Dolichyl phosphate biosynthesis

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). 

Further work in plant systems confirmed, by different methods (biosynthesis from mevalonate,27 28 mass spectrometry,33 and the techniques already mentioned30), that the polyprenyl phosphate involved in glycosyl-transfer reactions is dolichyl phosphate. Table II shows different sources from which dolichyl phosphate has been identified in plants. 

SCHEME 1.—The Biosynthesis and Metabolism of Dolichyl Phosphate and Other Plant Isoprenoids. 

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 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. 

Figure 6.5 The tentative coupling of PolyP and mannan biosynthesis in yeast Dol-PP, dolichyl diphosphate Dol-P, dolichyl phosphate GDPMan, GDPmannose Dol-PP-Man, dolichyl diphosphate mannose (Man) , mannan, where n is the number of mannose residues.

The putative pathway of coordination of mannan and PolyP biosynthesis by cell-wall formation has been proposed (Kulaev, 1994), which explains the presence of PolyP outside of the cytoplasmic membrane (Figure 7.4). Dolichyl-phosphates (Dol-Ps) act as transmembrane carriers of carbohydrate residues in glycoprotein biosynthesis. GDP-mannose at the... 

The biosynthesis of the N-linked oligosaccharide chain is initiated by the assembly of the high mannose type of oligosaccharide on the ER membrane (Figure 2). This assembly process involves the participation of a carrier molecule called dolichyl-phosphate (Dol-P), which is a component of the ER membrane, and is a lipid structure... 

Polyterpenoids.— C N.m.r. has been used to investigate the structure of the C55 polyprenol moraprenol-11 (190) from leaves of Moms alba. The main component has the trans-trans-trans-cis-cis-cis-cis-cis-cis-cis-OH configuration. The monophosphate of this compound was prepared in 34% yield by treatment with o-phenylenephosphorochloridate. Biosynthetic studies have provided evidence for the formation of 2,3-dehydrodolichyl phosphate (191) as an intermediate in dolichyl phosphate (192) biosynthesis. Most of the members of a series of C2S—C40 acyclic hydrocarbons obtained from crude oil have isoprenoid or degraded isoprenoid structures. The mokupalides from a marine sponge have been shown to be hexaprenoids with structures (193)—(195). ... 

S CTP + dolichol <2-4, 6, 7, 9> (<6> enzyme can function in augmenting the steady-state levels of dolichyl phosphate by phosphorylating preexisting dolichol molecules [4] <2> newly formed dolichyl monophosphate, phosphorylated via CTP is available for biosynthesis of mannosylphos-phoryldolichol and N-acetylglucosaminylpyrophosphoryldolichol [1] <4,7> CTP mediated phosphorylation of dolichol, the terminal step in dolichyl monophospate biosynthesis de novo [12]) (Reversibility <2-4,... 

Adair, W.L. Keller, R.K. (1982) Dolichol Metabolism in Rat Liver. Determination of the Subcellular Distribution of Dolichyl Phosphate and its Site and Rate of de novo Biosynthesis , Journal of Biological Chemistry, 257, 8990-6... 

The role of undecaprenyl phosphate (C55, allylic) as a carrier of glycosyl units in the biosynthesis of bacterial cells walls is well established. Dolichyl phosphate plays a similar role in the synthesis of glycoproteins in animal systems. Evidence that prenyl lipids serve as glycosyl carriers in the biosynthesis of glycoproteins in plants is accumulating rapidly (Pont Lezica et al., 1976 Delmer al., 1978 Erickson et al., 1978). Several authors have recently reviewed the literature on the role of polyprenols as glycosyl carriers in plants (Hemming, 1978 Pont Lezica, 1979 Elbein, 1979). 

In plant tissues two major forms of polyprenols are found, e.g. free alcohols and esters with fatty acids. In some species one form is highly dominating. Similarly in animal tissues dolichols and dolichyl esters were described, accompanied by usually small amounts of dolichyl phosphate. Biological role of this latter compound is well established. Dolichyl phosphate is involved as cofactor in biosynthesis of glycoproteins and GPI-anchored proteins [3]. Biological role of free alcohol and its acyl ester has not been defined yet. As the components of biological membranes these compounds modulate membrane properties. Dolichol increases fluidity and permeability and decreases the stability of model membranes [4]. Experiments with polyprenols point to the same direction polyprenols and polyprenyl phosphates increases the fluidity and permeability of model membranes [5], Both polyprenols and dolichols could also serve as a reserve isoprenoid pool utilized by corresponding kinases. 

Isoprenoid Lipids.—Lipids which contain a mono- or pyro-phosphate link between a polyisoprenoid alcohol and a carbohydrate residue are important intermediates in the biosynthesis of bacterial cell walls. The monophosphate derivatives are probably involved in the addition of single carbohydrate residues to existing polysaccharide chains while the pyrophosphates are probably intermediates in the biosynthesis of the main chain. Ficaprenyl (13 = 10), famesyl (13 = 2), dolichyl (14 n = 18), and citro-nellyl (14 n = 1) a-D-mannopyranosyl phosphates have now been... 

The biosynthesis and functions of the membrane glycoproteins of eukaryotic cells have been reviewed. The general features of the biosynthesis of these glycoproteins closely resemble those involved in the biosynthesis of complex bacterial glycans, especially the O-antigen of lipopolysaccharides. Syntheses of polyisoprenyl jS-o-mannopyranosyl phosphate and P -di-A-acetylchitobiosyl P -dolichyl pyrophosphate have been reported. A preparation from human-lymphocyte membranes was able to transfer D-mannose from GPD-o-mannose to the chitobiosyl derivative, whereas dolichyl D-mannosyl phosphate did not act as a D-mannosyl donor. It appears that D-mannosyl residues at various positions of the glycoprotein have different anomeric configurations, so that different D-mannosyl donors are required in their synthesis. 

Lipid intermediates are formed during the biosynthesis of glycoproteins in microsomes. Column chromatography was used to separate dolichyl D-[ C]mannosyI phosphate from a D-[ C]manno-oligosaccharide which was also bound to lipid after removal of the lipid, the oligosaccharide fraction appeared to be heterogeneous in its reaction with concanavalin A. 

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