An elusive electron shuttle from a facultative anaerobe.

Area of Science:

• Microbiology
• Biochemistry
• Bioenergetics

Background:

• Anaerobic bacteria utilize insoluble minerals as terminal electron acceptors.
• Extracellular electron transfer (EET) mechanisms are crucial for bacterial physiology and bioenergy.
• A previous study proposed an unidentified electron shuttle molecule in *Shewanella oneidensis* MR-1.

Purpose of the Study:

• To identify the specific molecule responsible for extracellular electron transfer in *Shewanella oneidensis* MR-1.
• To elucidate the role of this molecule in electron transport to insoluble extracellular acceptors.

Main Methods:

• Isolation and chemical synthesis of potential electron shuttle molecules.
• Anaerobic growth experiments with menaquinone-deficient mutants.
• Bioelectrochemistry analyses to determine redox potential and electron transfer capabilities.

Main Results:

• Identified 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) as the electron shuttle.
• ACNQ is derived from 1,4-dihydroxy-2-naphthoic acid (DHNA) via a non-enzymatic pathway.
• ACNQ and DHNA restored extracellular electron transfer in menaquinone-deficient mutants.
• ACNQ functions as an electron shuttle at -0.32 V_Ag/AgCl without affecting menaquinone production or cytochrome c expression.

Conclusions:

• ACNQ is the identified extracellular electron shuttle in *Shewanella oneidensis* MR-1.
• The non-enzymatic synthesis of ACNQ explains why genetic approaches failed to identify it.
• This finding has significant implications for understanding microbial electron transport and developing bioenergy technologies.