Globular - roll up to form balls, usually soluble, usually have metabolic roles, e.g enzymes found in all organisms, plasma proteins and antibodies found in the blood of mammals. Haemoglobins quaternary structure consists of four polypeptide subunits, two alpha cahins and two beta chains. The four subunits form one water soluble globular protein. A specialised part of each polypeptide called the haem group, a prosthetic group, contains an iron ion. the haem group is responsible for the colour. An oxygen molecule can bind to the to the iron in the haem group, one haemoglobin can bind to four oxygen molecules.
Fibrous - form fibres, usually insoluble, usually have structural roles, e.g collagen found in bone and cartilage, keratin found in hair and fingernails. Collagen is a fibrous protein made up of three polypeptide chains wound around each other. each of the three chains is a coil made up of around 1000 amino acids. hydrogen bonds form between the chains giving strength. Strength is increased as each collagen molecule forms covalent bonds with the collagen molecules next to it resulting in a collagen fibril, many fibrils form a fibre. collagen provides mechanical strength e.g in the walls of arteries, tendons connecting skeletal muscles to bones, cartilage, bones, connective tissue.
Quaternary structure refers to the fact that some proteins are made up of more than one polypeptide subunit joined together.
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