Biosynthesis of ubiquinones in animals

Coenzyme Q in Thraustochytrium sp. RT2316-16: Effect of the Medium Composition

Coenzyme Q (CoQ; ubiquinone) is an essential component of the respiratory chain. It is also a potent antioxidant that prevents oxidative damage to DNA, biological membranes, and lipoproteins. CoQ comprises a six-carbon ring with polar substituents that interact with electron acceptors and donors, and a hydrophobic polyisoprenoid chain that allows for its localization in cellular membranes. Human CoQ has 10 isoprenoid units (CoQ10) within the polyisoprenoid chain. Few microorganisms produce CoQ10. This work shows that Thraustochytrium sp. RT2316-16 produces CoQ10 and CoQ9. The CoQ10 content in RT2316-16 depended strongly on the composition of the growth medium and the age of the culture, whereas the CoQ9 content was less variable probably because it served a different function in the cell. Adding p-hydroxybenzoic acid to the culture media positively influenced the CoQ10 content of the cell. The absence of some B vitamins and p-aminobenzoic acid in the culture medium negatively affected the growth of RT2316-16, but reduced the decline in CoQ10 that otherwise occurred during growth. The highest content of CoQ9 and CoQ10 in the biomass were 855 μg g-1 and 10 mg g-1, respectively. The results presented here suggest that the thraustochytrid RT2316-16 can be a potential vehicle for producing CoQ10. Metabolic signals that trigger the synthesis of CoQ10 in RT2316-16 need to be determined for optimizing culture conditions.

Mechanisms and Therapeutic Effects of Benzoquinone Ring Analogs in Primary CoQ Deficiencies

Coenzyme Q (CoQ) is a conserved polyprenylated lipid composed of a redox-active benzoquinone ring and a long polyisoprenyl tail that serves as a membrane anchor. CoQ biosynthesis involves multiple steps, including multiple modifications of the precursor ring 4-hydroxybenzoic acid. Mutations in the enzymes involved in CoQ biosynthesis pathway result in primary coenzyme Q deficiencies, mitochondrial disorders whose clinical heterogenicity reflects the multiple biological function of CoQ. Patients with these disorders do not always respond to CoQ supplementation, and CoQ analogs have not been successful as alternative approaches. Progress made in understanding the CoQ biosynthesis pathway and studies of supplementation with 4-hydroxybenzoic acid ring analogs have opened a new area in the field of primary CoQ deficiencies treatment. Here, we will review these studies, focusing on efficacy of the different 4-hydroxybenzoic acid ring analogs, models in which they have been tested, and their mechanisms of action. Understanding how these compounds ameliorate biochemical, molecular, and/or clinical phenotypes of CoQ deficiencies is important to develop the most rational treatment for CoQ deficient patients, depending on their molecular defects.

Coenzyme Q Biosynthesis: An Update on the Origins of the Benzenoid Ring and Discovery of New Ring Precursors

Coenzyme Q (ubiquinone or CoQ) is a conserved polyprenylated lipid essential for mitochondrial respiration. CoQ is composed of a redox-active benzoquinone ring and a long polyisoprenyl tail that serves as a membrane anchor. A classic pathway leading to CoQ biosynthesis employs 4-hydroxybenzoic acid (4HB). Recent studies with stable isotopes in E. coli, yeast, and plant and animal cells have identified CoQ intermediates and new metabolic pathways that produce 4HB. Stable isotope labeling has identified para-aminobenzoic acid as an alternate ring precursor of yeast CoQ biosynthesis, as well as other natural products, such as kaempferol, that provide ring precursors for CoQ biosynthesis in plants and mammals. In this review, we highlight how stable isotopes can be used to delineate the biosynthetic pathways leading to CoQ.

