FIG 6.

Flux partitioning around threonine and isoleucine metabolism. The flux distribution through the metabolic routes around the threonine node was investigated using stable isotope tracer experiments with ¹³C-labeled threonine and ¹³C-labeled isoleucine. (A and D) Portion of consumed (cons) threonine (A) or isoleucine (D) recovered in proteinogenic (prot) threonine, glycine, or isoleucine. The labeled fraction (light color) corresponds to consumed threonine or isoleucine directly incorporated into proteins, while the unlabeled fraction (dark color) represents the amino acids in proteins synthesized de novo from CCM precursors. (B and E) Contribution of threonine (B) or isoleucine (E) catabolism to propanol synthesis. The labeled fraction (light color) corresponds to the fraction of propanol synthesized using the carbon backbone from consumed threonine (B) or isoleucine (E), while the unlabeled fraction (dark color) represents the part of propanol synthesized from CCM via aspartate. Isotopic enrichments (defined as the molar ratio of the quantity of labeled compound to the total quantity of compound) are shown above each bar. The raw data and details of the calculations are provided in the supplemental material. (C and F) Partitioning of fluxes involved in the use of threonine (C) or isoleucine (F) during fermentation. The fraction of consumed threonine or isoleucine catabolized through a pathway, reported in colored boxes, was assessed from the molar ratio of the amount of a labeled proteinogenic amino acid or volatile compound to the amount of consumed amino acid. The fluxes indicated by light blue arrows were not quantified because of the small amounts of metabolites produced, which prevented accurate measurement of labeling incorporation.