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218 Biopolymers

may be cross-linked to other polymers to give the struc-
tural unit in living organisms.
Collagen, a fibrous structural protein, is the most abun-
dant protein in the animal kingdom. It is a major compo-
nent of skin, cartilage, bone, tendon, and teeth. The basic
“unit” is the tropocollagen molecule, consisting of three
polypeptide chains, each approximately 1000 amino acid
residues long. Many tropocollagens exist, differing in the
amino acid sequences of the chains. In at least one col-
lagen, the collagen II of cartilage, all three polypeptide
chains in a molecule are identical; in other collagens this
is not the case. All collagens have, however, certain fea-
˚
tures in common. The molecules are about 2800 A long
˚
and 15 A in diameter. The amino acid composition of
tropocollagen is unusual, for one third of the residues
may be glycine, while proline and hydroxyproline may
constitute another quarter of the total protein. The se-
quence, –glycine–proline–hydroxyproline–, is common,
which means that neither α-helices nor β-structures can be
formed readily by the polypeptide chains, for neither pro-
line nor hydroxyproline residues possess the NH groups
necessary for hydrogen bond formation. The presence of
glycine as every third residue in long stretches of the
tropocollagen polypeptide chains allows another kind of
regular structure to be formed. This is a triple helix of the
three chains wound round each other; each single chain
is coiled as a left-handed helix (Fig. 7a). The bulky side
chains of proline and hydroxyproline project outward on
the surface of the triple helix, while the side chains of
glycine (H atoms) pack inside. The three chains are held
togetherbyhydrogenbondingbetweentheNHofaglycine
on one chain and C O of another residue on a second
chain. In addition there may be covalent cross-links be-
tween the three chains of the tropocollagen molecule near
their N-terminal ends. In some tropocollagens, the amino
acid sequence is less regular, and the molecules can consist
of interrupted triple helices. FIGURE 7 Structure of collagen. (a) The triple helix of a tropocol-
Heating collagen in water results in rupture of the hy- lagen molecule; the left-handed helix of a single polypeptide chain
is shown. (b) Packing of tropocollagen molecules in a collagen
drogen bonds and gives a denatured water-soluble protein
fibril.
with a random three-dimensional structure. This product
is known as gelatin, and on cooling some molecules re-
nature to give the original triple helical structure, while acid found in collagen. The importance of such covalent
others partially renature to produce a cross-linked net- bonding is indicated by the disease lathyrism, where se-
work, a gel. Because of its gel-forming abilities collagen vere skeletal deformities are found and which results from
is widely used in the food industry. inhibitionofthefirstenzymeinvolvedincatalysisofcross-
In structures such as bone and tendon, tropocollagen link formation.
molecules are packed head to tail parallel to one another Collagen fibrils are, in turn, packed side by side in tis-
to form collagen fibrils. In the fibrils the molecules are co- sues to form fibers. Fiber formation is probably influenced
valently bonded side by side and are probably staggered by the other polymers such as proteins and glycosamino-
by one-quarter of their length (Fig. 7b). The bonding is glycans (see Sections II.B.6 and 7) of a tissue. The long
similar but not identical to that holding the three chains of cylindrical shape of the tropocollagen molecules and their
one tropocollagen molecule together and involves the side packing into fibrils and fibers gives collagen the high
chains of lysine and hydroxylysine, a less common amino tensile strength necessary for a structural material. The