Classification of Proteins According to Shape and Solubility
Proteins can be broadly classified into three groups, based on
their shape and solubility.
proteins: these proteins have a rod like structure. They are
not soluble in water. is an example of a fibrous protein.
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.
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.
of Globular Proteins According to Secondary Structure
Proteins are also
classified based on the type of secondary structure present.
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
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
Proteins that contain alternating
alpha-helical and beta-sheet secondary structure elements.
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.
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.