Impact of Chemical Analogs of 4-Hydroxybenzoic Acid on Coenzyme Q Biosynthesis: From Inhibition to Bypass of Coenzyme Q Deficiency

Coenzyme Q is a lipid that participates to important physiological functions. Coenzyme Q is synthesized in multiple steps from the precursor 4-hydroxybenzoic acid. Mutations in enzymes that participate to coenzyme Q biosynthesis result in primary coenzyme Q deficiency, a type of mitochondrial disease. Coenzyme Q₁₀ supplementation of patients is the classical treatment but it shows limited efficacy in some cases. The molecular understanding of the coenzyme Q biosynthetic pathway allowed the design of experiments to bypass deficient biosynthetic steps with analogs of 4-hydroxybenzoic acid. These molecules provide the defective chemical group and can reactivate endogenous coenzyme Q biosynthesis as demonstrated recently in yeast, mammalian cell cultures, and mouse models of primary coenzyme Q deficiency. This mini review presents how the chemical properties of various analogs of 4-hydroxybenzoic acid dictate the effect of the molecules on CoQ biosynthesis and how the reactivation of endogenous coenzyme Q biosynthesis may achieve better results than exogenous CoQ₁₀ supplementation.

Mechanistic Details of Early Steps in Coenzyme Q Biosynthesis Pathway in Yeast

Coenzyme Q (Q) is a redox lipid that is central for the energetic metabolism of eukaryotes. The biosynthesis of Q from the aromatic precursor 4-hydroxybenzoic acid (4-HB) is understood fairly well. However, biosynthetic details of how 4-HB is produced from tyrosine remain elusive. Here, we provide key insights into this long-standing biosynthetic problem by uncovering molecular details of the first and last reactions of the pathway in the yeast Saccharomyces cerevisiae, namely the deamination of tyrosine to 4-hydroxyphenylpyruvate by Aro8 and Aro9, and the oxidation of 4-hydroxybenzaldehyde to 4-HB by Hfd1. Inactivation of the HFD1 gene in yeast resulted in Q deficiency, which was rescued by the human enzyme ALDH3A1. This suggests that a similar pathway operates in animals, including humans, and led us to propose that patients with genetically unassigned Q deficiency should be screened for mutations in aldehyde dehydrogenase genes, especially ALDH3A1.

para-Aminobenzoic acid is a precursor in coenzyme Q6 biosynthesis in Saccharomyces cerevisiae

Coenzyme Q (ubiquinone or Q) is a crucial mitochondrial lipid required for respiratory electron transport in eukaryotes. 4-Hydroxybenozoate (4HB) is an aromatic ring precursor that forms the benzoquinone ring of Q and is used extensively to examine Q biosynthesis. However, the direct precursor compounds and enzymatic steps for synthesis of 4HB in yeast are unknown. Here we show that para-aminobenzoic acid (pABA), a well known precursor of folate, also functions as a precursor for Q biosynthesis. A hexaprenylated form of pABA (prenyl-pABA) is normally present in wild-type yeast crude lipid extracts but is absent in yeast abz1 mutants starved for pABA. A stable (13)C(6)-isotope of pABA (p- amino[aromatic-(13)C(6)]benzoic acid ([(13)C(6)]pABA)), is prenylated in either wild-type or abz1 mutant yeast to form prenyl-[(13)C(6)]pABA. We demonstrate by HPLC and mass spectrometry that yeast incubated with either [(13)C(6)]pABA or [(13)C(6)]4HB generate both (13)C(6)-demethoxy-Q (DMQ), a late stage Q biosynthetic intermediate, as well as the final product (13)C(6)-coenzyme Q. Pulse-labeling analyses show that formation of prenyl-pABA occurs within minutes and precedes the synthesis of Q. Yeast utilizing pABA as a ring precursor produce another nitrogen containing intermediate, 4-imino-DMQ(6). This intermediate is produced in small quantities in wild-type yeast cultured in standard media and in abz1 mutants supplemented with pABA. We suggest a mechanism where Schiff base-mediated deimination forms DMQ(6) quinone, thereby eliminating the nitrogen contributed by pABA. This scheme results in the convergence of the 4HB and pABA pathways in eukaryotic Q biosynthesis and has implications regarding the action of pABA-based antifolates.

