Question

Do plastoquinone contains copper containing protein

Answer 1

Quinones and Quinone Enzymes, Part A
Ewa Swiezewska, in Methods in Enzymology, 2004
Introduction
Ubiquinone (UQ) and plastoquinone (PQ) function as carriers of electrons in the mitochondrial inner membrane and chloroplast thylacoids, respectively. In the reduced form, both UQ and PQ have been shown to act as antioxidants1,2 in plant cells. Plastoquinone is also involved in the chlororespiratory pathway3 and serves as the effector of carotenoid biosynthesis.4 In addition, activation of specific kinases was keyed to the redox status of the PQ pool.5 On the other hand, UQ is the obligatory cofactor of uncoupling proteins6 (responsible for the dissipation of the mitochondrial chemiosmotic gradient), producing heat rather than ATP; this observation has been made for bovine heart mitochondria, nevertheless, could be possibly generalized for plant uncoupling proteins described recently.7 Finally, UQ and PQ as the constituents of biological membranes modulate their physicochemical properties. Molecular dynamics simulations of UQ inside a lipid bilayer suggests two preferred positions of the molecule; one close to the phospholipid headgroups, the other in the membrane midplane.8 It has also been shown that the isoprenoid side chain length rather than the redox state of prenylquinones determines their effectiveness in perturbation of thermotropic properties of the lipid bilayer.9 As postulated recently, cation leak inhibition resulting in saving of metabolic energy might be attributed to certain isoprenoid compounds (e.g., UQ for mitochondria and PQ for the chloroplasts).10
Biosynthesis of PQ and UQ has been studied in detail, and sequences of reactions leading from the isoprenoid precursors, namely prenyl diphosphate (solanesyl diphosphate for nine-isoprene-unit long side chain) and aromatic “head” (4-hydroxybenzoate and homogentizate for UQ and PQ, respectively) have been described. Fundamental studies are summarized in this series by J. Soll for PQ11 and by F. Lutke-Brinkhaus and H. Kleining for UQ.12
A few enzymes of both pathways have been characterized at the molecular level. Recently published cloning of solanesyl diphosphate synthase13 of Arabidopsis thaliana provides a good platform for future studies. According to the computer prediction, this enzyme can be transported into chloroplasts and possibly mitochondria, whereas kinetic analysis indicates that geranylgeranyl diphosphate and farnesyl diphosphate are its preferred substrates. Examination of the total spinach microsomal fraction performed earlier14 reveals that farnesyl diphosphate in contrast to geranyl and geranylgeranyl diphosphates is not accepted as the precursor of the prenylated quinone intermediates. Whether these differences in substrate specificity indicate the existence of solanesyl diphosphate synthase isoenzymes in endoplasmic reticulum (ER) and chloroplasts remains a task for future investigations. The only eukaryotic hydroxybenzoate prenyltransferase purified15 and cloned16 so far is geranyl diphosphate:4-hydroxybenzoate geranyltransferase from Lithospermum erythrorhizon involved in shikonin biosynthesis; this enzyme is localized at the ER. Corresponding homogentisate phytyltransferase responsible for the biosynthesis of tocopherols (structurally related to PQ) in A. thaliana has been cloned17 recently. Of other enzymes catalyzing the biosynthetic reactions downstream the prenyl group transfer only the product of the A. thaliana gene homologous to yeast COq3 encoding a methyltransferase involved in UQ biosynthesis is characterized18 and found to be localized within mitochondrial membranes.
Although the structures of intermediates of the UQ and PQ biosynthetic pathways are generally accepted, questions concerning the intracellular localization of the biosynthetic reactions have been raised when UQ was proved to be formed in the ER in addition to mitochondria of the rat liver.19 In addition, involvement of the same precursor, all-trans-prenyl diphosphate, m