Dolichol, biosynthesis

The concentration of dolichyl phosphate in eukaryotic tissues is very low and is probably rate-limiting for the glycosylation processes. Variation of the concentration in the endoplasmic-reticulum membranes is a possible way of controlling the rate of glycosylation. It is important to point out that the early steps in the dolichol biosynthesis are common to such other prenyl derivatives in plants as steroids, essential oils, hormones, phytol, and carotenes (see Scheme 1), and parameters affecting those reactions that may control the dolichol to dolichyl phosphate step could be another mechanism for regulation of the level of dolichyl phosphate. 

Sterol and dolichol biosynthesis of cultured L cells exhibit differing sensitivities to 25-hydroxycholesterol [217]. 

J.S. Rush, J.G. Schelling, N.S. Zingg, P.H. Ray, and C.J. Waechter, Mannosyl-phosphoryldolichol-mediated reactions in oligosaecharide-P-P-dolichol biosynthesis. Recognition of the saturated alpha-isoprene unit of the mannosyl donor by pig brain mannosyl-transferases, / Biol. Chem. 1993,268, 13110-13117. 

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). 

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. 

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.

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 47-7. 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.)...

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... 

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... 

In Section II (on the biosynthesis of oligosaccharides), emphasis is placed on the biosynthesis of the dolichol-linked sugars, because the inhibitors mainly interfere with steps in the dolichol cycle of protein glycosylation. Modification of the oligosaccharide side-chain, once it has been transferred from the carrier lipid to the protein, is not discussed in any great detail. 

The initial demonstration that isopentenyl diphosphate may be used in the biosynthesis of dolichol phosphate in animals and plants... 

Scheme 1.—Pathway of the Biosynthesis oi Dolichol Phosphate and ot Doliehol-1 inked Saccharides. (Reactions requiring more than one enzyme are indicated by dashed arrows. The Scheme is based on work cited in Refs. 1, 3, 8—25, 35, and 50.)...

The dolichol-linked D-mannosyl, D-glucosyl, and 2-acetamido-2-deoxy-D-glucosyl residues are used in the glycosylation of proteins and in the biosynthesis of D-m an nan.2,35 GlcNAc-PP-Dol may also play a role in the synthesis of glycosaminoglycans. Reaction 4 is the first step in the formation of lipid-linked precursors for cellulose.46... 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

Figure 20-6 Biosynthesis of the dolichol diphosphate-linked oligosaccharide precursor to glycoproteins. The site of inhibition by tunicamycin is indicated.

Little is known as to the biosynthesis and metabolism of plant dolichols. Nevertheless, this is an interesting field if the variety of polyprenols synthesized by plants (see Table I and Scheme 1), as compared with animals, is considered. The early knowledge on the stereochemistry of E, Z-polyprenol biosynthesis was obtained by Gough and Hemming63 with a plant system. They found that the 4-5 hydrogen atom of mevalonate is retained when a Z- double bond is formed, whereas this hydrogen atom is lost in the biosynthesis of an -double bond. The same authors found similar results for rat-liver dolichol.64 With this technique, [3H]dolichol was found in rat-liver mitochondria when the animals were injected with65 4-(S)-[3H]meval-... 

Grange and Adair73 isolated 2,3-dehydrodolichyl phosphate from in vitro studies on hen oviduct. It may be derived from 2,3-dehydrodolichyl diphosphate, a likely intermediate in the biosynthesis of dolichyl phosphate. Many questions still remain unanswered, such as is the substrate for the a-isoprene reductase, the diphosphate or the monophosphate of 2,3-dehy-drodolichol Are the unsaturated polyprenols a reserve pool for the synthesis of dolichol in plants What is the function of unsaturated, plant polyprenols How is the synthesis of saturated and unsaturated polyprenols regulated ... 

The study of the biosynthesis of D-mannoproteins in other fungi is less developed. It has already been mentioned that the participation of Man-P-Dol as the glycosyl donor in O-glycosylation was demonstrated in Neuro-spora crassa97 and in Fusarium solani f. pisi.56 In the latter, the lipid acceptor was identified as C90- to C110-dolichols. In Hansenula species, O-glycosyla-tion follows the same pathway that is D-mannose linked to L-serine or L-threonine is incorporated by way of Man-P-Dol, and further elongation... 

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