CHAPTER
11
Heteropolysaccharides II:
Proteoglycans and Peptidoglycans
11.1
Protein Fibers and Proteoglycans
Connective tissues are composed of insoluble protein
fibers (the glycoprotein
collagen
and the nonglycoprotein
elastin)
embedded in a matrix of proteoglycans (ground
substance). The connective tissues bind tissues together
and provide support for the organs and other structures
of the body. Their properties depend on the proportion of
different components present. A tissue of very high ten-
sile strength, the Achilles tendon, is composed of about
32% collagen and 2.6% elastin, whereas an elastic tissue,
the ligamentum nuchae, is composed of about 32% elastin
and 7% collagen. The proteins and proteoglycans are syn-
thesized by connective tissue cells:
fibroblasts
(gener-
alized connective tissue),
chondroblasts
(cartilage), and
osteoblasts
(bone). Connective tissue also contains blood
and lymphs vessels and various transient cells including
macrophages and mast cells. Adipose tissue is a special-
ized form of connective tissue consisting of a collection of
adipocytes
(stores of triacylglycerol) that cluster between
the protein fibers.
Collagen
Collagens are extracellular proteins of connective tis-
sue and they make up about one third of all body pro-
tein. They are a family of related glycoproteins in which
hydroxylysyl residues provide the sites for attachment
of glucose, galactose, or an a (l
—>
2)
glucosylgalactose
residue via a /3-O-glycosidic linkage (Figure 11-1). In
brief, the synthesis of collagen can be considered to
occur in two stages: intracellular and extracellular. The
intracellular stage consists of the production of procol-
lagen from precursor polypeptide chains that undergo,
in sequence, hydroxylation, glycosylation, formation of
a triple helix, and secretion. The extracellular stage con-
sists of the conversion of procollagen to tropocollagen by
limited proteolysis from the amino and carboxyl termini,
self-assembly of tropocollagen molecules into fibrils, and
finally cross-linking of the fibrils to form collagen fibers
(Chapter 25).
Collagen exists predominantly as fibrous protein; how-
ever, in the basement membrane of many tissues, includ-
ing kidney glomeruli and the lens capsule, it is present in a
nonfibrous form. The unique property of each connective
tissue (e.g., the flexibility of skin, rigidity of bone, elas-
ticity of large arteries, and strength of tendons) depends
on the composition and organization of collagen and other
matrix components.
Collagen Types
More than 16 different types of collagen have been re-
ported. They constitute the most abundant family of pro-
teins in the human body. The collagens are encoded by
28 genes dispersed in at least
1 2
different chromosomes.
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