Enzymatic formation of dehydrodolichal and dolichal, new products related to yeast dolichol biosynthesis

A pseudoautosomal glycosylation disorder prompts the revision of dolichol biosynthesis

Dolichol is a lipid critical for N-glycosylation as a carrier for activated sugars and nascent oligosaccharides. It is commonly thought to be directly produced from polyprenol by the enzyme SRD5A3. Instead, we found that dolichol synthesis requires a three-step detour involving additional metabolites, where SRD5A3 catalyzes only the second reaction. The first and third steps are performed by DHRSX, whose gene resides on the pseudoautosomal regions of the X and Y chromosomes. Accordingly, we report a pseudoautosomal-recessive disease presenting as a congenital disorder of glycosylation in patients with missense variants in DHRSX (DHRSX-CDG). Of note, DHRSX has a unique dual substrate and cofactor specificity, allowing it to act as a NAD+-dependent dehydrogenase and as a NADPH-dependent reductase in two non-consecutive steps. Thus, our work reveals unexpected complexity in the terminal steps of dolichol biosynthesis. Furthermore, we provide insights into the mechanism by which dolichol metabolism defects contribute to disease.

Isoprenoid Alcohols are Susceptible to Oxidation with Singlet Oxygen and Hydroxyl Radicals

Isoprenoids, as common constituents of all living cells, are exposed to oxidative agents—reactive oxygen species, for example, singlet oxygen or hydroxyl radicals. Despite this fact, products of oxidation of polyisoprenoids have never been characterized. In this study, chemical oxidation of isoprenoid alcohols (Prenol-2 and -10) was performed using singlet oxygen (generated in the presence of hydrogen peroxide/molybdate or upon photochemical reaction in the presence of porphyrin), oxygen (formed upon hydrogen peroxide dismutation) or hydroxyl radical (generated by the hydrogen peroxide/sonication, UV/titanium dioxide or UV/hydrogen peroxide) systems. The structure of the obtained products, hydroxy-, peroxy- and heterocyclic derivatives, was studied with the aid of mass spectrometry (MS) and nuclear magnetic resonance (NMR) methods. Furthermore, mass spectrometry with electrospray ionization appeared to be a useful analytical tool to detect the products of oxidation of isoprenoids (ESI–MS analysis), as well as to establish their structure on the basis of the fragmentation spectra of selected ions (ESI–MS/MS analysis). Taken together, susceptibility of polyisoprenoid alcohols to various oxidizing agents was shown for the first time.

Genetic defects in dolichol metabolism

Congenital disorders of glycosylation (CDG) comprise a group of inborn errors of metabolism with abnormal glycosylation of proteins and lipids. Patients with defective protein N-glycosylation are identified in routine metabolic screening via analysis of serum transferrin glycosylation. Defects in the assembly of the dolichol linked Glc(3)Man(9)GlcNAc(2) glycan and its transfer to proteins lead to the (partial) absence of complete glycans on proteins. These defects are called CDG-I and are located in the endoplasmic reticulum (ER) or cytoplasm. Defects in the subsequent processing of protein bound glycans result in the presence of truncated glycans on proteins. These defects are called CDG-II and the enzymes involved are located mainly in the Golgi apparatus. In recent years, human defects have been identified in dolichol biosynthesis genes within the group of CDG-I patients. This has increased interest in dolichol metabolism, has resulted in specific recognizable clinical symptoms in CDG-I and has offered new mechanistic insights in dolichol biosynthesis. We here review its biosynthetic pathways, the clinical and biochemical phenotypes in dolichol-related CDG defects, up to the formation of dolichyl-P-mannose (Dol-P-Man), and discuss existing evidence of regulatory networks in dolichol metabolism to provide an outlook on therapeutic strategies.

