FIG 4.
Carbon metabolism in the plastids of E. longa and E. gracilis. (a) The Calvin-Benson cycle (CBC) resident to this organelle is central to the plastid carbon metabolism, regulated by the ferredoxin/thioredoxin (Fd/Trx) system. Reduction of disulfide bonds by the Fd/Trx system activates several CBC enzymes (FBP, SBP, RPIA, PRK, RBCL; abbreviations explained below). FTR and FD of the Fd/Trx system require for their functioning a posttranslationally added Fe-S prosthetic group provided by the Fe-S assembly system (for details on the SUF pathway of Fe-S cluster assembly in the E. longa plastid, see reference 38). GapN apparently mediates the shuttling of reducing equivalent (NADPH) through the exchange of DHAP/GA3P and 3PG, reflecting the cytosolic NADPH/NADP+ ratio and thus the overall metabolic state of the cell. In contrast, the E. gracilis plastid is an energy-converting organelle, harvesting light into chemical energy bound as NADPH and ATP, and subsequently using this bound energy to fix CO2 into organic carbohydrates via the CBC. Enzyme abbreviations are color coded according to their inferred evolutionary origin; see the key. (b) Stoichiometric comparison of reactions converting glyceraldehyde 3-phosphate to 3-phosphoglycerate via glycolysis and the Calvin-Benson pathway. Abbreviations for the CB pathway: 3PG, 3-phosphoglycerate; ALDO, aldolase; DHAP, dihydroxyacetone-phosphate; FBP, fructose-1,6-bisphosphatase; GA3P, glyceraldehyde-3-phosphate; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PGK, 3-phosphoglygerate kinase; PGP, phosphoglycolate phosphatase; PLGG1, plastid glycolate/glycerate transporter; PRK, phosphoribulokinase; RBCL/RBCS, RuBisCO large/small subunit; RCA, RuBisCO activase; RPE, ribulose-5-phosphate epimerase; RPIA, ribulose-phosphate isomerase A; SBP, sedoheptulose-1,7-bisphosphatase; TCA, tricarboxylic acid; TKTL, transketolase; TPI, triose-phosphate isomerase; TPT, triose-phosphate translocator; ATPC, ADP/ATP translocase. Abbreviations for the Fd/Trx system: FD, ferredoxin; FNR, FD/NADP+ oxidoreductase; FTR, FD/TRX oxidoreductase; TRX, thioredoxin; ATPS, ATP synthase; LHC, light-harvesting complex.