A New Member of the Family of Di-iron Carboxylate Proteins

Coenzyme Q (Q) is a lipid that functions as an electron carrier in the mitochondrial respiratory chain in eukaryotes. There are eight complementation groups of Q-deficient Saccharomyces cerevisiae mutants, designated coq1-coq8. Here we have isolated the COQ6 gene by functional complementation and, in contrast to a previous report, find it is not an essential gene. coq6 mutants are unable to grow on nonfermentable carbon sources and do not synthesize Q but instead accumulate the Q biosynthetic intermediate 3-hexaprenyl-4-hydroxybenzoic acid. The Coq6 polypeptide is imported into the mitochondria in a membrane potential-dependent manner. Coq6p is a peripheral membrane protein that localizes to the matrix side of the inner mitochondrial membrane. Based on sequence homology to known proteins, we suggest that COQ6 encodes a flavindependent monooxygenase required for one or more steps in Q biosynthesis.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

The Saccharomyces cerevisiae COQ6 Gene Encodes a Mitochondrial Flavin-dependent Monooxygenase Required for Coenzyme Q Biosynthesis

Gin

Hsu

Rothman

et al. 2003

“…clk-1 encodes a highly conserved (3,4) mitochondrial (5,6) protein that is required for ubiquinone (UQ) 1 biosynthesis in yeast (7) and worms (8,9). Recent evidence suggests that CLK-1 is a hydroxylase that converts demethoxyubiquinone (DMQ) into 5-hydroxy-UQ (10). Indeed, a bacterial CLK-1 homologue is capable of replacing the function of UbiFp, the unrelated enzyme that carries out this function in Escherichia coli.…”

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confidence: 99%

Ubiquinone Is Necessary for Mouse Embryonic Development but Is Not Essential for Mitochondrial Respiration

Levavasseur

Miyadera

Sirois

et al. 2001

121

140

Ubiquinone (UQ) is a lipid found in most biological membranes and is a co-factor in many redox processes including the mitochondrial respiratory chain. UQ has been implicated in protection from oxidative stress and in the aging process. Consequently, it is used as a dietary supplement and to treat mitochondrial diseases. Mutants of the clk-1 gene of the nematode Caenorhabditis elegans are fertile and have an increased life span, although they do not produce UQ but instead accumulate a biosynthetic intermediate, demethoxyubiquinone (DMQ). DMQ appears capable to partially replace UQ for respiration in vivo and in vitro. We have produced a vertebrate model of cells and tissues devoid of UQ by generating a knockout mutation of the murine orthologue of clk-1 (mclk1). We find that mclk1؊/؊ embryonic stem cells and embryos accumulate DMQ instead of UQ. As in the nematode mutant, the activity of the mitochondrial respiratory chain of ؊/؊ embryonic stem cells is only mildly affected (65% of wild-type oxygen consumption). However, mclk1؊/؊ embryos arrest development at midgestation, although earlier developmental stages appear normal. These findings indicate that UQ is necessary for vertebrate embryonic development but suggest that mitochondrial respiration is not the function for which UQ is essential when DMQ is present.clk-1 mutants of Caenorhabditis elegans are being studied for their pleiotropic phenotype, in which the rates of many biological processes are deregulated and slowed down on average (1, 2). clk-1 encodes a highly conserved (3, 4) mitochondrial (5, 6) protein that is required for ubiquinone (UQ) 1 biosynthesis in yeast (7) and worms (8, 9). Recent evidence suggests that CLK-1 is a hydroxylase that converts demethoxyubiquinone (DMQ) into 5-hydroxy-UQ (10). Indeed, a bacterial CLK-1 homologue is capable of replacing the function of UbiFp, the unrelated enzyme that carries out this function in Escherichia coli. Consistent with this finding, clk-1 mutants in yeast and worms accumulate DMQ 9 instead of producing UQ 9 (7, 9) (the subscript refers to the length of the isoprenoid side chain). In E. coli, DMQ 8 is able to sustain respiration in isolated membranes although at a lower rate than Q 8 (11). Similarly, DMQ 9 also appears to be capable of sustaining electron transport in clk-1 mutants at almost wild-type levels (6, 9). Furthermore, synthetic DMQ 2 can function as a co-factor for electron transport from Complex I and, albeit more poorly, from Complex II (9).It is not clear how the absence of UQ relates to the other phenotypes of clk-1 mutants as there is no correlation between the biochemical phenotype and the severity of the overall phenotype. Indeed, the quinone phenotype is identical for all three known clk-1 alleles (e2519, qm30, and qm51); UQ 9 is undetectable in the mitochondria in all three cases, and all three accumulate the same amount of DMQ. Yet, most of the features affected in clk-1 mutants are slowed down much more severely in the putative null alleles qm30 and qm51 than they are in the part...

“…In yeast, the COQ7 gene product was found to be responsible for the penultimate step in Q biosynthesis, the hydroxylation of demethoxy-Q (DMQ) to demethyl-Q (7-9). The predicted structure of Coq7p suggests that it is a di-iron carboxylate protein (10). C. elegans with mutations in the clk-1 gene, the COQ7 homologue, show slowed adult behaviors, including defecation cycles and pharyngeal pumping, slowed embryonic and post-embryonic development, and decreased brood sizes (11,12).…”

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confidence: 99%

“…The polyprenyldiphosphate synthase gene ispB was disrupted, and homologues from different organisms were introduced on plasmids, generating E. coli that produced Q 7 , Q 8 , Q 9 , or Q 10 . We examined the effects of feeding these different Q isoforms on reproductive fitness and progeny viability in both N2 and clk-1 animals.…”

mentioning

confidence: 99%

Reproductive Fitness and Quinone Content of Caenorhabditis elegans clk-1 Mutants Fed Coenzyme Q Isoforms of Varying Length

Jonassen

Davis

Larsen

et al. 2003

Caenorhabditis elegans clk-1 mutants lack coenzyme Q 9 and accumulate the biosynthetic intermediate demethoxy-Q 9 . A dietary source of ubiquinone (Q) is required for larval growth and development of the gonad and germ cells. We considered that uptake of the shorter Q 8 isoform present in the Escherichia coli food may contribute to the Clk phenotypes of slowed development and reduced brood size observed when the animals are fed Q-replete E. coli. To test the effect of isoprene tail length, N2 and clk-1 animals were fed E. coli engineered to produce Q 7 , Q 8 , Q 9 , or Q 10 . Wild-type nematodes showed no change in reproductive fitness regardless of the Q n isoform fed. clk-1(e2519) fed the Q 9 diet showed increased egg production; however, this diet did not improve reproductive fitness of the clk-1(qm30) animals. Furthermore, animals with the more severe clk-1(qm30) allele become sterile and their progeny inviable when fed Q 7 -containing bacteria. The content of Q 7 in the mitochondria of clk-1 animals was decreased relative to Q 8 , suggesting less effective transport of Q 7 to the mitochondria, impaired retention, or decreased stability. Additionally, regardless of E. coli diet, clk-1(qm30) animals contain a dysfunctional dense form of mitochondria. The gonads of clk-1(qm30) worms fed Q 7 -containing food were severely shrunken and disordered. The differential fertility of clk-1 mutant nematodes fed Q isoforms may result from changes in Q localization, altered recognition by Q-binding proteins, and/or potential defects in mitochondrial function resulting from the mutant CLK-1 polypeptide itself.