Globular proteins are proteins are tightly packed and spherical. In other words, these proteins are circular in shape. Globular proteins are mostly transport or messenger proteins. For example, hemoglobin is a globular protein that transports oxygen in the body. Globular proteins have alpha-helices that contributes to their coiling.
Structural proteins or fibrous proteins are proteins that lack structure elements that globular proteins have like strong bonds. As a result, structural proteins are tubular and long in structure. Most structural proteins used to strengthen skin, bones, and collagen. Structural proteins also has beta pleated sheets in the teritary and quarternary structures
A globular protein has a fixed specific sequence of amino acids that are non-repetitive while a fibrous protein has a repetitive regular sequence of amino acid.
For example, haemoglobin, a globular protein is made up of 4 polypeptide chains to form a tetramer (?2?2), composed of two identical alpha-beta (??) dimers. Collagen, a fibrous protein, has a primary structure characterized by a repeating tripeptide sequence of Glycine - X - Y. (X is proline, Y is either hydroxyproline or hydroxylysine)
A globular protein has a more compact structure owing to highly contorted pattern of folding, bending and twisting along polypeptide chain to give the protein a spherical 3D shape while a fibrous protein is usually formed with elongated polypeptide chains wrapped around to form multi-molecular paralleled filaments to strands.
For example, haemoglobin is a tetramer made up of 4 polypeptide chains of 2? chains and 2? chains. These four subunits are packed to form an overall spherically shaped molecule. However, collagen, a fibrous protein, is formed with three polypeptide chains lie parallel and wind round one another, forming a tropocollagen. The tropocollagen molecules lie side by side and are linked to each other giving a collagen fibril.
A globular protein has its length of polypeptide always identical in two examples of the same globular protein where else a fibrous protein has its length of polypeptide chain varying in two examples of the same fibrous protein.
For example, haemoglobin, a globular protein is always made up of 4 polypeptide chains, namely ?chain(141 amino acids) and ßchain(146 amino acids). Collagen, a fibrous protein, is made up of a primary structure of a polypeptide chain characterized by Glycine - X - Y. Y rotates around hydroxyproline and hydroxylysine so the length of polypeptide chains differ.
A globular protein is held primarily by numerous intra- and inter-molecular non-convalent bonds, such as hydrogen, ionic bonds while a fibrous proteins are held due to numerous intra- and inter- molecular hydrogen and disulphide bonds.
For example, haemoglobin, a globular protein, is basically held together by interchain hydrophobic interatctions, with hydrogen and ionic bonds occurring. On the other hand, collagen, a fibrous protein, has its triple helix held together by extensive network of hydrogen bonds. Tropocollagen of collagen is also held by covalent cross-links to give the collagen fibre strength and rigidity.