Classification of Proteins According to Shape and Solubility

Proteins can be broadly classified into three groups, based on their shape and solubility.

Fibrous proteins: these proteins have a rod like structure. They are not soluble in water. is an example of a fibrous protein.

Globular proteins: these proteins more or less spherical in nature. Due to their distribution of amino acids (hydrophobic inside, hydrophillic outside) they are very soluble in aqueous solution. is an example of a globular protein.

Membrane proteins: these are protein which are in association with lipid membranes. Those membrane proteins that are embedded in the lipid bilayer have extensive hydrophobic amino acids that interact with the non-polar environment of the bilayer interior. Membrane proteins are not soluble in aqueous solution. is an example of a membrane protein. Note that rhodopsin is an integral membrane protein and is embedded in the bilayer. The lipid membrane is not shown in the structure presented here.


Classification of Globular Proteins According to Secondary Structure

Proteins are also classified based on the type of secondary structure present.

All alpha:
Proteins that contain only (in some exceptional instances there may be isolated beta-sheets) alpha helical secondary structure.

is an example of an all alpha protein.

All beta:
Protein that contain only (in some exceptional instances there may be isolated alpha helices) beta-sheet secondary structure. is an example of an all beta protein.

Alpha/beta:
Proteins that contain alternating alpha-helical and beta-sheet secondary structure elements. is an example of an alpha/beta protein. This structure is known as a TIM barrel. The alternating helical and sheet segments form a closed barrel like structure.

Alpha + Beta:
In these proteins the alpha helical and beta sheet regions occur in independent regions of the molecule. is an example of an alpha+beta protein.