The correct answer is (B) MQ > DMQ > UQ.

Quinone Roles in E. coli Respiration

E. coli uses three quinones in its electron transport chain: ubiquinone (UQ, high redox potential +113 mV), menaquinone (MK or MQ, low -74 mV), and demethylmenaquinone (DMK or DMQ, intermediate +36 mV). UQ dominates aerobic respiration, while MK and DMK support anaerobic processes like fumarate reduction by fumarate reductase.

Anaerobic Fumarate Growth Specifics

Under anaerobic conditions with fumarate as electron acceptor, low-potential quinones prevail to match fumarate’s reduction potential. MK and DMK levels rise significantly (up to 9-fold vs. aerobic), while UQ drops sharply (e.g., from 1090 nmol/g DCW aerobic to ~150 nmol/g anaerobic in wild-type).

Quinone Abundance Data

Studies quantify anaerobic pools: MK at ~47-61% (highest), DMK at ~26-39%, UQ minimal (~27% oxidized form, low total). MK is primary for formate dehydrogenase and fumarate reductase; DMK aids broadly; UQ is marginal.

Option Analysis

• (A) UQ > DMQ > MQ: Incorrect; UQ is lowest anaerobically.

• (B) MQ > DMQ > UQ: Correct; matches quantified order (MK highest, then DMK, UQ least).

• (C) MQ = DMQ > UQ: Incorrect; MK exceeds DMK.

• (D) MQ > UQ > DMQ: Incorrect; UQ < DMK anaerobically.

E. coli anaerobic fumarate respiration relies on specific quinone abundance where menaquinone (MQ) predominates over demethylmenaquinone (DMQ) and ubiquinone (UQ). This MQ > DMQ > UQ order optimizes electron transfer in low-oxygen conditions.

Quinone Biosynthesis and Redox Potentials

E. coli synthesizes UQ (aerobic primary, +113 mV), MQ (-74 mV), and DMQ (+36 mV, MQ precursor). Anaerobically, menA/menB pathways boost MQ/DMQ; ubi genes downregulate UQ.

Fumarate Respiration Mechanism

Fumarate reductase uses quinol oxidation (prefers MQ/DMQ) to reduce fumarate, generating proton motive force. Low-potential MQ/DMQ match fumarate’s E_m (-33 mV); UQ mismatches.

Experimental Evidence on Abundance

Anaerobic cultures show MQ ~47-61%, DMQ ~26-39%, UQ ~27% (mostly oxidized, low total). Mutants confirm: UQ-only strains fail succinate production; MQ/DMQ-only grow normally.

CSIR NET Exam Relevance

This highlights adaptive respiration for competitive exams. MQ dominance ensures efficiency in fumarate as terminal acceptor, unlike aerobic UQ reliance.