Dolichol

Dolichol (from pig liver) [11029-02-0] Cgo-Cios polyprenols. Cryst 6 times from pet ether/EtOH at -20°C. Ran as entity on a paper chromatogram on paraffin impregnated paper, with acetone as the mobile phase. [Burgos Biochem J 88 470 1963.]... 

Long-chain polyisoprenoid. molecules with a terminal alcohol moiety are called, polyprenols. The dolichols, one class of polyprenols (Figure 8.18), consist of 16 to 22 isoprene units and, in the form of dolichyl phosphates, function to carry carbohydrate units in the biosynthesis of glycoproteins in animals. Polyprenyl groups serve to anchor certain proteins to biological membranes (discussed in Chapter 9). 

FIGURE 8.18 Dolichol phosphate is an initiation point for the synthesis of carbohydrate polymers in animals. The analogous alcohol in bacterial systems, undecaprenol, also known as bactoprenol, consists of 11 isoprene units. Undecaprenyl phosphate delivers sugars from the cytoplasm for the synthesis of cell wall components such as peptidoglycans, lipopolysaccharides, and glycoproteins. Polyprenyl compounds also serve as the side chains of vitamin K, the ubiquinones, plastoquinones, and tocopherols (such as vitamin E). 

The search for inhibitors of this pathway began with the first key regulatory enzyme, HMG CoA reductase. Several clinically useful inhibitors of HMG CoA reductase are now known. One of the most successful, Mevacor, produced by Merck, is one of the pharmaceutical industry s best selling products. However, the problem with inhibiting a branched biosynthetic pathway at an early point is that the biosynthesis of other crucial biomolecules may also be inhibited. Indeed, there is some evidence that levels of ubiquinone and the dolichols are affected by some HMG CoA reductase inhibitors. Consequently, efforts have recently been directed towards finding inhibitors of squalene synthase, the enzyme controlling the first step on the route to cholesterol after the FPP branch point. 

Lipids, e.g. sitosterol, geraniol, dolichol, squalene, cholesterol 200 C, 15 min Induced fluorescence detection limits <1 pg cholesterol. [3]... 

Coenzymes serve as recyclable shuttles—or group transfer reagents—that transport many substrates from their point of generation to their point of utilization. Association with the coenzyme also stabilizes substrates such as hydrogen atoms or hydride ions that are unstable in the aqueous environment of the cell. Other chemical moieties transported by coenzymes include methyl groups (folates), acyl groups (coenzyme A), and oligosaccharides (dolichol). 

The enzymes in peroxisomes do not attack shorter-chain fatty acids the P-oxidation sequence ends at oc-tanoyl-CoA. Octanoyl and acetyl groups are both further oxidized in mitochondria. Another role of peroxisomal P-oxidation is to shorten the side chain of cholesterol in bile acid formation (Chapter 26). Peroxisomes also take part in the synthesis of ether glycerolipids (Chapter 24), cholesterol, and dolichol (Figure 26-2). 

Farnesyl Diphosphate Gives Rise to Dolichol Ubiquinone... 

The polyisoprenoids dolichol (Figure 14-20 and Chapter 47) and ubiquinone (Figure 12-5) are formed from farnesyl diphosphate by the further addition of up to 16 (dolichol) or 3-7 (ubiquinone) isopentenyl diphosphate residues, respectively. Some GTP-binding proteins in the cell membrane are prenylated with farnesyl or geranylgeranyl (20 carbon) residues. Protein prenylation is believed to facilitate the anchoring of proteins into lipoid membranes and may also be involved in protein-protein interactions and membrane-associated protein trafficking. 

Figure26-2. Biosynthesis of squalene, ubiquinone, dolichol, and other polyisoprene derivatives. (HMG, 3-hydroxy-3-methylglutaryl x, cytokinin.) A farnesyl residue is present in heme a of cytochrome oxidase. The carbon marked with asterisk becomes C or C,2 in squalene. Squalene synthetase is a microsomal enzyme all other enzymes indicated are soluble cytosolic proteins, and some are found in peroxisomes.

Dolichol-P-P-oligosaccharide is not involved, nor are gly-cosidases and the reactions are not inhibited by tuni-camycin. 

The Biosynthesis of N-Linked Glycoproteins Involves Dolichol-P-P-Oligosaccharide... 

Before it participates in the biosynthesis of Dol-P-P-oligosaccharide, dolichol must first be phosphorylated to form dolichol phosphate (Dol-P) in a reaction catalyzed by dolickol kinase and using ATP as the phosphate donor. 

Figure 47-6. The structure of dolichol. The phosphate in dolichol phosphate is attached to the primary alcohol group at the left-hand end of the molecule. The group within the brackets is an isoprene unit (n = 17-20 isoprenoid units).

It should be noted that the first seven sugars (two GlcNAc and five Man residues) are donated by nucleotide sugars, whereas the last seven sugars (four Man and three Glc residues) added are donated by dolichol-P-sugars. The net result is assembly of the compound illustrated in Figure 47-8 and referred to in shorthand as Dol-P-P-GlcNAcjManjGlcj. 

Figure 47-7. Pathway of biosynthesis of dolichol-P-P-oligosaccharide. The specific linkages formed are indicated in Figure 47-8. Note that the first five internal mannose residues are donated by GDP-mannose, whereas the more external mannose residues and the glucose residues are donated by dolichol-P-mannose and dolichol-P-glucose. (UDP, uridine diphosphate Dol, dolichol P, phosphate UMP, uridine monophosphate GDP, guanosine diphosphate M, mannose G, glucose.)...

Figure47-8. Structure of dolichol-P-P-oligosaccharide. (Reproduced, with permission, from Lennarz WJ The Biochemistry of Glycoproteins and Proteoglycans. Pienum Press, 1980.)...

The oligosaccharide GICjMangCGIcNAcjj is transferred from dolichol-P-P-oligosaccharide in a reaction catalyzed by oligosaccharide protein transferase, which is inhibited by tunicamycin. 

Tunicamycin Inhibits GIcNAc-P transferase, the enzyme catalyzing addition of GIcNAc to dolichol-P, the first step in the biosynthesis of oligosaccharide-P-P-dolichol... 

Type I disorders are due to mutations in genes encoding enzymes (eg, phosphomannomutase-2 [PMM-2], causing CDG la) involved in the synthesis of dolichol-P-P-oligo-saccharide... 

C. Nucleoside Diphosphate Sugars.—A polyprenol phosphate containing eleven isoprene units is involved in the biosynthesis of various bacterial cell-wall components.As mentioned in last year s Report, another isoprenoid phosphate, dolichol monophosphate (40), is an intermediate in sugar... 

If the synthesis starts from glucose molecules, then the initial step is the transfer of glucose residues from UDP-glucose onto an intermediary acceptor-dolichol phosphate (membrane-bound polyprenol phosphate). Dolichol phosphate assists in the synthesis of an... 

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