Fig. 1: The rGlyP compared with the CBB cycle.
From: One-carbon fixation via the synthetic reductive glycine pathway exceeds yield of the Calvin cycle
a, In the CO2 module, formate is generated from electrochemical reduction of CO2. Formate can then be used to form pyruvate as indicated by the rGlyP or the CBB cycle, resulting in different formate requirements. In the rGlyP, the C1 module activates and reduces formate to 5,10-methylene-THF via three heterologous enzymes from M. extorquens: FtfL (Ft), FchA (Fc) and MtdA (M). Next, methylene-THF is converted to glycine by the glycine cleavage system (GCS, composed of the enzymes GcvT, H and P) operating in the reductive direction (C2 module). Finally, glycine is condensed with another 5,10-methylene-THF via serine hydroxymethyltransferase (GlyA, G) to yield serine, which is then dehydrated by serine deaminase (SdaA, S) to pyruvate. NADH is regenerated from formate via formate dehydrogenase. NADPH is regenerated by proton-translocating, membrane-bound transhydrogenase (PntAB). Formate and pyruvate are referred to as such in the text, but are here depicted in their protonated forms, in which they are metabolized. In the CBB cycle, three ribulose-5-phosphate molecules and CO2 are converted into 3-phosphoglycerate via the CBB cycle signature enzymes phosphoribulokinase (Prk) and RuBisCO. Subsequently, one of six generated 3-phosphoglycerate molecules can be used in metabolism, for example, via conversion to pyruvate. The other five 3-phosphoglycerate molecules are recycled into ribulose-5-phosphate via gluconeogenesis and the pentose phosphate pathway. For formatotrophic growth via the CBB cycle, all formate is oxidized into NADH and CO2 to supply energy and carbon to the CBB cycle. b, Comparison of the rGlyP with the CBB cycle for the number of enzymatic reactions required, cost of ATP and reducing equivalents, minimal thermodynamic driving force MDF (Supplementary Fig. 1) and biomass yield predicted by the model at a doubling time of 14 h and assuming GAM = 135 mmol gCDW−1 and NGAM = 3 mmol gCDW−1 h−1). Values were calculated for both routes from formate to pyruvate at 10% CO2 (3.4 mM). The GAPDH reaction was set as NAD+-dependent for the CBB cycle, and the standard NADH/NAD+ ratio of 0.1 was used, as the ratio has not been determined for C. necator grown on formate.