Dolichol biosynthesis: the occurrence of epoxy dolichol in skipjack tuna liver

Polyisoprenoid alcohols from the livers of temperate sea fish (skipjack tuna, chub mackerel, red sea bream and rainbow trout) were analyzed by using 2D-TLC, electrospray ionization (ESI) mass spectrometry and NMR methods. Dolichols (Dols) were detected in all the fish livers, and they were composed of 19-22 isoprene units with Dol-20 as the predominant prenolog. In addition, Dol-like family compounds were found by using 2D-TLC on skipjack tuna samples. These compounds were found to have a larger molecular mass than the Dol family by 16 mass units. NMR analysis indicated that the Dol-like compounds were consistent with the terminal epoxide structure of Dols (the ω-oxirane derivatives of Dols). ESI analysis also revealed the occurrence of dehydro molecules in both Dols and epoxy Dols (Dol-like) fractions. The occurrence of epoxy Dols in fish is discussed in context with the biosynthesis of Dols, which is responsible for forming Dol phosphate, which lead to Dol-PP-oligosaccharide.

Conservation of the PTEN catalytic motif in the bacterial undecaprenyl pyrophosphate phosphatase, BacA/UppP

Isoprenoid lipid carriers are essential in protein glycosylation and bacterial cell envelope biosynthesis. The enzymes involved in their metabolism (synthases, kinases and phosphatases) are therefore critical to cell viability. In this review, we focus on two broad groups of isoprenoid pyrophosphate phosphatases. One group, containing phosphatidic acid phosphatase motifs, includes the eukaryotic dolichyl pyrophosphate phosphatases and proposed recycling bacterial undecaprenol pyrophosphate phosphatases, PgpB, YbjB and YeiU/LpxT. The second group comprises the bacterial undecaprenol pyrophosphate phosphatase, BacA/UppP, responsible for initial formation of undecaprenyl phosphate, which we predict contains a tyrosine phosphate phosphatase motif resembling that of the tumour suppressor, phosphatase and tensin homologue (PTEN). Based on protein sequence alignments across species and 2D structure predictions, we propose catalytic and lipid recognition motifs unique to BacA/UppP enzymes. The verification of our proposed active-site residues would provide new strategies for the development of substrate-specific inhibitors which mimic both the lipid and pyrophosphate moieties, leading to the development of novel antimicrobial agents.

Sugar availability modulates polyisoprenoid and phytosterol profiles in Arabidopsis thaliana hairy root culture

Sugars are recognized as signaling molecules regulating the biosynthesis of secondary metabolites in plants. Here, a modulatory effect of sugars on dolichol and phytosterol profiles was noted in the hairy roots of Arabidopsis thaliana. Arabidopsis roots contain a complex dolichol mixture comprising three groups ('families') of dolichols differing in the chain-length. These dolichols, especially the longest ones are accompanied by considerable amounts of polyprenols of the same length. The spectrum of polyisoprenoid alcohols, i.e. dolichols and polyprenols, was dependent on sugar type (glucose or sucrose) and its concentration in the medium. Among the long-chain dolichols Dol/Pren-20 (dolichol or prenol molecule composed of 20 isoprene residues) and Dol/Pren-23 were the main components at 0.5% and 2% glucose, respectively. Moreover, the ratio of polyprenols versus respective dolichols was also modulated by sugar in this group of polyisoprenoids, with polyprenols dominating at 3% sucrose and dolichols at 2% glucose. Glucose concentration affected the expression level of genes encoding cis-prenyltransferases, enzymes responsible for elongation of the polyisoprenoid chain. The most abundant phytosterols of the A. thaliana roots, β-sitosterol, stigmasterol and campesterol, were accompanied by corresponding stanols and traces of brassicasterol, stigmast-4,22-dien-3-one and stigmast-4-en-3-one. Similar to the polyisoprenoids, sterol profile responded to the sugar present in the medium, β-sitosterol dominating in roots grown on 3% or lower glucose concentrations and stigmasterol in 3% sucrose. These results indicate an involvement of sugar signaling in the regulation of cis-prenyltransferases and phytosterol pathway enzymes.

