Proteins

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. Collagen 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. Myoglobin 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. Rhodopsin 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.

A. Simple Proteins

1. Albumins: blood (serumbumin); milk (lactalbumin); egg white (ovolbumin); lentils (legumelin); kidney beans (phaseolin); wheat (leucosin). Globular protein; soluble in water and dilute salt solution; precipitated by saturation with ammonium sulfate solution; coagulated by heat; found in plant and animal tissues.

2. Globulins: blood (serum globulins); muscle (myosin); potato (tuberin); Brazil nuts (excelsin); hemp (edestin); lentils (legumin). Globular protein; sparingly soluble in water; soluble in neutral solutions; precipitated by dilute ammonium sulfate and coagulated by heat; distributed in both plant and animal tissues.

3. Glutelins: wheat (glutenin); rice (oryzenin). Insoluble in water and dilute salt solutions; soluble in dilute acids; found in grains and cereals.

4. Prolamines: wheat and rye (gliadin); corn (zein); rye (secaline); barley (hordein). Insoluble in water and absolute alcohol; soluble in 70% alcohol; high in amide nitrogen and proline; occurs in grain seeds.

5. Protamines: sturgeon (sturine); mackerel (scombrine); salmon (salmine); herring (clapeine). Soluble in water; not coagulated by heat; strongly basic;