Globular protein

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 pratically insoluble.

Globular structure and solubility

The term globular protein is quite old (dating probably from the 19th century) and even predates the discovery of secondary structural motifs . That is to say, 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 globular structure is induced by the proteins' primary structure. As a matter of fact, the molecule's apolar groups are bounded towards the molecule's interior whereas polar ones are bounded outwards, allowing dipole-dipole interactions with the solvant , which explains the molecule's solubility.

A wide range of roles in the organism

Unlike fibrous proteins whose 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. insuline etc.
• Transporters of other molecules throughout membranes
• Stocks of amino acids.

Members

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.