Glycosylation is an important posttranslational protein modification
occurring in the cytoplasm, the endoplasmic reticulum, and the Golgi
apparatus. A rapidly growing family of genetic diseases is due to
defects in protein glycosylation (congenital disorders of glycosylation [CDG]). Most
CDG are severe, multisystem diseases with important neurological
involvement. Some 30 CDG have been identified. CDG due to an N-glycosylation
defect (there are 18 disorders) comprise two groups: CDG-I (with
absence of one or more glycans; CDG-Ia through CDG-IL) and CDG-II
(with incomplete glycans; CDG-IIa through CDG-IIf). Six disorders
have been identified in O-glycosylation, including some long-known
diseases such as hereditary multiple exostoses; another six disorders
have a combined N- and O-glycosylation defect. Important tools in
the diagnosis are transferrin isoelectric focusing, analysis of
lipid-linked oligosaccharides and of protein-linked glycans, and
Congenital disorders of glycosylation (CDG) are a rapidly growing
family of genetic diseases caused by defects in the synthesis of
the glycan moiety of glycoconjugates (glycoproteins and glycolipids).
There are two main types of protein glycosylation: N-glycosylation
and O-glycosylation. N-glycosylation (N-glycans attached to an amino
group of asparagine of proteins) comprises an assembly part and
a processing part and extends over three cellular compartments:
the cytosol, the endoplasmic reticulum (ER), and the Golgi.
The assembly part of the N-glycosylation starts on the cytosolic
side of the ER, with the transfer of N-acetylglucosamine (GlcNAc) phosphate
from UDP-GlcNAc to membrane-bound dolichyl monophosphate (Dol-P), forming
GlcNAc-pyrophosphate-dolichol (GlcNAc-PP-Dol). One GlcNAc and five mannose
(Man) residues are subsequently attached to this lipid-linked monosaccharide
in a stepwise manner (Fig. 163-1). The donor
of these mannoses is a nucleotide-activated sugar, GDP-Man, which
is synthesized from fructose 6-phosphate, an intermediate of the glycolytic
pathway (Fig. 163-2). The lipid-linked heptasaccharide
Man5GlcNAc2 is translocated by a flippase across
the ER membrane and is elongated at the lumenal side by the attachment
of four mannose residues and subsequently of three glucose residues.
The four mannosyltransferases and three glucosyltransferases involved
require dolichyl-phosphate-bound monosaccharides (Dol-P-Man and
Dol-P-Glc). The completed Glc3Man9GlcNAc2 oligosaccharide
is then transferred to selected asparagine residues of the nascent
proteins by the oligosaccharyltransferase complex.
Scheme of the endoplasmic reticulum part of the N-glycosylation
pathway (see text for explanation). The black bar beside ALG6 indicates
the defect in CDG-Ic (ALG6 or glucosyltransferase I defect).
Scheme of the synthesis of guanosine diphosphate (GDP)-mannose
from fructose 6-phosphate. Vertical red bars indicate defects in
CDG-Ia (PMM2 defect) and in CDG-Ib (PMI defect).
The processing part of the N-glycosylation starts in the ER by
trimming the glucoses (catalyzed by glucosidases I and II) and one
mannose (catalyzed by α-mannosidase I). The residual
glycoprotein intermediate is directed to the cis-Golgi, where the
processing pathway branches. A minor branch ...