MetaCyc Reaction:

Superclasses: Reactions Classified By Conversion TypeSimple ReactionsChemical Reactions
Reactions Classified By SubstrateSmall-Molecule Reactions

EC Number:

Enzymes and Genes:

Escherichia coli K-12 substr. MG1655: bifunctional 3-demethylubiquinone-8 3-O-methyltransferase and 2-octaprenyl-6-hydroxyphenol methylaseInferred from experiment: ubiG
Homo sapiens: hexaprenyldihydroxybenzoate methyltransferase: COQ3

In Pathway: ubiquinol-8 biosynthesis (prokaryotic)

Note that this reaction equation differs from the official Enzyme Commission reaction equation for this EC number, which can be found here .

The direction shown, i.e. which substrates are on the left and right sides, is in accordance with the direction in which it was curated.

Most BioCyc compounds have been protonated to a reference pH value of 7.3. Please see the PGDB Concepts Guide for more information.

Mass balance status: Balanced.

Direct generic reaction:
a 3-(all-trans-polyrenyl)benzene-1,2-diol + S-adenosyl-L-methionine → a 2-methoxy-6-(all-trans-polyprenyl)phenol + S-adenosyl-L-homocysteine + H+ (

Enzyme Commission Primary Name: 2-polyprenyl-6-hydroxyphenol methylase

Enzyme Commission Synonyms: ubiG (gene name, ambiguous), ubiG methyltransferase (ambiguous), 2-octaprenyl-6-hydroxyphenol methylase

Standard Gibbs Free Energy (ΔrG in kcal/mol): -4.359993Inferred by computational analysis [Latendresse13]

The enzyme is involved in the pathway of ubiquinone-8 in Escherichia coli. COQ3 from human, yeast and rat also catalyse the reaction although this is not a step in biosynthesis of ubiquinone in these organisms [Poon99]. The enzyme also catalyses the methylation of 3-demethylubiquinone-8 (this activity is classified as EC [Hsu96].

Enzyme Commission Summary:
UbiG catalyses both methylation steps in ubiquinone biosynthesis in Escherichia coli. The second methylation is classified as EC (3-demethylubiquinol 3-O-methyltransferase) [Hsu96]. In eukaryotes Coq3 catalyses 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 (polyprenyldihydroxybenzoate methyltransferase). The substrate of the eukaryotic enzyme (3,4-dihydroxy-5-all-trans-polyprenylbenzoate) differs by an additional carboxylate moiety.

Gene-Reaction Schematic

Gene-Reaction Schematic

Instance reactions of [a 3-(all-trans-polyrenyl)benzene-1,2-diol + S-adenosyl-L-methionine → a 2-methoxy-6-(all-trans-polyprenyl)phenol + S-adenosyl-L-homocysteine + H+] (
i1: 3-(all-trans-decaprenyl)benzene-1,2-diol + S-adenosyl-L-methionine → 2-methoxy-6-(all-trans-decaprenyl)phenol + S-adenosyl-L-homocysteine + H+ (

i2: 3-(all-trans-octaprenyl)benzene-1,2-diol + S-adenosyl-L-methionine → 2-methoxy-6-(all-trans-octaprenyl)phenol + S-adenosyl-L-homocysteine + H+ (
i3: 3-(all-trans-nonaprenyl)benzene-1,2-diol + S-adenosyl-L-methionine → 2-methoxy-6-(all-trans-nonaprenyl)phenol + S-adenosyl-L-homocysteine + H+ (

i4: 3-(all-trans-heptaprenyl)benzene-1,2-diol + S-adenosyl-L-methionine → 2-methoxy-6-(all-trans-heptaprenyl)phenol + S-adenosyl-L-homocysteine + H+ (

Unification Links: KEGG:R04988, Rhea:27770

Relationship Links: BRENDA:EC:, ENZYME:EC:, IUBMB-ExplorEnz:EC:

Revised 11-Oct-2010 by Caspi R, SRI International


Hsu96: Hsu AY, Poon WW, Shepherd JA, Myles DC, Clarke CF (1996). "Complementation of coq3 mutant yeast by mitochondrial targeting of the Escherichia coli UbiG polypeptide: evidence that UbiG catalyzes both O-methylation steps in ubiquinone biosynthesis." Biochemistry 1996;35(30);9797-806. PMID: 8703953

Latendresse13: Latendresse M. (2013). "Computing Gibbs Free Energy of Compounds and Reactions in MetaCyc."

Poon99: Poon WW, Barkovich RJ, Hsu AY, Frankel A, Lee PT, Shepherd JN, Myles DC, Clarke CF (1999). "Yeast and rat Coq3 and Escherichia coli UbiG polypeptides catalyze both O-methyltransferase steps in coenzyme Q biosynthesis." J Biol Chem 274(31);21665-72. PMID: 10419476

Report Errors or Provide Feedback
Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by SRI International Pathway Tools version 19.5 on Sun Nov 29, 2015, BIOCYC14A.