2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 579
Table 12.7. Types of collagen
Peptide Molecular
Type chains a composition Occurrence
I α 1 , α 2 [α 1 (I)] 2 α 2 (I) Skin, tendons, bones,
muscle (epimysium)
II α 1 [α 1 (II)] 3 Cartilage
III α 1 [α 1 (III)] 3 Fetal skin,
cardiovascular system,
synovial membranes,
inner organs, muscle
(perimysium)
IV α 1 , α 2 [α 1 (IV)] 3 (?) b Basal membranes,
capsule of lens,
glomeruli
(?) Placental membrane,
lung, muscle
(endomysium)
V αA, αB, [αB] 2 α A Placental membrane,
αC (?) or(αB) 3 + cardiovascular
(αA) 3 system, lung, muscle
(endomysium),
(αC) 3 (?) secondary component
of many tissues
a Since the α chains of various types of collagen differ,
they are called α 1 (I), α 1 (II), αAetc.
b (?) Not completely elucidated.
Fig. 12.19. Collagen fibers of bovine muscle (Extensor
carpi radialis); transmission electron microscopy; sample
fixed with 2% glutaraldehyde/paraformaldehyde,
bar: 0.5 µm. (according to Elkhalifa et al., 1988)
This reaction yields an α-aminoadipic acid
semialdehyde residue bound to the existing
peptide chain (R = H or OH):
Figure 12.19 shows an electron micrograph of
collagen fibers of bovine muscle.
During maturation or aging, collagen fibers
strengthen and are stabilized, primarily by
covalent cross-linkages. Thus, cross-links confer
mechanical strength to collagen fibers.
Cross-link formation involves the following reactions:
• Enzymatic oxidation of lysine and hydroxylysine
to the corresponding ω-aldehydes.
• Conversion of these aldehydes to aldols and
aldimines.
• Stabilization of these primary products by
additional reduction or oxidation reactions.
It appears that only certain residues undergo reaction,
mainly in the terminal, non-helical regions
of the peptide chains.
Lysine and hydroxylysine residues within the
peptide chain are oxidized by an enzyme that
requires Cu 2+ and pyridoxal phosphate for its
activity and which is related to amine oxidase.
(12.10)
The two aldehyde-containing chains may interact
through an aldol condensation followed by
elimination of water, forming a cross-link:
(12.11)