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Proteins
5. Proteins in structures Page 19
photo of horses Figure 3
Ligaments in the necks of grazing animals are rich in elastin . The tension in the stretched ligament counterbalances the mass of the animal’s head, rather as large springs counterbalance the mass in an ’up and over’ garage door mechanism. This tension reduces the muscular effort needed to raise the head. 		5.2 Ligaments and arteries
Ligaments are fibrous tissues that hold the body’s organs in place and hold the joints of the skeleton together. In both these applications the ligament must be elastic to allow movement (Figure 3).

The main component in ligaments is a protein called elastin. Smaller contributions are made by collagen and polysaccharides. Elastin fibres can stretch to several times their length and return quickly to their original dimensions when the pulling force is released.

Arteries, especially the aorta, are rich in elastin too (Figure 4). When the heart beats the aorta stretches and absorbs some of the energy passed on to the blood flow. Before the next beat the aorta contracts again and helps the blood flow along. In this way the aorta evens out the rate of blood flow by damping the beats.
diagram of artery
Figure 4
Stretchy elastin helps to make artery walls.

Maggot to chrysalis

Self tanning insects
Insects wear their skeletons on the outside: they have exoskeletons. These structures are mostly made up of protein material. Maggots are the larvae that hatch from insect eggs. At first a maggot feeds and grows rapidly, then it changes into a pupa from which an insect will finally emerge.

Development of the insect may take up to three weeks. During this time the developing insect is protected by a rigid protein coat (cuticle). The cuticle starts off as a soft, flexible white skin - ideal for the maggot that is foraging about, but not at all good for long term protection. How does the skin change into the rigid cuticle? In what way is the protein in the skin different from that in the rigid cuticle?

In the cuticle the polypeptide chains are locked together by covalent cross-links . This gives a hard, rigid 3D structure, rather like a thermosetting plastic.

To make the cross-links the maggot releases phenol-like compounds into its skin. These cause some of the a-amino acid side chains to react together.

The new phenol-like groups are hydrophobic so water is expelled as the cross-links form. The cross-linking process is similar to the changes that happen when animal hide is tanned and turned into leather.

Phenol-like compounds are also responsible for the dark coloration in the cuticle. These are similar to the compounds that form when cut apples go brown or the pigment that forms when we sunbathe.

Cross-linked protein molecules give the pupa its protective coat.
photo of maggot chrysalis
Cross-linked protein molecules give the pupa its protective coat.
Unilever Education Advanced Series: Proteins
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