MetaCyc Compound Class: a plastoquinol

Superclasses: an acceptora redox electron carrierMembrane-Electron-Carriersan electron-transfer quinol
an aromatic compounda quinolan electron-transfer quinol

Plastoquinones are found in the chloroplasts of higher plants, red, brown and green algae, and in cyanobacteria, and are important members of the photosynthetic electron transfer chain, where they carry electrons from photosystem II to the cytochrome b6f complex. In fact, it has been shown that in cyanobacteria essentially all electron transport through unbound quinones proceeds via PQ in cyanobacterial thylakoids. Apart from PQ the only quinone found in thylakoids is vitamin K1, which is associated with PS I [Vermaas01].

Plastoquinones also serve as cofactors involved in the desaturation of phytoene in the synthesis of carotenoids (see trans-lycopene biosynthesis II (plants)).

They are analogs of ubiquinones (Coenzyme Q), with two methyl groups substituting the methoxy groups found in Q. Plastoquinone can function as acceptor/donor for one or two electrons. The fully reduced form is called a plastoquinol.

Plastoquinone was isolated originally in 1946 from alfalfa, but was not identified at the time. It was rediscovered in 1959 and its structure was described in the same year (see [Collins81]).

Please note that the structure shown here is that of plastoquinol-9 and is not accurate for this generic class of compounds, which contains plastoquinols with different side-chain lengths.

a plastoquinol compound structure

Chemical Formula: C53H82O2


Molecular Weight: 751.23 Daltons

Monoisotopic Molecular Weight: 750.6314818764 Daltons


InChI: InChI=1S/C53H82O2/c1-40(2)21-13-22-41(3)23-14-24-42(4)25-15-26-43(5)27-16-28-44(6)29-17-30-45(7)31-18-32-46(8)33-19-34-47(9)35-20-36-48(10)37-38-51-39-52(54)49(11)50(12)53(51)55/h21,23,25,27,29,31,33,35,37,39,54-55H,13-20,22,24,26,28,30,32,34,36,38H2,1-12H3/b41-23+,42-25+,43-27+,44-29+,45-31+,46-33+,47-35+,48-37+

Standard Gibbs Free Energy of Change Formation (ΔfG in kcal/mol): 1039.152Inferred by computational analysis [Latendresse13]

Reactions known to consume the compound:

photosynthesis light reactions :
2 an oxidized plastocyanin[chloroplast thylakoid lumen] + a plastoquinol[chloroplast thylakoid membrane] + 2 H+[chloroplast stroma] → 2 a reduced plastocyanin[chloroplast thylakoid lumen] + a plastoquinone[chloroplast thylakoid membrane] + 4 H+[chloroplast thylakoid lumen]

methane oxidation to methanol II :
methane + an electron-transfer quinol + oxygen → methanol + an electron-transfer quinone + H2O

partial TCA cycle (obligate autotrophs) :
succinate[in] + an electron-transfer quinone[membrane] ← fumarate[in] + an electron-transfer quinol[membrane]

Not in pathways:
2 an oxidized c-type cytochrome[out] + an electron-transfer quinol[membrane] → 2 a reduced c-type cytochrome[out] + an electron-transfer quinone[membrane] + 2 H+[in]

Reactions known to produce the compound:

photosynthesis light reactions :
2 a plastoquinol[chloroplast thylakoid membrane] + 4 H+[chloroplast thylakoid lumen] + oxygen[chloroplast thylakoid lumen] ← hν + 2 a plastoquinone[chloroplast thylakoid membrane] + 4 H+[chloroplast stroma] + 2 H2O[chloroplast thylakoid lumen]

trans-lycopene biosynthesis II (plants) :
15-cis-phytoene + a plastoquinone → 15,9'-di-cis-phytofluene + a plastoquinol
15,9'-di-cis-phytofluene + a plastoquinone → 9,15,9'-tri-cis-ζ-carotene + a plastoquinol

(5R)-carbapenem carboxylate biosynthesis , L-citrulline biosynthesis , L-Nδ-acetylornithine biosynthesis , L-proline degradation :
L-proline + an electron-transfer quinone[inner membrane] → (S)-1-pyrroline-5-carboxylate + an electron-transfer quinol[inner membrane] + H+

4-hydroxymandelate degradation :
(S)-4-hydroxymandelate + an electron-transfer quinone → 2-(4-hydroxyphenyl)-2-oxoacetate + an electron-transfer quinol

L-alanine degradation I :
D-alanine + an electron-transfer quinone[inner membrane] + H2O → ammonium + pyruvate + an electron-transfer quinol[inner membrane]

sulfide oxidation I (sulfide-quinone reductase) :
hydrogen sulfide + an electron-transfer quinone → intracellular S0 + an electron-transfer quinol

TCA cycle I (prokaryotic) :
(S)-malate + an electron-transfer quinone[inner membrane] → oxaloacetate + an electron-transfer quinol[inner membrane]

TCA cycle IV (2-oxoglutarate decarboxylase) , TCA cycle V (2-oxoglutarate:ferredoxin oxidoreductase) :
succinate[in] + an electron-transfer quinone[membrane] → fumarate[in] + an electron-transfer quinol[membrane]