Taurine in development and nutrition

Taurine is an amino acid that is widely distributed in the fluids and tissues of man. In mammals, taurine is a major end-product of methionine metabolism. Taurine is found in most mammalian tissues but is only present in trace amounts in many plants. During fetal development of the brain in man and other mammals taurine is present in high concentrations and declines to lower, adult concentrations during neonatal life. However, during this time there is a net accumulation of taurine when the amount per brain rather than per gram of tissue is calculated. In man, taurine is apparently an essential nutrient, unlike in other animals which have a much greater capacity to synthesize this compound. The human infant, is particular, needs a dietary supply of taurine to synthesize the bile salt taurocholate. Thus, taurine appears to be an important component of the developing brain and must be supplied to man in the diet.

Ubiquinone biosynthesis by the microsomal fraction from rat liver

The distribution and biosynthesis of ubiquinone were investigated in vivo in rats and using liver slices. In addition to mitochondria, Golgi vesicles and lysosomes also contain large amounts of this lipid, and even the plasma membrane, peroxisomes and microsomes demonstrate easily measurable amounts. The spectral and chromatographic properties of microsomal ubiquinone were identical to those of its mitochondrial counterpart. When pentane was used to deplete beef heart submitochondrial particles of ubiquinone, NADH and succinate oxidase activities could be restored by reincorporation of microsomal ubiquinone. Injection of [3H]mevalonate into the portal vein of rats and incubation of liver slices with [3H]mevalonate and [3H]- and [14C]tyrosine demonstrated that labeling of mitochondrial ubiquinone was initially much lower than labeling of the microsomal lipid. Furthermore, intraportal injection of [3H]mevalonate resulted in the rapid appearance of labeled ubiquinone in the blood. These results indicate that ubiquinone is synthesized not only in mitochondria, but also on the endoplasmic reticulum of rat liver.

The modulating effect of an inhibitor of cholesterolgenesis present in bovine milk upon the synthesis of cholesterol, dolichol and ubiquinone

Bovine milk contains two inhibitors of hepatic cholesterol genesis. One of these, identified as orotic acid, influences the early segment of the cholesterol biosynthetic pathway and suppresses the conversion of acetate to mevalonate. In this study the other inhibitor was shown to curtail the formation of compounds past farnesyl pyrophosphate on the squalene-cholesterol branch of the pathway. Thus cholesterol synthesis may be suppressed while the production of two other products of the branched pathway, dolichol and ubiquinone, is allowed to continue. The possible role of these ingested regulators in the metabolism of the young until they achieve sufficient development is discussed.

Identification of 3,4-dihydroxy-5-hexaprenylbenzoic acid as an intermediate in the biosynthesis of ubiquinone-6 by Saccharomyces cerevisiae

The mutant strain of Saccharomyces cerevisiae E3-24 is unable to synthesize ubiquinone-6. When this mutant is grown in the presence of p-hydroxy[U-14C]benzoate or p-hydroxy[carboxy-14C]benzoate, a radioactive compound accumulates. This new metabolite has been isolated and identified as 3,4-dihydroxy-5-hexaprenylbenzoate (3,4-DHHB). Aerobically grown prototrophic strains of S. cerevisiae were found to contain only low levels of this compound. When strain X963-18C, blocked at homoserine O-transacetylase (in methionine biosynthesis), was deprived of methionine, ubiquinone biosynthesis ceased, and 3,4-DHHB was observed to accumulate. This suggested that S-adenosylmethionine (SAM) could be the methyl donor for 3,4-DHHB. Restoration of methionine to the cultures released this block and resulted in the conversion of 3,4-DHHB to ubiquinone-6, demonstrating a precursor--product relationship. The identification of 3,4-DHHB as an intermediate in ubiquinone biosynthesis in yeast establishes an alternate pathway for ubiquinone biosynthesis in eukaryotes.