloxazine rings from the SFX dark structure (blue; molecule A) and the synchrotron structures. From D. Sorigué et al., Mechanism and dynamics of fatty acid photodecarboxylase,

Science 372, 6538 (2021). Reprinted with permission from AAAS.