Tracing the Path from 4-Hydroxyphenylpyruvate to the Benzoquinone Ring of Q6 and the p-aminobenzoate pathway in Yeast

Coenzyme Q (ubiquinone, CoQ) is an essential component of the mitochondrial electron transport chain and a major lipid antioxidant in eukaryotic cells. Formation of its benzoquinone ring requires aromatic precursors whose metabolic origin remains incompletely defined. Here, we elucidate the biochemical link between tyrosine metabolism and the synthesis of the benzoquinone head group of coenzyme Q6 (Q6) in Saccharomyces cerevisiae through the 4-hydroxymandelate (4HMA) pathway. Using isotopic tracing with 13C6-tyrosine, 13C6-4-hydroxybenzoate, and 13C6-p-aminobenzoate (pABA), we demonstrate that tyrosine-derived 4-hydroxyphenylpyruvate is converted into 4-hydroxybenzaldehyde via benzoylformate decarboxylation, defining a functional 4HMA pathway in yeast. Chemical inhibition of benzoylformate decarboxylase with methylbenzoylphosphonate led to accumulation of pathway intermediates, which were identified by GCMS. Consistently, mutants lacking ARO10, DLD1, or DLD2 exhibited strongly decreased 4-hydroxybenzaldehyde formation. Despite disruption of the 4HMA pathway, the pABA route from chorismate compensated, demonstrating S. cerevisiae's metabolic flexibility to use pABA or 4 HB and maintain Q6 ring biosynthesis. Our results provide a mechanistic framework linking aromatic amino acid metabolism to respiratory quinone biosynthesis in eukaryotes and support the evolutionary conservation of the 4HMA-derived pathway as a source of 4-hydroxybenzoate for Q synthesis in higher organisms.