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chapter 11
Heteropolysaccharides II: Proteoglycans and Peptidoglycans
Elastolytic proteinases (elastases) are found in pancre-
atic tissue, polymorphonuclear leukocytes, macrophages,
and platelets. These enzymes exhibit broad specificity.
They catalyze preferential cleavage of peptide bonds
adjacent to aliphatic amino acids, namely, glycine, ala-
nine, and valine, which are present in high amounts in
elastin. Elastases degrade elastin at neutral or slightly
alkaline pH. The pancreas secretes elastase as an inac-
tive precursor (zymogen) known as
proelastase,
which
is converted to its active form by trypsin in the duo-
denum; elastase takes part in the digestion of dietary
proteins (Chapter 12). This enzyme has a structure ho-
mologous to that of other pancreatic serine proteinases,
including the amino acid sequence at the active site
(Chapter
6
). Polymorphonuclear leukocyte elastase is
also a serine proteinase. Elastases are inactivated by
serum a i-proteinase inhibitor and a
2
-macroglobulin;
thus, their proteolytic action may be checked to prevent
indiscrimination digestion of elastin-containing tissues
(Chapter
6
). An enzyme that degrades only soluble forms
of elastin and exhibits trypsin-link activity has been de-
scribed. Its biological significance is unknown. Several
heritable and acquired disorders of elastin are known (see
Chapter 25).
Proteoglycans
Proteoglycans are high-molecular-weight, complex mole-
cules with diverse structures and functions. They are
polyanionic substances containing a core protein to
which at least one
g lyco sa m in o g lyca n
(also known as
mucopolysaccharide) chain is covalently attached. Pro-
teoglycans are major components of connective tissue
and participate with other structural protein constituents,
namely, collagen and elastin, in the organization of the
extracellular matrix.
Types, S tru ctu res, a n d F u n ctio n s
o f G lyco sa m in o g lyca n s
Six classes of glycosaminoglycans have been des-
cribed. All are heteropolysaccharides and contain repeat-
ing disaccharide units. The compositions and structures of
the repeating disaccharide units are shown in Table 11-2
and
Figure
11-7.
In
five
glycosaminoglycans,
the
TABLE 11-2
C om position , P rop erties, a n d D istrib u tio n o f G lyco sa m in o g lyca n s
Glycosaminoglycan
(and range of
molecular weight)
Amino Sugar
Uronic Acid
Type of Sulfate
Linkage (extent of
sulfation is variable)
Tissue
Distribution
Hyaluronate
(4 to 80 x 106)
D-Glucosamine
D-Glucuronate
None
Connective tissues,
cartilage, synovial
uid, vitreous humor,
umbilical cord
Chondroitin sulfate
(5,000 to 50,000)
D-Galactosamine
D-Glucuronate
4-0- and/or 6-0-sulfate
on galactosamine
Cartilage, bone, skin,
cornea, blood vessel
walls
Dermatan sulfate
(15,000 to 40,000)
D-Galactosamine
L-Iduronate,
D-Glucuronate
4-0-sulfate on
galactosamine;
2
-
0
-sulfate on iduronate
Skin, heart valve,
tendon, blood vessel
walls
Keratan sulfate
(4,000 to 19,000)
D-Glucosamine
None (but contains
D-Galactose)
6
-
0
-sulfate on both
carbohydrate
residues
Cartilage, cornea,
intervertebral disks
Heparan sulfate
(
1 0 4
to
1 0
5)
D-Glucosamine
D-Glucuronate
(major),
L-Iduronate
(minor)
6-0-sulfate and N-sulfate
(or N-acetyl) on
glucosamine;
2
-
0
-sulfate on iduronate
Lung, blood vessel
walls, many cell
surfaces
Heparin
(
1 0 3
to
1 0
6)
D-Glucosamine
L-Iduronate
(major)
D-Glucuronate
(minor)
2
-
0
-sulfate on iduronate;
6-0-sulfate and N-sulfate
(or N-acetyl) on
glucosamine
Lung, liver, skin,
intestinal mucosa
(mast cells)
