Food Chemistry

12.3 Muscle Tissue: Composition and Function 579

Table 12.7.Types of collagen

Peptide Molecular
Type chainsa 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 αA Placental membrane,
αC(?) or(αB) 3 + cardiovascular
(αA) 3 system, lung, muscle
(endomysium),
(αC) 3 (?) secondary component
of many tissues

aSince theαchains of various types of collagen differ,

they are calledα^1 (I),α^1 (II),αAetc.
b(?) Not completely elucidated.

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 reac-
tions:

• Enzymatic oxidation of lysine and hydroxyly-
  sine to the correspondingω-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 reac-
tion, 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.

Fig. 12.19.Collagen fibers of bovine muscle (Extensor

carpi radialis); transmission electron microscopy; sam-

ple fixed with 2% glutaraldehyde/paraformaldehyde,

bar: 0.5 μm. (according toElkhalifaet al., 1988)

This reaction yields an α-aminoadipic acid

semialdehyde residue bound to the existing

peptide chain (R=H or OH):

(12.10)

The two aldehyde-containing chains may inter-

act through an aldol condensation followed by

elimination of water, forming a cross-link:

(12.11)