Keratins are structural proteins found in hair, wool, claws, hooves, scales, shells, beaks and feathers.
Keratins are made of mainly small amino acids such as glycine and alanine which allows for the protein strands to easily align next to each other and readily form hydrogen bonds with each other. Also they contain a large amount of cysteine which will form disulfide bridges between different protein strands. These create a insoluble, strong protein useful for hair, claws etc. Also just the simple fact that its made of long chains rather than having a globular structure like enzymes or hormones which are therefore soluble.
Collagen is found in all the connective and supportive structures of the body. It is found in particularly high amounts in tissues that need to be strong, flexible or both, including cartilage, bones and tendons. Collagen can allow a tissue to withstand immense pulling pressure without stretching -- for example in a tendon transmitting the pull of a muscle to a bone. It can also be elastic, as in skin, and flexible, as in a tendon bending around a joint. Each fiber of collagen contains thousands of individual collagen molecules that are bound together by cross-linking and staggered covalent bonds. Covalent bonds are the strongest bonds that can exist among protein molecules. The collagen molecules themselves are made from 3 individual polypeptides or strings of amino acids. The strands wind around one another in an alpha-helix. The helix forms because of the regular amino acid sequence of the strands. The sequence is a repeating pattern of glycine-proline-X, where X can be any amino acid.
Levels of Protein Structure
Structural features of proteins are usually described at four levels of complexity:
Primary structure: the linear arrangement of amino acids in a protein and the location of covalent linkages such as disulfide bonds between amino acids. Secondary structure: areas of folding or coiling within a protein; examples include alpha helices and pleated sheets, which are stabilized by hydrogen bonding. Tertiary structure: the final three-dimensional structure of a protein, which results from a large number of non-covalent interactions between amino acids. Quaternary structure: non-covalent interactions that bind multiple polypeptides into a single, larger protein. Hemoglobin has quaternary structure due to association of two alpha globin and two beta globin polyproteins.
