The primary structure of any protein is determined by the number and sequence of amino acids. Each sequence of amino acids is specific for each protein. The secondary structure of a protein is based on hydrogen bonding. The C=O and -NH group on neighbouring amino acids will form many hydrogen bonds due to an unequal distribution of electrons in the bonds. The bonds causes polypeptide chains to twist into a three dimensional shape such as an alpha helix or a beta sheet. The tertiary structure of a protein is based upon the polypeptide chain folding to form a precise 3D shape. This shape is determined by hydrophilic amino acids shielding hydrophobic amino acids in aqueous solution, hydrogen, ionic and disulphide bonds. Quaternary structure of a protein refers to how different sub units of protein link together in various ways. Often large proteins form complex molecules which may have a non protein component referred to as a prosthetic group.
Collagen is a naturally occurring protein, found exclusively in animals. It is the most abundant protein in animals and the main structural protein and is found in fibrous tissues such as tendons, ligaments and skin. The primary structure of collagen is an unbranched polypeptide chain. Largely composed of triplet repeat sequences in which the first residue (amino acid molecule which has lost its water molecule) is hydrophobic glycine or alanine, the second is proline and the third is often hydroxyproline. The triplet repeats to form a stable, narrow left handed helix- not an alpha helix. This defines collagen’s secondary structure. In the tertiary structure of collagen, three polypeptide chains are hydrogen bonded to each other. This occurs between the NH groups on glycine residues and CO groups of residues on other chains.
The three collagen molecules form the quaternary structural unit of collagen fibers. They are formed in the endoplasmic reticulum, where three collagen chains wrap around one another to form a right handed triple helical structure. The strength of the triple helix is maximised when the three collagen chains are parallel to each other and are staggered by one residue. The staggered arrangement means that a glycine residue is present at each point along the helix. The side chain of glycine is a hydrogen atom meaning it is small enough to fit inside the collagen triple helix. Any other side chain would be too large and could destabilise the triple helical structure. The tight packing of the collagen chains adds to the tensile strength of the collagen fibres. Covalent cross-links are present between adjacent triple helices, further increasing the tensile strength of the fibrils which go on to form collagen fibres.
Collagen diseases in humans include scurvy. This is caused by a vitamin C (ascorbic acid) deficiency. The person suffering is unable to add hydroxyl groups to proline residues. This leads to the inadequate synthesis of collagen. This explains why the symptoms of scurvy include muscle and joint pain.
Catalase is a common protein found in nearly all living organisms exposed to oxygen. It catalyses the decomposition of hydrogen peroxide to water and water. Due to the toxic nature of the hydrogen peroxide, the function of catalase is especially important to living humans. As an enzyme, catalase has an exceptionally high turnover number - with one catalase molecule converting millions of hydrogen peroxide to water and oxygen each second. The structure of a catalase enzyme is known a dumbbell-shaped tetramer of four polypeptide chains, with each chain being over 500 amino acids long. Each monomer of catalase contains a heme prosthetic group at the active centre and most catalase monomers also contain one NADP molecule. As catalase is an enzyme, it is globular and therefore soluble in water. The secondary structure of catalase is irregular and consists of alpha helices and beta sheets. This structure means…