section 25.11
Collagen Biosynthesis and Its Disorders
587
TABLE 25-6
Osteogenesis Imperfecta
Type
Inheritance
Biochemical Defect
I
II
III
IV
AD
Decreased synthesis of pro
a l
(I),
leading to less production of
type I collagen because of
several defects in the gene.
AR
Markedly diminished synthesis of
type I collagen; presence of
hydroxylysine-enriched
collagens associated with
delayed helix formation.
AR
Delayed secretion of type I
procollagen associated with
excess mannose residues in the
carboxyterminal propeptide
region; absence of a
2
(I)
synthesis.
AD
Point mutations in a
2
(I) gene,
resulting in substitution for
glycine residues.
AD = Autosomal dominant; AR=autosomal recessive.
synthesis. Osteogenesis imperfecta is a genetically and
clinically heterogeneous disorder (Table 25-6). It is one
of the most common inherited connective tissue disorders
and has an incidence of 1:15,000. It is characterized clini-
cally by defects in bone, tendon, dentin, ligament, and
skin.
Intracellular Posttranslational Modifications
Hydroxylations o f Selected Prolyl and Lysyl Residues
The hydroxylation reactions are catalyzed by three en-
zymes: prolyl 4-hydroxylase (usually known as prolyl hy-
droxylase), prolyl 3-hydroxylase, and lysyl hydroxylase.
These enzymes are located within the cistemae or rough
endoplasmic reticulum; as the procollagen chains enter
this compartment, the hydroxylations begin.
All three enzymes have the same cofactor requirements:
ferrous ion, o'-kctoglutarate, molecular oxygen, and ascor-
bate (vitamin C). The reducing equivalents required for the
hydroxylation reaction are provided by the decarboxyla-
tion of equimolar amounts of a-ketoglutarate to succi-
nate and carbon dioxide. One atom of the O
2
molecule
is incorporated into succinate while the other is incorpo-
rated into the hydroxyl group. The data on enzyme kinet-
ics and mechanism of reaction are consistent with the or-
dered binding of Fe2+, a-ketoglutarate, O
2
, and the peptide
substrate and an ordered release of hydroxylated peptide,
CO
2
, succinate, and Fe2+. The Fe2+ does not necessarily
dissociate from the enzyme during each catalytic cycle.
The activation of oxygen is required and may involve the
formation of superoxide (0
2
). The requirement for ascor-
bate is specific with purified enzyme preparations. It is not
consumed stoichiometrically during the hydroxylation re-
action. The exact role of ascorbate is not known, but pre-
sumably it reduces either the enzyme iron complex or the
free enzyme.
Humans, other primates, and guinea pigs lack the en-
zyme required for the conversion of gluconate to ascorbate
(Chapter 15). However, most mammals do possess this en-
zyme and are able to synthesize ascorbate from glucose
by way of glucuronic acid. Other reducing agents, such
as dithiothreitol, L-cysteine, and some reduced pteridines,
in high concentrations, can partially replace ascorbate in
in vitro
assays. The generalized hydroxylation reaction is
shown below.
C O O '
I
C H
2
I
C H
2
S u b s t r a t e - H + Ö
2
+ I
C — O
C O O '
C H
2
Enzyme-> S u b s t r a t e - O H + 1
+ C 0
2
Fe2*
C H
2
A s c o r b a te
1
C O O '
C O O '
a - K e to g lu ta r a te
S u c c in a te
The substrates for all three hydroxylases are highly spe-
cific. Free proline and lysine are not substrates. The
residue to be hydroxylated must be in a peptide link-
age (the minimum is a tripeptide) and in the correct
X or Y position in the chain. The proline hydroxylase
and the lysyl hydroxylase catalyze the hydroxylation of
only prolyl or lysyl residues in the Y positions of pep-
tides with the sequence -X-Y-Gly-, whereas the pro-
lyl 3-hydroxylase catalyzes the hydroxylation of prolyl
residues at the X position only if Y is 4-hydroxyproline.
The conformation of the substrate is also important for
catalytic activity. The substrates have to be nonhelical;
triple-helical polypeptides do not function as substrates.
Thus, the hydroxylation reactions must occur before triple-
helix formation. Many important functions are associ-
ated with the hydroxylation of prolyl and lysyl residues.
The 4-hydroxyprolyl residues participate in interchain
hydrogen bonding that aids in the maintenance of triple-
helical structures. For example, nonhydroxylated polypep-
tide chains cannot form stable triple-helical structures at
body temperature. The biological role of 3-hydroxyprolyl
residues is not known. The hydroxyl groups of hydrox-
ylysyl residues are sites of attachment of carbohydrate
