Based on the composition and solubility, proteins are broadly classified into three major groups.

1. Simple proteins: These proteins are composed of only amino acid residues. These are further classified into two classes.
   1. Globular proteins: These are spherical or oval in shape, soluble in water or other solvents and digestible.
      1. Albumins: These are soluble in water and dilute salt solutions and are coagulated by heat; examples are serum albumin, ovalbumin (egg) and lactalbumin (milk).
      2. Globulin: These are soluble in neutral and dilute salt solutions; examples are serum globulin and vitelline (egg yolk).
      3. Glutelins: These are soluble in dilute acids and alkalis and are mostly found in plants; examples are glutelin (wheat) and oryzenin (rice).
      4. Prolamines: These are soluble in 70 per cent alcohol; examples are gliadin (wheat) and zein (maize).
      5. Histones: These are strongly basic proteins and are soluble in dilute ammonium hydroxide; examples are thymus histones and histones of codfish sperm.
      6. Globins: These are generally considered along with histones. However, globins are basic proteins and are not precipitated by ammonium hydroxide.
      7. Protamines: These are strongly basic and resemble histones but are smaller in size and soluble in ammonium hydroxide. Protamines are also found in association with nucleic acids. Examples are sperm proteins.
   2. Fibrous proteins: These are fibre-like in shape, insoluble in water and resistant to digestion. Albuminoids or scleroproteins constitute the most predominant group of fibrous proteins.
      1. Collagens: These are connective tissue proteins lacking tryptophan. On boiling with water or dilute acids, collagens yield gelatin, which is soluble and digestible.
      2. Elastins: These proteins are found in elastic tissues such as tendons and arteries.
      3. Keratins: These are present in exoskeletal structures such as hair, nails and horns. Human hair keratin contains as much as 14 per cent of cysteine.
2. Conjugated proteins: Besides the amino acids, these proteins contain a non-protein moiety known as prosthetic group or conjugating group.
   1. Nucleoproteins: Nucleic acid (DNA or RNA) is the prosthetic group. Examples are nucleohistones and nucleoprotamines.
   2. Glycoproteins: The prosthetic group is carbohydrate, which has less than four per cent protein. The term mucoprotein is used if the carbohydrate content is more than four per cent. Examples are mucin (saliva) and ovomucoid (egg white).
   3. Lipoproteins: Proteins found in combination with lipids are the prosthetic group. Examples are serum lipoproteins and membrane lipoproteins.
   4. Phosphoproteins: Phosphoric acid is the prosthetic group. Examples are casein (milk) and vitelline (egg yolk).
   5. Chromoproteins: The prosthetic group is coloured in nature. Examples are haemoglobins and cytochromes.
   6. Metalloproteins: These proteins contain metal ions such as iron, cobalt, zinc, copper and magnesium. Examples are ceruloplasmin (Cu) and carbonic anhydrase (Zn).
3. Derived proteins: These proteins are the denatured or degraded products of simple and conjugated proteins. Derived proteins are of two types. The primary derived are the denatured, coagulated or first hydrolysed products of proteins. The secondary derived are the degraded (due to breakdown of peptide bonds) products of proteins.
   1. Primary derived proteins:
      1. Coagulated proteins: These are the denatured proteins produced by agents such as heat, acids, and alkalis. Examples are cooked proteins and coagulated albumin (egg white).
      2. Proteans: These are the earliest products of protein hydrolysis by enzymes, dilute acids, alkalis, and so on, which are insoluble in water. Examples are fibrins formed from fibrinogen.
      3. Metaproteins: These are the second-stage products of protein hydrolysis obtained by treatment with slightly stronger acids and alkalis. Examples are acid and alkali metaproteins.
   2. Secondary derived proteins: These are the progressive hydrolytic products of protein hydrolysis. These include proteoses, peptones, polypeptides and peptides.