Enzymes in glycopeptide, glycolipid, and glycoprotein synthesis
In addition to glycosidic bond–forming catalysts, other enzymatic transformations are valuable for the facile construction of complex glycoconjugates. Relevant posttranslational modifications of oligosaccharides and glycopeptides often provide required biological recognition elements. Enzymes have also had great utility in protecting group removal or in the synthesis of unnatural sugar derivatives. The combination of glycosyltransferases with other biological catalysts or synthetic strategies often provides an efficient route toward complex glycoconjugates other than oligosaccharides.
Taking advantage of the wealth of knowledge and techniques provided by the exquisitely developed methods for solid-phase peptide synthesis, solid-phase glycopeptide synthesis has been very successful. However, one of the main hurdles in establishing this procedure has been cleavage of the glycopeptide product from the resin without destruction of acid- or base-sensitive glycan functionality. When performing enzymatic reactions, the solid phase selected also must have specific properties, such as water compatibility and appreciable swelling that will allow the macromolecular enzyme access to the tethered substrate. Monitoring solid-phase glycosyltransferase reactions can also be difficult, often requiring that cleavage from the support be achieved before reaction progress can be assessed.
To resolve some of these issues, solid-phase linkers that can be cleaved under essentially neutral conditions have been developed (Gewehr and Kunz, 1997). The protease chymotrypsin has been utilized as a cleaving reagent following glycosyltransferase-catalyzed assembly of a solid-phase bound 3′-SLN glycopeptide (Schuster et al., 1994). In the synthesis of glycopeptides from MAdCAM-1, a Pd(0)-labile HYCRON linker was employed for facile release following the enzymatic construction of sLex attached to threonine (Figure 7A; Seitz and Wong, 1997). A combination of chemical, solid-phase, and enzymatic strategies can also be efficient, as was shown in the synthesis of sulfated glycopeptides from PSGL-1 (Figure 7B; Koeller et al., 2000a,b). In this case, a pre-glycosylated threonine residue was incorporated into the solid-phase synthesis. Following peptide assembly, the construct was cleaved from the resin, sulfated by chemical means, and enzymatically glycosylated in solution.
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