On the contributions of photorespiration and compartmentation to the contrasting intramolecular H profiles of C and C plant sugars.

Compartmentation of C photosynthetic biochemistry into bundle sheath (BS) and mesophyll (M) cells, and photorespiration in C plants is predicted to have hydrogen isotopic consequences for metabolites at both molecular and site-specific levels. Molecular-level evidence was recently reported (Zhou et al., 2016), but evidence at the site-specific level is still lacking. We propose that such evidence exists in the contrasting H distribution profiles of glucose samples from naturally grown C, C and CAM plants: photorespiration contributes to the relative H enrichment in H and relative H depletion in H & H (the average of the two pro-chiral Hs and in particular H) in C glucose, while H-enriched C mesophyll cellular (chloroplastic) water most likely contributes to the enrichment at H; export of (transferable hydrogen atoms of) NADPH from C mesophyll cells to bundle sheath cells (via the malate shuttle) and incorporation of H-relatively unenriched BS cellular water contribute to the relative depletion of H & H respectively; shuttling of triose-phosphates (PGA: phosphoglycerate dand DHAP: dihydroacetone phosphate) between C bundle sheath and mesophyll cells contributes to the relative enrichment in H & H (in particular H) in C glucose.