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| Proteins help to make up many of the structures in organisms. In this chapter we will look at what these protein materials do and how their molecules control the way they work.
Reinforced concrete, fibreglass and carbon fibre are all examples of man-made structural materials. Although their chemical make-up differs widely they do have one feature in common - they are all composites. In other words they are mixtures of two or more materials, put together in such a way that the combination has the useful properties of all its components. For example, concrete resists crushing forces but is poor at resisting stretching. Construction engineers improve the resistance to stretching by placing steel rods or grids in the concrete before it sets. The steel resists stretching and shares this property with the reinforced concrete. Structural biomaterials are often composites. They may contain several different proteins together, or proteins can form composites with other biomolecules such as carbohydrates. Proteins may even combine with inorganic materials such as calcium phosphate in bone. Not only do the biomaterials have many different components but their molecules can be grouped in different complex arrangements of strands, bundles, sheets or layers. Three types of sstructure are common in structural proteins. In this chapter we will look at the collagen triple helix and add this to the a-helix and b-pleated sheet (Chapter 3). These regular molecular arrangements are often embedded in a matrix of protein molecules or parts of molecules with no obvious, tidy organisation. |
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 Figure 1 Cross-linking between tropocollagen molecules |
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 Figure 2 The tropocollagen superhelix |
5.1 Tendons and bone Collagen is the most common protein in mammals, making up approximately 30% by mass of the total protein. It occurs in a variety of environments, often where insoluble materials that are strong and resist stretching are needed. Tendon and bone are examples of these. Tendons connect muscles to bone. Collagen is made up of long-stranded molecules, called tropocollagen, organised in small bundles ( microfibrils and fibrils ) so that each strand has a large overlap with others. The tropocollagen molecules cross-link covalently to each other using lysine side chains (Figure 1). These cross-links are unusual and occur only in collagen and elastin , a related protein. The fibrils may contain millions of tropocollagen strands. They run parallel along the length of a tendon. The secondary structure of tropocollagen is unusual. Each strand of tropocollagen is made from three individual polypeptide chains. Each of these chains is a left-handed helix (unlike the right-handed a-helix), but the three chains are twisted together in a right handed superhelix (Figure 2). When a muscle contracts, collagen fibres in the connecting tendons have the job of passing the effect on to the skeleton to bring about some movement. It is important that the collagen fibres in the tendons are strong and do not stretch significantly. A collagen fibre of 1 mm diameter can support a mass of at least 10 kg before it breaks. When collagen is pulled in a direction parallel to its fibres it is the covalent bonds that provide the strength and resistance. |
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Unilever Education Advanced Series: Proteins | ||||||||||
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