Fig. 6: Proposed biochemical model of the ‘Photosynthetic C1 pathway’ in California poplar.
From: The ‘photosynthetic C1 pathway’ links carbon assimilation and growth in California poplar
Photosynthetic C1 pathway initiated in chloroplasts by the Calvin-Benson cycle linked to NH4+ assimilation via the glutamine synthetase/glutamine oxoglutarate aminotransferase (GS/GOGAT) cycle11. Enzymes abbreviations shown in red include (1) ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), (2) 3-phosphoglycerate dehydrogenase (PGDH), (3) 3-phosphoserine aminotransferase (PSAT), (4) 3-phosphoserine phosphatase (PSP), (5) Serine hydroxymethyltransferase (SHMT), (6) methylenetetrahydrofolate reductase (MTHFR), and (7) methionine synthase (MS). PGDH, PSAT and PSP are enzymes of the phosphorylated serine pathway9. All enzymes of the photosynthetic C1 pathway and GS/GOAGT cycle are chloroplast localized, except MTHFR which is thought to be in the cytosol. Also shown is the AdoMet recycling pathway in the cytosol as a source of cellular methylation reactions with enzymes including methionine adenosyltransferase (MAT), methyltransferases (MT), and S-adenosyl-homocysteine hydrolase (SAHH). The blue balls represent carbon atoms recently assimilated by RuBisCO via the Calvin-Benson cycle during photosynthesis as CO2. Note, the regeneration of THF and homocysteine intermediates, and the lack of ATP and NADPH requirements for methionine synthesis in the chloroplast. However, the GS/GOAGT cycle for NH4+ assimilation requires both ATP and NAPDH. * Large kinetic isotope effect leading a natural 13C-depletion of C3 biomass relative to CO2. ** Large kinetic isotope effect leading to natural 13C-depletion of C1 carbon pools relative to C3 biomass.
