Synthetic glycosylphosphatidylinositol (GPI) anchors: how these complex molecules have been made

Glycosylphosphatidylinositols (GPIs) are a class of natural glycosylphospholipids that anchor proteins, glycoproteins and lipophosphoglycans to the membrane of eukaryotic cells. GPI anchors are widely present in parasitic protozoa, where GPI-anchored mucins and phosphoglycans are abundant and form a dense protective layer (glycocalyx) on the surface of the parasites. This type of anchor appears to be present in these organisms with a much higher frequency than in higher eukaryotes. Since the first full assignment of a GPI structure in 1988, more than 50 glycosylphosphatidylinositols have been structurally characterised. The functions of GPI anchors (in addition to the clear one of linking the above biopolymers to membranes) have been extensively discussed. The high lateral mobility of GPIs and GPI-anchored polymers seems to actively facilitate the selective release of molecules from the cell surface and the exchange of membrane proteins between cells. There is also evidence that GPIs and/ or their metabolites can act as secondary messengers, modulating biological events including insulin production, insulin-mediated signal transduction, cellular proliferation and cell-cell recognition. Their discovered role as mediators of regulatory processes makes the chemical preparation of these compounds and their analogues of great interest. This comprehensive review highlights the progress in the chemical synthesis of GPI anchors and related glycoconjugate structures from protozoan parasites, yeast and mammals in the last two decades. The synthesis of a structurally related prokaryotic glycoconjugate of Mycobacterium tuberculosis is also discussed.