The interactions between the hydrophobic R groups inside the molecule are important in holding it in its correct 3-D shape. The outward pointing hydrophilic R groups on the surface of the molecule are important in maintaining its solubility.
Each polypeptide chain contains a haem group. A group like this, which is an important, permanent, part of a protein molecule but is not made of amino acids, is called a prosthetic group.
Each haem group contains an iron ion, Fe2+. One oxygen molecule 02 can bond with each iron ion. So a complete haemoglobin molecule, with four haem groups, can carry four oxygen molecules at a time.
It is the haem group which is responsible for the colour of haemoglobin. This colour changes depending on whether or not the iron ions are combined with oxygen. If they are, the molecule is known as oxyhaemoglobin, and is bright red. If not the colour is purplish.
Collagen
Collagen is a fibrous protein that is found in skin, tendons, cartilage, bones, teeth and the walls of blood vessels. It is an important structural protein, not only in humans, but in almost all animals, and is found in structures ranging from the body wall of sea anemones to the egg cases of dogfish.
A collagen molecule consists of three polypeptide chains, each in the shape of a helix. (This is not an α helix as it is not tightly xound.) the three helical polypeptides then wind around each other to form a three stranded rope. Almost every third amino acid in each polypeptide chain is glycine. Its small size allows the three strands to lie close together and so form a tight coil. Any other amino acid would be too large. The three strands are held together by hydrogen bonds.
Each complete, three-stranded molecule of collagen interacts with other collagen molecules running parallel to it. Bonds from between the R groups of lysines in molecules lying next to each other. These cross-links hold many collagen molecules side by side, forming fibres. The ends of the parallel molecules are staggered; if they were not, there would be a weak spot running right across the collagen fibre. As it is, collagen has tremendous tensile strength, that is it can withstand large pulling forces. The human Achilles tendon, which is almost all collagen fibres can withstand a huge pulling force, almost one quarter the tensile strength of steel.
