The C21-formyl group in chlorophyll f originates from molecular oxygen

Chlorophylls (Chls) are the most important cofactors for capturing solar energy to drive photosynthetic reactions. Five spectral types of Chls have been identified to date, with Chl f having the most red-shifted absorption maximum because of a C2(1)-formyl group substitution of Chl f. However, the biochemical provenance of this formyl group is unknown. Here, we used a stable isotope labeling technique (O-18 and H-2) to determine the origin of the C2(1)-formyl group of Chl f and to verify whether Chl f is synthesized from Chl a in the cyanobacterial species Halomicronema hongdechloris. In the presence of either (H2O)-O-18 or O-18(2), the origin of oxygen atoms in the newly synthesized chlorophylls was investigated. The pigments were isolated with HPLC, followed by MS analysis. We found that the oxygen atom of the C2(1)-formyl group originates from molecular oxygen and not from H2O. Moreover, we examined the kinetics of the labeling of Chl a and Chl f from H. hongdechloris grown in 50% D2O-seawater medium under different light conditions. When cells were shifted from white light D2O-seawater medium to far-red light H2O-seawater medium, the observed deuteration in Chl f indicated that Chl(ide) a is the precursor of Chl f. Taken together, our results advance our understanding of the biosynthesis pathway of the chlorophylls and the formation of the formyl group in Chl f.