Vanillic Acid Restores Coenzyme Q Biosynthesis and ATP Production in Human Cells Lacking COQ6

Figure 6

Proposed models of the CoQ₁₀ biosynthetic pathway in mammals showing two alternative sequences of reactions (the decaprenyl chain is abbreviated as R). Path (a) corresponds to the traditional model in which COQ6 acts after the condensation of the ring with the isoprene tail, while path (b) shows the unknown decarboxylase and hydroxylase (in blue) acting before COQ6, consistent with the accumulation of 4-HP₁₀ in COQ6-deficient cells. Path (c) depicts the same pathway when VA is employed instead of 4-HB. Biosynthesis can occur even in the absence of COQ6 because carbon C5 of the ring is already bound to a methoxyl group in VA.