588
chapter 25
RNA
and
Protein Synthesis
and also participate in the intermolecular collagen cross-
links.
The importance of lysyl hydroxylation is seen in pa-
tients with the type VI variant of Ehlers-Danlos syndrome
(Table 25-5). The collagen in these individuals has a de-
creased fibril diameter and profound changes in mechani-
cal properties. Skin fibroblasts show virtually no lysyl hy-
droxylase activity. Furthermore, hydroxylysine formation
can be severely affected in some tissues, mildly affected
in others, and unaffected in still others (e.g., cartilage).
These observations suggest the presence of tissue-specific
lysyl hydroxylases.
Glycosylation o f Hydroxylysyl Residues
The glycosylation occurs as the N-terminal ends of the
polypeptide chains move into cisternae at specific hydrox-
ylysyl residues. The carbohydrates found are galactose and
the disaccharide, glucosylgalactose. These reactions are
catalyzed by two specific enzymes: hydroxylysyl galacto-
syltransférase and galactosylhydroxylysyl glucosyltrans-
ferase. The first enzyme catalyzes the transfer of galac-
tose from UDP-galactose to hydroxylylsyl residues, and
the second enzyme transfers a glucose from UDP-glucose
to galactosylhydroxylysyl residues. Both enzymes require
a bivalent cation, preferably manganese. The substrate has
to be in the nonhelical conformation, and the glycosyla-
tion ceases when the collagen propeptides fold into a triple
helix. Thus, both hydroxylations and glycosylation must
occur before triple-helix formation, which is an intracel-
lular process.
The exact function of the carbohydrate unit in the colla-
gen molecule is not clear, but it may have some role in the
organization of fibrils. For example, an inverse relation-
ship between carbohydrate content and fibril diameter has
been observed. Deficiency of galactosylhydroxylysyl glu-
cosyltransferase has been seen in the members of kindred
with a dominantly inherited disease known as
epidermol-
ysis buflosa simplex.
This disease belongs to a group of
inherited disorders characterized by blister formation in
response to minor skin trauma. Further investigation of
this disorder may provide information about the role of
glycosylation of collagen.
Collagen propeptides contain oligosaccharide units typ-
ical of glycoproteins in the proregions of the polypeptide
chain (Figure 25-19). These oligosaccharide units may
be present in either the N- or C-terminal proregions of
the peptide or both. In type I procollagen, the oligosac-
charide units are at the C-terminal, whereas in type II
procollagen, the units are found at both ends, These
oligosaccharides do not appear in the finished colla-
gen molecule because the proregions are excised. The
oligosaccharide units probably are synthesized through
• Glc
Igqi
f Goi
(M«n)n
(GIcNftc).
N -T erm in a l
p r o p e p tid e s
C o lla g e n
C -T e rm in a l
p ro p e p tid e s
O c c u r in p a rt in
O c c u r in g lo b u la r
g lo b u la r c o n fo rm a tio n
c o n fo rm a tio n
a n d in s h o rt trip le -
h e lic al s tr u c tu r e
F I G U R E 2 5 - 1 9
Structure of type I procollagen molecule, which consists of two proa i (I)
and one proa
2
(I) chains. Note the presence of oligosaccharide units at the
C-terminal propeptide region. In other collagen types, the oligosaccharide
units may be present in either one of the regions or both. The disulfide
linkages are also present in these regions. These linkages may be either
intra- or interchain linkages and may be involved in the triple-helical
formation. In the finished collagen molecule, the N- and C-propeptide
regions are absent because they are excised (shown by vertical dotted
lines). Glc, Glucose; Gal, galactose; Man, mannose; GIcNAc,
N-acetylglucosamine. [Reproduced with permission from D. J. Prockop
and K. I. Kivirikko, Heritable diseases of collagen.
N Engl J M ed
311:376
(1984).]
dolichol intermediates (Chapter 16) and linked to the
nascent peptide chains within the rough endoplasmic retic-
ulum. The function of these asparaginyl-linked oligosac-
charide units is not known.
Since ascorbate is required for hydroxylation reac-
tions, its deficiency causes accumulation of the collagen
polypeptides in the endoplasmic reticulum and their even-
tual secretion. However, these proteins lack the proper
modifications and therefore cannot be used for assembly
of collagen fibrils. Thus, the role of ascorbate in the hy-
droxylation of collagen is one of its major physiological
roles (see also Chapter 38).
Scurvy,
a connective tissue
disorder, is due to vitamin C deficiency in the diet. Most
pathological changes of scurvy can be attributed to fail-
ure to synthesize collagen. Scurvy patients have defective
blood vessels and poor intravascular support, leading to
frequent hemorrhages, defective formation of bone and
teeth, and poor wound healing. All of these manifestations
can be corrected by administration of vitamin C.
Formation o f Disulfide Linkages and Assembly of
Procollagen Polypeptides into a Triple Helix
The propeptide regions of procollagen polypeptides
contain cysteine residues that can form both intra- and in-
terchain disulfide linkages (Figure 25-19). As is the case
for other proteins that contain disulfide linkages, it is not
known whether synthesis of these linkages requires an en-
zyme or whether it occurs spontaneously. The assembly
of procollagen polypeptides into a triple helix appears to
have two requirements:
