Dolichol synthase

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. 

R Mazhari-Tabrizi, V Eckert, M Blank, R Muller, D Mumberg, M Funk, RT Schwarz. Cloning and functional expression of glycosyltransferases from parasitic protozoans by heterologous complementation in yeast the dolichol phosphate mannose synthase from Trypanosoma brucei. Biochem J 316 853-858, 1996. 

Orlean, P., Albright, C., and Robbins, P. W. (1988). Cloning and sequencing of the yeast gene for dolichol phosphate mannose synthase, an essential protein./. Biol. Chem. 263,17499-17507. 

Squalene synthase is an enzyme catalyzing the formation of squalene from farnesyl diphosphate which is a committed step in the cholesterol biosynthetic pathway. Therefore, squalene synthase is considered a better target than HMG-CoA reductase because farnesyl pyrophosphate, a downstream product of HMG-CoA reductase, is needed for prenylation of proteins and for the biosyntheses of ubiquinone and dolichol (Fig. 2). Before squalestatins and zaragozic acids were discovered, a number of squalene synthase inhibitors were synthesized that showed respectable inhibitory potencies in vitro, but none were successful in animal testing [41]. It was the discovery of squalestatins and zaragozic acids that renewed interest in this biological target, and at picomolar potencies they were the most active inhibitors of squalene synthase. 

Fig. 20.1. Abbreviated scheme of the synthesis of a diantennary N-glycan. Fucose modification has been omitted. 20.1, Phosphomannomutase 2 (PMM2) 20.2, phosphoman-nose isomerase (PMI) 20.3, glucosyltransferase I (GT I) 20.4, mannosyltransferase VI (MT VI) 20.5, dolichol phosphate-mannose synthase-1 (DPM synthase-1) 20.5, N-acet-ylglucosaminyltransferase II (GnT II) 20.7, glucosidase I (G I) GDP, guanosine diphosphate Marly mannose GlcNACy N-acetylglucosamine Do/, dolichol P, phosphate G/c, glucose Gal galactose SiUy sialic acid. Dotted arrow indicates multiple steps...

Table 20 5 Dolichol phosphate-mannose synthase-1 deficiency (CDG-Ie)...

Kim S, Westphal V, Srikrishna G, et al. Dolichol phosphate mannose synthase (DPMI) mutations define congenital disorder of glycosylation le (CDG-Ie). / Clin Invest 2000 105 191-198. 

Imbach T, Schenk B, Schollen E, et al. Deficiency of dolichol-phosphate-mannose synthase-1 causes congenital disorder of glycosylation type le. / Clin Invest 2000 105 233-239. 

Dol-P-Man synthase catalyzes transfer of Man from GDPMan to Dol-P with inversion of configuration to form Dol-P-PMan. This enzyme activity is ubiquitous in eukaryotes, and Dol-P-Man serves as a donor in the dolichol pathway [2, 3], in GPI biosynthesis [1], in O-mannosylation [5], and in C-mannosylation [16], Related enzyme activities have been detected in eubacteria, for example Mycobacterium [14], and in archaea [45], polyprenol-P and Dol-P being used, respectively, as acceptors. 

P.A. Colussi, C.H. Taron, J.C. Mack, and P. Orlean, Human and Saccharomyces cerevisiae dolichol phosphate mannose synthases represent two classes of the enzyme, but both function in Schizosaccharomyces pomhe, Proc. Natl Acad. Sci. USA 1997, 94, 7873-7878. 

---