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Proteins
5. Proteins in structures Page 23
electron micrograph
Figure 12
Electron micrograph of human squames.
5.4 Hair, wool, nails and skin (continued)

Nails and skin
The molecules of
a-keratin in finger and toe nails are organised in sheets stacked in layers. The proteins still fall into two groups; the helical and matrix, but in nail there is less sulphur in the matrix than in the hair matrix.

Skin is a more complicated structure (Figure 11). Only a small percentage of skin is a-keratin and this makes up the outermost, cornified layer of the epidermis. Cells in the epidermis make

a-keratin. When the cells die they form an interlocking mat of squames (Figure 12). The a-keratin fibres in these are surrounded by a cornified envelope and the gaps between the squames are filled with fatty molecules to give the skin its waterproofing.

Collagen fibres make up the bulk of the proteins in skin and are produced by cells in the dermis. Here they are woven into a relatively loose network where the fibres can slide over one another without stretching themselves, giving the skin its elasticity. Leather is produced by tanning animal skins. In this process most of the material apart from collagen is removed and additional cross-links between the tropocollagen molecules are introduced.

Question:

Tanning makes the collagen more resistant to heat. Explain why.

diagram of beta keratin
Figure 13
b-keratin - the b-sheet twists gradually.
5.5 Beaks, feathers and silk
Birds and reptiles have their own keartins , very different from the a-keratins in mammals. Like a-keratins, bird and reptile keratins are composites made up from both fibrous and matrix components. The fibrous feather keratin can only stretch some 6% before breaking, unlike hair a-keratin that can stretch to twice its length. Protein chemists think that the main secondary structure in bird and reptile keratin is the b-sheet. The b-sheet does not lie flat but twists gradually (Figure 13). Each polypeptide chain in these b-keratins has a central helical section with less regular regions at each end. These regions contribute to the matrix component and have some -S-S- cross-links.Silks too have much b-sheet in their structures. They are different from the b-keratins, though, since they have no -S-S- links. The sheets are antiparallel, so that polypeptide chains next to each

other run in opposite directions. The sheets stack together in layers like a pack of playing cards (Figure 14).

diagram of forces in silk
Figure 14
The forces holding silk together. Side views of the b-sheets in silks. The polypeptide chains in these sheets have the sequence -GlyAlaGlyAla- repeated many times. (a) represents alanine side chains, (g) represents glycine side chains.
Question:

Explain why a- and b- keratin behave differently when pulled.

Unilever Education Advanced Series: Proteins
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