TCA cycle VII (acetate-producers) :
(S)-malate + an electron-transfer quinone[inner membrane] → oxaloacetate + an electron-transfer quinol[inner membrane]
succinate[in] + an electron-transfer quinone[membrane] → fumarate[in] + an electron-transfer quinol[membrane]

trans-4-hydroxy-L-proline degradation I :
trans-4-hydroxy-L-proline[in] + an electron-transfer quinone[membrane] → (3R,5S)-1-pyrroline-3-hydroxy-5-carboxylate[in] + an electron-transfer quinol[membrane] + H+[in]

trans-lycopene biosynthesis II (plants) :
7,9,9'-cis-neurosporene + an electron-transfer quinone → prolycopene + an electron-transfer quinol
9,9'-di-cis-ζ-carotene + an electron-transfer quinone → 7,9,9'-cis-neurosporene + an electron-transfer quinol

UMP biosynthesis :
(S)-dihydroorotate + an electron-transfer quinone[inner membrane] → orotate + an electron-transfer quinol[inner membrane]

Not in pathways:
D-glucopyranose[out] + an electron-transfer quinone[membrane] + 2 H+[in] → D-glucono-1,5-lactone[out] + an electron-transfer quinol[membrane] + 2 H+[out]
NADPH + an electron-transfer quinone[inner membrane] + H+ → NADP+ + an electron-transfer quinol[inner membrane]
an electron-transfer quinone[inner membrane] + NAD(P)H + H+an electron-transfer quinol[inner membrane] + NAD(P)+
sn-glycerol 3-phosphate[in] + an electron-transfer quinone[membrane] → glycerone phosphate[in] + an electron-transfer quinol[membrane]
a D-amino acid[in] + an electron-transfer quinone[membrane] + H2O[in] → a 2-oxo carboxylate[in] + ammonium[in] + an electron-transfer quinol[membrane]

phenylacetate degradation II (anaerobic) :
phenylacetyl-CoA + 2 a quinone + H2O → 2 a quinol + phenylglyoxylyl-CoA

quinate degradation I :
L-quinate + a quinone → 3-dehydroquinate + a quinol

shikimate degradation I :
shikimate + a quinone → 3-dehydroshikimate + a quinol

thiosulfate oxidation II (via tetrathionate) :
2 thiosulfate + a quinone → tetrathionate + a quinol

Not in pathways:
a cyclic alcohol + a quinone → a cyclic ketone + a quinol

Reactions known to both consume and produce the compound:

superpathway of photosynthetic hydrogen production :
a plastoquinone + NAD(P)H + H+a plastoquinol + NAD(P)+

nitrate reduction I (denitrification) , nitrate reduction VII (denitrification) :
nitrate + an electron-transfer quinol[inner membrane] ↔ nitrite + an electron-transfer quinone[inner membrane] + H2O

In Reactions of unknown directionality:

Not in pathways:
15-cis-phytoene + 2 a plastoquinone = 9,15,9'-tri-cis-ζ-carotene + 2 a plastoquinol

Not in pathways:
an oxidized coenzyme F420 + an electron-transfer quinol = a reduced coenzyme F420 + an electron-transfer quinone
NADH + an electron-transfer quinone + H+ = NAD+ + an electron-transfer quinol
an aldehyde + an electron-transfer quinone + H2O = a carboxylate + an electron-transfer quinol + H+
an (R)-2-hydroxyacid + an electron-transfer quinone = a 2-oxo acid + an electron-transfer quinol
formate + an electron-transfer quinone + H+ = CO2 + an electron-transfer quinol
9,9'-di-cis-ζ-carotene + 2 an electron-transfer quinone = prolycopene + 2 an electron-transfer quinol

Not in pathways:
1-(β-D-ribofuranosyl)-1,4-dihydronicotinamide + a quinone + H+ = a quinol + 1-(β-D ribofuranosyl)nicotinamide

In Redox half-reactions:
a plastoquinone[membrane] + 4 H+[in] + 2 e-[membrane]a plastoquinol[membrane] + 2 H+[out],
a plastoquinone[membrane] + 2 H+[in] + 2 e-[membrane]a plastoquinol[membrane],
a plastoquinone[membrane] + 2 H+[out] + 2 e-[membrane]a plastoquinol[membrane],
an electron-transfer quinone[membrane] + 2 H+[in] + 2 e-[membrane]an electron-transfer quinol[membrane]

Revised 17-Aug-2009 by Caspi R, SRI International


Collins81: Collins MD, Jones D (1981). "Distribution of isoprenoid quinone structural types in bacteria and their taxonomic implication." Microbiol Rev 45(2);316-54. PMID: 7022156

Kawamukai02: Kawamukai M (2002). "Biosynthesis, bioproduction and novel roles of ubiquinone." J Biosci Bioeng 94(6);511-7. PMID: 16233343

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

Vermaas01: Vermaas WFJ (2001). "Photosynthesis and Respiration in Cyanobacteria." Encyclopedia of Life Sciences, Macmillan Publishers Ltd, Nature Publishing Group.

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
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