Abstract
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 C21-formyl group substitution of Chl f However, the biochemical provenance of this formyl group is unknown. Here, we used a stable isotope labeling technique (18O and 2H) to determine the origin of the C21-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 H218O or 18O2, 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 C21-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.
Highlights
Chlorophylls (Chls) are the most important cofactors for capturing solar energy to drive photosynthetic reactions
The results indicate that four oxygen atoms in Chl a are derived from water (Fig. 2A), which is consistent with previous reports [13, 15]
Similar to the Chl a, the newly synthesized Chl f was 8 Da heavier with a maximum mass of 914.5 m/z indicating that four oxygen atoms in Chl f are derived from water (Fig. 2B)
Summary
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 C21formyl group substitution of Chl f. Chlorophylls (Chls) are very effective photo-capturing cofactors in oxygenic photosynthetic organisms as they have very strong absorption bands in the visible region of the spectrum, enabling them to absorb solar energy efficiently [1]. The different Chls possess distinct spectral properties and absorb light energy at different wavelength regions of the solar spectrum, primarily because of their side chain substitutions, excepting the c-type Chls, which differ in the saturation level of the macrocycle, as well as modifications at C-17 [2]
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