Biomedical Engineering Reference
In-Depth Information
(a)
(b)
H
H
R
R
O
H
C
N
N
C
N
H
H
O
H
R
Hydrogen
bond
R
H
O
H
O
C
N
C
N
C
R
H
O
R
H
H
Right-handed helix
0.72 nM
FIGURE 6.1
(a) Hypothetical flat sheet structure of a protein. (b) Helical arrangement of a protein chain.
polypeptide
chain to adapt a less constraining conformation by rotating the bulky R groups away from
the crowded interactions, forming a helix, where the large R groups are directed toward the surface of
the helix (Figure 6.1b). The hydrogen bonds are allowed to form within a helix between the hydrogen
attached to nitrogen in one amino acid residue and the oxygen attached to a second amino acid residue.
Thus, the final conformation of a protein, which is directly related to its function, is governed primarily
by the amino acid sequence of the particular protein.
Collagen is a protein comprised of three polypeptides (α chains), each having a general amino acid
sequence of (-Gly-
X
-
Y
-)
n
, where
X
is any other amino acid and is frequently
proline
(Pro) and
Y
is any
other amino acid and is frequently
hydroxyproline
(Hyp). A typical amino acid composition of collagen
is shown in Table 6.1. The application of helical diffraction theory to a high-angle collagen x-ray diffrac-
tion pattern (Rich and Crick, 1961) and the stereochemical constraints from the unusual amino acid
composition (Eastoe, 1967) led to the initial triple-helical model and subsequent modified triple helix of
the collagen molecule. Thus, collagen can be broadly defined as a protein which has a typical triple helix
extending over the major part of the molecule. Within the triple helix, glycine must be present as every
third amino acid, and proline and hydroxyproline are required to form and stabilize the triple helix.
To date, 19 proteins can be classified as collagen (Fukai et al., 1994). Among the various colla-
gens, type I collagen is the most abundant and is the major constituent of bone, skin, ligament, and
TABLE 6.1
Amino Acid Content of Collagen
Content, Residues/1000
Residues
a
Amino Acids
Gly
334
Pro
122
Hyp
96
Acid polar (Asp, Glu, Asn)
124
Basic polar (Lys, Arg, His)
91
Other
233
Source:
From Eastoe, J.E. 1967.
Treatise on Collagen
,
pp. 1-72, Academic Press, New York. With permission.
a
Reported values are average values of 10 different
determinations for tendon tissue.