SRD5A3 is required for converting polyprenol to dolichol and is mutated in a congenital glycosylation disorder

N-linked glycosylation is the most frequent modification of secreted and membrane-bound proteins in eukaryotic cells, disruption of which is the basis of the congenital disorders of glycosylation (CDGs). We describe a new type of CDG caused by mutations in the steroid 5alpha-reductase type 3 (SRD5A3) gene. Patients have mental retardation and ophthalmologic and cerebellar defects. We found that SRD5A3 is necessary for the reduction of the alpha-isoprene unit of polyprenols to form dolichols, required for synthesis of dolichol-linked monosaccharides, and the oligosaccharide precursor used for N-glycosylation. The presence of residual dolichol in cells depleted for this enzyme suggests the existence of an unexpected alternative pathway for dolichol de novo biosynthesis. Our results thus suggest that SRD5A3 is likely to be the long-sought polyprenol reductase and reveal the genetic basis of one of the earliest steps in protein N-linked glycosylation.

Yeast Saccharomyces cerevisiae has two cis-prenyltransferases with different properties and localizations. Implication for their distinct physiological roles in dolichol synthesis

BACKGROUND

Dolichol is a family of long-chain polyprenols, which is utilized as a sugar carrier in protein glycosylation in the endoplasmic reticulum (ER). We have identified a key enzyme of the dolichol synthesis, cis-prenyltransferase, as Rer2p from Saccharomyces cerevisiae. We have also isolated a multicopy suppressor of an rer2 mutant and named it SRT1. It encodes a protein similar to Rer2p but its function has not been established.

RESULTS

The cis-prenyltransferase activity of Srt1p has been proved biochemically in the lysate of yeast cells lacking Rer2p. The polyprenol product of Srt1p is longer in chain length than that of Rer2p and is not sufficiently converted to dolichol and dolichyl phosphate, unlike that of Rer2p. The subcellular localization of these two isozymes has been examined by immunofluorescence microscopy and by the use of GFP fusion proteins. Whereas GFP-Rer2p is localized to the continuous ER and some dots associated with the ER, GFP-Srt1p shows only punctate localization patterns. Immunofluorescence double staining with Erg6p, a marker of lipid particles in yeast, indicates that Srt1p is mainly localized to lipid particles (lipid bodies). RER2 is mainly expressed in the early logarithmic phase, while the expression of SRT1 is induced in the stationary phase.

CONCLUSIONS

We have shown that yeast has two active cis-prenyltransferases with different properties. This result implies that the two isozymes have different physiological roles during the life cycle of the yeast.

Quantitation by (1)H-NMR of dolichol, cholesterol and choline-containing lipids in extracts of normal and phathological thyroid tissue

Proton magnetic resonance spectroscopy at 1.9 T was used to quantify dolichols, cholesterols, choline-containing phospholipids and double bonds in unsaturated acyl chains in lipid extracts of four types of thyroid tissue [normal (n = 27), papillary cancer (n = 15), adenoma (n = 13) and Basedow disease (n = 6)]. In normal thyroid the mean concentrations of dolichol, cholesterol and phospholipids were 1.2, 3.6 and 2.1 micromol/g wet weight, respectively. The concentrations of these lipids exhibited positive mutual correlations and positive correlations with patient age. The increase in dolichol in elderly human thyroid may be due to the accumulation of lysosomes and may help to compensate for the decrease in the activity of lysosomal enzymes and in thyroid hormone production and release. Dolichol concentrations were significantly lower in papillary cancer (0.4 micromol/g) and Basedow disease (0.3 micromol/g) compared to normal thyroid (p < 0.01 and p < 0.05, respectively), while cholesterol was enhanced only in cancer tissue (10.7 micromol/g). Benign adenoma exhibited normal levels of both dolichol and cholesterol. These results suggest that the synthesis and accumulation of isoprenoids are normal in adenoma but not in cancer.

