Page 79 - Color Atlas of Biochemistry

P. 79

70 Biomolecules

Structural proteins two latter amino acids are only formed during
collagen biosynthesis as a result of posttrans-
The structural proteins give extracellular lational modification (see p. 344).
structures mechanical stability, and are in- The triplet Gly-X-Y (2)is constantly re-
volved in the structure of the cytoskeleton peated in the sequence of collagen, with the
(see p. 204). Most of these proteins contain a X position often being occupied by Pro and
high percentage of specific secondary struc- the Y position by Hyp. The reason for this is
tures (see p. 68). For this reason, the amino that collagen is largely present as a triple helix
acid composition of many structural proteins made up of three individual collagen helices
is also characteristic (see below). (1). In triple helices, every third residue lies
on the inside of the molecule, where for steric
reasons there is only room for glycine resi-
A. D Keratin
dues (3; the glycine residues are shown in
D-Keratin is a structural protein that predom- yellow). Only a small section of a triple helix
inantly consists of α helices. Hair (wool), is illustrated here. The complete collagen
feathers, nails, claws and the hooves of ani- molecule is approximately 300 nm long.
mals consist largely of keratin. It is also an
important component of the cytoskeleton
(cytokeratin), where it appears in intermedi- C. Silk fibroin
ate filaments (see p. 204). Silk is produced from the spun threads from
In thekeratins, largeparts of thepeptide silkworms (thelarvaeof the moth Bombyx
chain show right-handed α-helical coiling. mori and related species). The main protein
Two chains each form a left-handed super- in silk, fibroin, consists of antiparallel pleated
helix, asisalsoseen in myosin (see p. 65). sheet structures arranged one on top of the
The superhelical keratin dimers join to form other in numerous layers (1). Since the amino
tetramers, and these aggregate further to acid side chains in pleated sheets point either
form protofilaments,with a diameter of straightuporstraightdown (seep. 68),only
3 nm. Finally, eight protofilaments then compact side chains fit between the layers. In
form an intermediate filament,witha diam- fact, more than 80% of fibroin consists of gly-
eter of 10 nm (see p. 204). cine, alanine, and serine, the three amino
Similar keratin filaments are found in hair. acids with the shortest side chains. A typical
In a single wool fiber with a diameter of about repetitive amino acid sequence is (Gly-Ala-
20 µm, millions of filaments are bundled to- Gly-Ala-Gly-Ser). The individual pleated sheet
gether within dead cells. The individual kera- layers in fibroin are found to lie alternately
tin helices are cross-linked and stabilized by 0.35 nm and 0.57 nm apart. In the first case,
numerous disulfide bonds (see p. 72). This fact only glycine residues (R = H) are opposed to
is exploited in the perming of hair. Initially, oneanother. Theslightly greater distance of
the disulfide bonds of hair keratin are dis- 0.57 nm results from repulsion forces be-
rupted by reduction with thiol compounds tween the side chains of alanine and serine
(see p. 8). The hair is then styled in the desired residues (2).
shape and heat-dried. In the process, new
disulfide bonds are formed by oxidation,
which maintain the hairstyle for some time.

B. Collagen

Collagen is the quantitatively most important
proteininmammals, makingup about 25%of
the total protein. There are many different
types of collagen, particularly in connective
tissue. Collagen has an unusual amino acid
composition. Approximately one-third of the
amino acids are glycine (Gly), about 10% pro-
line (Pro), and 10% hydroxyproline (Hyp). The

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