Lactic acid fermentation of xylose by Escherichia coli: carbon tracer studies on the C₂ + C₁ condensation reaction

The ubiquitous distribution of the pentose molecule in nature and particularly its presence in certain enzymes and in nucleic acids emphasizes the metabolic significance of these carbohydrates. In living systems the pentoses are undergoing continuous metabolic changes. It thus appeared that investigations concerning the metabolic decomposition of the pentose molecule would be important from a comparative biochemical point of view. The advantages of a microbial system as a working model for biochemical investigations are well known. Investigations concerning pentose metabolism were, therefore, carried out with a washed bacterial cell suspension utilizing xylose as a sole substrate.

Previous investigators have obtained evidence that one of the first reactions in the fermentation of pentoses was a carbon bond cleavage resulting in the production of a C₃ and a C₂ fragment. The importance of the C₂ fragment in enzymatic systems is well recognized and it thus seemed plausible that investigations on bacterial pentose fermentations would be of significant value to the field of intermediary metabolism.

Preliminary investigations revealed that cells of Escherichia coli K-12 grown in the presence of pentose possessed the ability to ferment pentoses in the nonproliferating cell state. Additional experiments concerning the anaerobic decomposition of xylose re-emphasized the metabolic importance of the C₂ fragment. In fermentations conducted at low pH, lactic acid was produced in a ratio of approximately 1.3 moles per mole of xylose fermented. Since a maximum of only 1.0 moles of lactic acid could have been derived from the C₃ portion of the xylose molecule this was taken as a priori evidence that the C₂ portion of the C₅ molecule was also involved in the formation of lactic acid. Furthermore, at low pH, there was a net fixation of C0₂ which indicated that a direct participation of C0₂ was involved in the production of lactate. There were a number of pathways by which lactate could have been formed from C₂ and carbon tracer experiments were conducted in order to determine the main mechanism of C₂⟶C₃ in this system. These experiments demonstrated that C₂ tracers (C¹⁴H₃C00H and C¹⁴H₃CH₂0H) were converted to the CH₃-CH0H-portion of lactate while C₁ tracers (c¹³0₂ and HC¹⁴OOH) appeared in the lactate carboxyl. This latter piece of evidence was a further indication that lactate was formed via a C₂ + C₁ condensation. This condensation functioned at pH 7.4 as well as at pH 5.3. With C¹⁴H₃C00H as tracer succinate was labeled exclusively in the methylene carbons and it was concluded that the lactate was not in close equilibrium with succinate.

The production of lactate via C₂ + C₁ condensation further emphasizes the general role of this reaction in intermediary metabolism. The fact that C₂ produced from pentoses apparently can be converted to C₃ also provides a mechanism for the conversion of pentoses into hexoses and vice versa.