Globular proteins, or spheroproteins are one of the two main protein classes, comprising globelike proteins that are more or less soluble in aqueous solutions (where they form colloidal solutions). This main characteristic helps distinguishing them from fibrous proteins (the other class), which are practically insoluble.
Globular structure and solubility
The term globular protein is quite old (dating probably from the 19th century) and is now somewhat archaic given the hundreds of thousands of proteins and more elegant and descriptive structural motif vocabulary. The globular nature of these proteins can be determined without the means of modern techniques, but only by using ultracentrifuges or dynamic light scattering techniques.
The spherical structure is induced by the protein's primary structure. The molecule's apolar (hydrophobic tails) groups are bounded towards the molecule's interior whereas polar (hydrophilic tails) ones are bound outwards, allowing dipole-dipole interactions with the solvent, which explains the molecule's solubility. Additionally, globular proteins have non-covalent bonds within one polypeptide, as opposed to fibrous proteins, which have non-covalent bonds between adjacent polypeptide chains.
A wide range of roles in the organism
Unlike fibrous proteins which only play a structural function, globular proteins can act as:
- Enzymes, by catalyzing organic reactions taking place in the organism in mild conditions and with a great specificity. Different esterases fulfil this role.
- Messengers, by transmitting messages to regulate biological processes. This function is done by hormones, i.e. insulin etc.
- Transporters of other molecules through membranes
- Stocks of amino acids.
Among the most known globular proteins is hemoglobin, a member of the globin protein family. Other globular proteins are the immunoglobulins (IgA, IgD, IgE, IgG and IgM), and alpha, beta and gamma globulins. See protein electrophoresis for more information on the different globulins. Nearly all enzymes with major metabolic functions are globular in shape, as well as many signal transduction proteins.