Chemistry Reference
In-Depth Information
9.1.2
Biosynthesis of GPI Anchors
The basic feature of the biosynthesis of GPI anchors, in contrast to the elucidation
of their nature and structures (which took nearly two decades), was elucidated in
a few years. The rapid progress was due to the knowledge and experience gained
during studying
N
- and
O
-glycan biosynthesis (see Chapters 6 - 8 ). In general, co -
and posttranslational modifi cations of proteins by various glycans are initiated in
the ER and completed in the Golgi apparatus. Glycosyltransferases in the ER are
smaller in number than those in the Golgi, but they are relatively well conserved
evolutionarily in a wide variety of eukaryotic cells (for details, please see Chapter
6). The biosynthesis of GPI as well as
N -
and
O
-glycans (in yeast) is characterized
alike by sequential addition of mannose residues donated in part by dolichyl man-
nosyl phosphate synthase (dolichol plays a central role in
N
- glycan biosynthesis;
please see Chapter 6). In contrast to the lipid- linked high - mannose oligosaccharide
precursor Dol - PP - (GlcNAc)
2
Man
9
Glc
3
(for
N
- glycan synthesis), preassembled GPIs
are not activated compounds (such as GDP-Man), as the energy required for the
transfer to proteins is generated during the transamidase step (see below). The
cloning of genes encoding the enzymes implicated in the biosynthesis of the GPI
anchors has been done mostly by using complementation of yeast or mammalian
cell mutants lacking GPI proteins at their surface (please see below for further
details; Chapter 22.5 provides details on relation to disease). The creation or selec-
tion for these cell lines allowed the isolation of PIG and gpi mutants in mam-
malian cell lines and yeast, respectively. Table 9.2 presents an overview of most
of these genes and encoded enzymes. In many cases heterologous expression
experiments have shown that genes derived from parasites are also functional in
mammalian and yeast cells.
The synthesis of GPI anchors requires at least 12 steps and 23 genes. The num-
bering of steps in Table 9.2 indicates the most likely sequence of enzymatic reac-
tions based on the structure of the yeast and mammalian GPI intermediates
accumulating in gpi or PIG mutants (for further details, please see [1-5]). This
numbering correlates with the graphic scheme of biosynthesis presented in Figure
9.3, which summarizes the biosynthetic route step by step. The key roles of GPIs
in growth and virulence make this posttranslational modifi cation of protein a
potential antimicrobial or antiparasitic target and random screens have already
yielded inhibitors of GPI assembly. Indeed, the GPI pathway is an excellent target
for the development of new drugs against eukaryotic microbes, although many of
the enzymes involved in these steps are conserved. Nonetheless, differences do
occur that are critical for protozoa or for yeast, but which are absent from, or of
diminished importance in mammals. In this sense, a new set of therapeutics can
emerge, underlining the potential of sugars as pharmaceuticals (please see Chapter
28). In what follows, the biosynthetic route will be explained in a stepwise manner
(please follow each step in Figure 9.3 and the text) and, if you understand the fol-
lowing steps, you will grasp the working of GPI biosynthesis.
•
Step 1: generation of GlcNAc-PI from UDP- GlcNAc and PI (GlcNAc - PI - transferase)
.
The fi rst step of GPI anchor biosynthesis occurs at the cytosolic surface of the
