This review examines ubiquinones (UQ), membrane-bound quinone molecules critical to microbial energy metabolism. While traditionally viewed as passive electron carriers in respiratory chains, ubiquinones play multifaceted roles in oxidative stress management and potentially gene regulation. The article synthesizes knowledge of UQ structure, biosynthesis, and function across prokaryotes and lower eukaryotes, with emphasis on Escherichia coli and Saccharomyces cerevisiae. UQ biosynthesis involves a complex pathway derived from the shikimate pathway; the pathway is oxygen-dependent under aerobic conditions but uses alternative mechanisms anaerobically. Key genes (ubi genes in E. coli, COQ genes in yeast) encode enzymes catalyzing sequential modifications to form the quinone nucleus and isoprenoid side chains. The length of the isoprenoid side chain determines which ubiquinone species is produced. Transcriptional regulation of ubiquinone biosynthesis appears responsive to oxygen availability and glucose concentration. Recent genetic engineering efforts have successfully produced UQ-8 and UQ-10 in various bacterial hosts by overexpressing biosynthetic genes and substrate precursors, with important implications for clinical applications.

Key findings

• Ubiquinones function not only as electron carriers in respiration but also in oxidative stress management and possibly gene regulation in microbial cells
• The ubiquinone biosynthetic pathway diverges between prokaryotes and eukaryotes after synthesis of 3-polyprenyl-4-hydroxybenzoate; the specific polyprenyl diphosphate synthase determines the isoprenoid side chain length and species produced
• E. coli ubiquinone levels are highly regulated by oxygen availability, with aerobic cells containing more UQ-8 while anaerobic cells accumulate menaquinone, likely through post-translational mechanisms
• Genetic engineering approaches successfully increase ubiquinone production by co-expressing key biosynthetic genes (ubiCA, ispB) with substrate precursors in various bacterial hosts
• Evidence suggests reduced ubiquinone may serve as a signaling molecule regulating gene expression through uncharacterized regulatory pathways in bacteria