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Underground isoleucine biosynthesis pathways in E. coli.

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Enzyme promiscuity enables E. coli to create new metabolic pathways. Underground metabolism allows compensatory biosynthesis of isoleucine when essential pathways are deleted, showcasing metabolic flexibility.

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Area of Science:

  • Microbial Metabolism
  • Synthetic Biology
  • Enzymology

Background:

  • Enzyme promiscuity is a key driver for the evolution of novel metabolic functions.
  • Underground metabolism refers to alternative biochemical pathways that can be activated under specific conditions.
  • E. coli possesses a complex metabolic network that can be reprogrammed for new functions.

Purpose of the Study:

  • To investigate E. coli's capacity to utilize underground metabolism for compensating essential gene deletions.
  • To identify novel biosynthetic pathways for isoleucine production in E. coli.
  • To explore the conditions under which these alternative pathways are activated.

Main Methods:

  • Systematic deletion of threonine deaminases in E. coli to block isoleucine biosynthesis.
  • Cultivation of the engineered strain under aerobic and anaerobic conditions.
  • Metabolomic analysis to identify novel 2-ketobutyrate biosynthesis routes.

Main Results:

  • Aerobic conditions activated a novel pathway for 2-ketobutyrate biosynthesis via cystathionine γ-synthase (MetB) promiscuous cleavage of O-succinyl-L-homoserine.
  • Anaerobic conditions enabled 2-ketobutyrate biosynthesis from propionyl-CoA and formate, mediated by pyruvate formate-lyase.
  • The anaerobic pathway contributed significantly to isoleucine production in wild-type E. coli when propionate was supplied.

Conclusions:

  • E. coli can effectively harness underground metabolism to bypass essential pathway deletions, demonstrating significant metabolic plasticity.
  • Promiscuous enzyme activities are crucial for the emergence of alternative metabolic routes, providing survival advantages.
  • This study highlights the potential of metabolic engineering to exploit and engineer alternative pathways for biotechnological applications.