Novel dolichyl derivatives in rat spleen

Novel dolichyl derivatives were found in rat spleen. The compounds were eluted from reverse phase HPLC after eluting dolichyl fatty acid ester. The elution profiles of the unsaponified forms of the unknown compounds were coincident with that of dolichol from spleen on reverse phase HPLC. The compounds were not dolichyl dolichoate, which are present in bovine thyroid. The compounds were not found in young rats (4 months of age) but were found in old rats (above 12 months of age), and they were not detected in other tissues under our conditions.

The dolichol pathway of N-linked glycosylation

The oligosaccharide substrate for the N-linked protein glycosylation is assembled at the membrane of the endoplasmic reticulum. Dolichyl pyrophosphate serves as a carrier in this biosynthetic pathway. In this review, we discuss the function of the lipid carrier dolichol in oligosaccharide assembly and give an overview of the biosynthesis of the different sugar donors required for the building of the oligosaccharide. Yeast genetic techniques have made it possible to identify many different loci encoding specific glycosyltransferases required for the precise and ordered assembly of the dolichyl pyrophosphate-linked oligosaccharide. Based on the knowledge obtained from studying this pathway in yeast, we compare it to the process of N-linked protein glycosylation in archaea. We suggest that N-linked glycosylation in eukaryotes and in archaea share a common evolutionary origin.

Metabolism of dolichol, dolichoic acid and nordolichoic acid in cultured cells

The uptake and metabolism of [1-(14)C]-labelled dolichol, dolichoic acid and nordolichoic acid were investigated in MDCK and HepG2 cells. Each of the three isoprenoids, bound to human serum albumin, was taken up effectively. None of the compounds was broken down in HepG2 cells, although these converted dolichol into fatty acid esters. In MDCK cells dolichoic acid gave rise to the formation of [14C]CO2 and radiolabelled formic acid, indicating that dolichoic acid can be broken down by alpha-oxidation. Dolichoic acid was also converted to a mixture of polar compounds, possibly polyols. MDCK cells generated radiolabelled CO2 from nordolichoic acid, presumably through beta-oxidation, although we could not find any labelled propionic acid. No oxidative breakdown of dolichol was found, apparently due to the lack of or very low conversion to dolichoic acid.

A partial deficiency of dehydrodolichol reduction is a cause of carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein (CDG) syndrome type I is a congenital disorder that involves the underglycosylation of N-glycosylated glycoproteins (Yamashita, K., Ideo, H., Ohkura, T., Fukushima, K., Yuasa, I., Ohno, K., and Takeshita, K. (1993) J. Biol. Chem. 268, 5783-5789). In an effort to further elucidate the biochemical basis of CDG syndrome type I in our patients, we investigated the defect in the multi-step pathway for biosynthesis of lipid-linked oligosaccharides (LLO) by the metabolic labeling method using [3H]glucosamine, [3H]mannose, and [3H]mevalonate. The LLO levels in synchronized cultures of fibroblasts from these patients were severalfold lower than those in control fibroblasts in the S phase, and the oligosaccharides released from LLO showed the same structural composition, Glc1 approximately 3.Man9.GlcNAc.GlcNAc, in the case of both the patients and controls. The amount of [3H]mannose incorporated into mannose 6-phosphate, mannose 1-phosphate, and GDP-mannose was greater in fibroblasts from these patients than in the control fibroblasts in the G1 period, although the ratios of these acidic mannose derivatives as indicated by the relative levels of radioactivity were the same for the two types of fibroblasts. Furthermore, upon metabolic labeling with [3H]mevalonate, the level of [3H]dehydrodolichol in fibroblasts from these patients increased in the S phase, and the levels of [3H]dolichol and [3H]dolichol-PP oligosaccharides concomitantly decreased, although the chain length distribution of the respective dolichols and dehydrodolichols was the same in the two types of fibroblasts. These results indicate that the conversion of dehydrodolichol to dolichol is partially defective in our patients and that the resulting loss of dolichol leads directly to underglycosylation.