-!- This enzyme is involved in ubiquinone biosynthesis. -!- Ubiquinones from different organisms have a different number of prenyl units (for example, ubiquinone-6 in Saccharomyces, ubiquinone- 9 in rat and ubiquinone-10 in human), and thus the natural substrate for the enzymes from different organisms has a different number of prenyl units. -!- However, the enzyme usually shows a low degree of specificity regarding the number of prenyl units. -!- For example, the human COQ3 enzyme can restore biosynthesis of ubiquinone-6 in coq3 deletion mutants of yeast. -!- The enzymes from yeast, Escherichia coli and rat also catalyze the methylation of 3,4-dihydroxy-5-all-trans-polyprenylbenzoate (a reaction that is classified as EC 2.1.1.114).

-!- UbiG catalyzes both methylation steps in ubiquinone biosynthesis in Escherichia coli. -!- The second methylation is classified as EC 2.1.1.64. -!- In eukaryotes Coq3 catalyzes the two methylation steps in ubiquinone biosynthesis. -!- However, while the second methylation is common to both enzymes, the first methylation by Coq3 occurs at a different position within the pathway, and thus involves a different substrate and is classified as EC 2.1.1.114. -!- The substrate of the eukaryotic enzyme (3,4-dihydroxy-5-all-trans- polyprenylbenzoate) differs by an additional carboxylate moiety.