On the contributions of photorespiration and compartmentation to the contrasting intramolecular 2H profiles of C3 and C4 plant sugars

Abstract

Compartmentation of C4 photosynthetic biochemistry into bundle sheath (BS) and mesophyll (M) cells, and photorespiration in C3 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 2H distribution profiles of glucose samples from naturally grown C3, C4 and CAM plants: photorespiration contributes to the relative 2H enrichment in H5 and relative 2H depletion in H1 & H6 (the average of the two pro-chiral Hs and in particular H6, pro-R) in C3 glucose, while 2H-enriched C3 mesophyll cellular (chloroplastic) water most likely contributes to the enrichment at H4; export of (transferable hydrogen atoms of) NADPH from C4 mesophyll cells to bundle sheath cells (via the malate shuttle) and incorporation of 2H-relatively unenriched BS cellular water contribute to the relative depletion of H4 & H5 respectively; shuttling of triose-phosphates (PGA: phosphoglycerate dand DHAP: dihydroacetone phosphate) between C4 bundle sheath and mesophyll cells contributes to the relative enrichment in H1 & H6 (in particular H6, pro-R) in C4 glucose.