Classification of Proteins:
Proteins are classified on the basis of their chemical composition, shape and solubility into two major categories as discussed below.
(i) Simple proteins:
Simple proteins are those which, on hydrolysis, give only amino acids. According to their solubility, the simple proteins are further divided into two major groupsâ€™ fibrous and globular proteins.
(a) Fibrous Proteins: These are water insoluble animal proteins eg.collagen (major protein of connective tissues), elastins (protein of arteries and elastic tissues), keratins (proteins of hair, wool, and nails) are good examples of fibrous proteins. Molecules of fibrous proteins are generally long and thread like.
(b) Globular Proteins: These proteins are generally soluble in water,acids, bases or alcohol. Some examples of globular proteins are albumin of eggs, globulin (present in serum), and haemoglobin. Molecules of globular proteins are folded into compact units which are spherical in shape.
(ii) Conjugated proteins:
Conjugated proteins are complex proteins which on hydrolysis yield not only amino acids but also other organic or inorganic components. The non-amino acid portion of a conjugated protein is called prosthetic group.
Unlike simple proteins, conjugated proteins are classified on the basis of the chemical nature of their prosthetic groups. These are
a) Nucleoproteins (protein + nucleic acid)
b) Mucoproteins and glycoproteinâ€™s (protein+ carbohydrates)
c) Chromo proteins (proteins + a colored pigment)
d) Lipoproteins (proteins + lipid)
e) Metalloproteinase (metal binding proteins combined with iron, copper or zinc)
f) Phosphoproteins (proteins attached with a phosphoric acid group).
Proteins can also be classified on the basis of functions they perform, as summarized in table 2.