Figure 5

Unique carbon-carbon bond re-arrangements explain abnormal isotope abundances from the 100% 1-¹³C Glc ILE. (a) Five different Metabolic Models (V-IX) were constructed to explain the ILE data; each column represents a different model. Pathways included in a model are denoted by a “Y”, color-coded according to the color of the pathway in Figure 3. (b) Vertical bars represent the SSR of each model and horizontal lines represent the acceptable SSR (65) corresponding to the number of redundant isotopomer measurements (45) in each model. (c) The carbon rearrangements of SGAT and SHMT demonstrate how 3-¹³C triose phosphates (derived from 1-¹³C glucose through glycolysis) result in 2-¹³C serine and glycine. Serine is conventionally known to be synthesized directly from 3-phosphoglycerate without carbon rearrangements via PSAT shown with the dashed arrow. (d) The photorespiratory action of RuBisCO yields glyoxylate from pentose phosphate, whereas the PPK pathway yields acetate that is converted to glyoxylate via the glyoxylate shunt. Each pathway yields 2-¹³C glycine and serine from 1-¹³C pentose phosphate arising from the reductive PPP. (e) Anaplerotic fixation of a mixture of intracellular ¹²CO₂ and ¹³CO₂ results in 1-¹³C pyruvate through reversible reactions in the TCA cycle. Succinate is a symmetric molecule; therefore C-1 and C-4 are equivalent. Oxaloacetate is 6% ¹³C-enriched at the C-4 position and 13% enriched at C-1. In comparison, pyruvate is 18% enriched at C-1, indicating that anaplerotic fixation cannot fully account for the labeling on pyruvate. (f) The oxidative PPP yields ¹³CO₂ and U-¹²C ribulose 5-phosphate from 1-¹³C glucose-6-phosphate. Photosynthetic fixation of CO₂ via RuBisCO then results in 1-¹³C pyruvate. (g) The predominant isotopomers of serine and glycine that were simulated in the poorly fit Model VII and the well fit Model IX are compared against the experimental abundance of each isotopomer.