Page 353 - Color Atlas of Biochemistry
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344 Tissues and organs
Collagens are seen to have a characteristic banding pat-
tern of elements that are repeated every
Collagens are quantitatively the most abun- 64–67 nm.
dant of animal proteins, representing 25% of Tropocollagen molecules are firmly linked
the total. They form insoluble tensile fibers together, particularly at their ends, by cova-
that occur as structural elements of the ex- lent networks of altered lysine side chains.
tracellular matrix and connective tissue The number of these links increases with
throughout the body. Their name (which lit- age. Type IV collagens form networks with a
erally means “glue-producers”) is derived defined mesh size. The size-selective filtering
from the gelatins that appear as a decompo- effect of the basal membranes in the renal
sition product when collagen is boiled. glomeruli is based on this type of structure
(see p. 322).
A. Structure of collagens
B. Biosynthesis
Nineteen different collagens are now known,
and they are distinguished using roman nu- The precursor molecule of collagen (prepro-
merals. They mostly consist of a dextro- collagen), formed in the rER, is subject to
rotatory triple helix made up of three poly- extensive post-translational modifications
peptides (α-chains) (see p. 70). (see p. 232) in the ER and Golgi apparatus.
The triplet Gly-X-Y is constantly repeated in Cleavage of the signal peptide gives rise to
the sequence of the triple-helical regions— procollagen, which still carries large propep-
i. e., every third amino acid in such sequences tides at each end [1]. During this phase, most
is a glycine. Proline (Pro) is frequently found in proline residues and some lysine residues of
positions X or Y; the Y position is often occu- procollagen are hydroxylated [2]. The procol-
pied by 4-hydroxyproline (4Hyp), although lagen is then glycosylated at hydroxylysine
3-hydroxyproline (3Hyp) and 5-hydroxylysine residues [3]. Intramolecular and intermolecu-
(5Hyl) also occur. These hydroxylated amino lar disulfide bonds form in the propeptides
acids are characteristic components of colla- [4], allowing correct positioning of the pep-
gen. They are only produced after protein tide strands to form a triple helix [5]. It is only
biosynthesis by hydroxylation of the amino after these steps have been completed that
acids in the peptide chain (see p. 62). procollagen is secreted into the extracellular
The formation of Hyp and Hyl residues in space by exocytosis. This is where the N-and
procollagen is catalyzed by iron-containing C-terminal propeptides are removed proteo-
oxygenases (“proline and lysine hydrox- lytically [6], allowing the staggered aggrega-
ylase,” EC 1.14.11.1/2). Ascorbate is required tion of the tropocollagen molecules to form
to maintain their function. Most of the symp- fibrils [7]. Finally, several ε-amino groups in
toms of the vitamin C deficiency disease lysine residues are oxidatively converted into
scurvy (see p. 368) are explained by disturbed aldehyde groups [8]. Covalent links between
collagen biosynthesis. the molecules then form as a result of con-
Thehydroxyprolineresidues stabilize the densation [9]. In this way, the fibrils reach
triple helix by forming hydrogen bonds be- their final structure, which is characterized
tween the α-chains, while the hydroxyl by its high tensile strength and proteinase re-
groups of hydroxylysine are partly glycosy- sistance.
lated with a disaccharide (–Glc–Gal).
The various types of collagen consist of
different combinations of α-chains (α1to α3
and other subtypes). Types I, II, and III repre-
sent 90% of collagens. The type I collagen
shown here has the structure [α1(I)] 2 α2(1).
Numerous tropocollagen molecules (mass
285 kDa, length 400 nm) aggregate extracell-
ularly into a defined arrangement, forming
cylindrical fibrils (20–500 nm in diameter).
Under the electron microscope, these fibrils
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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