Classification of protein

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Source: foodrevolution.org/blog/plant-based-protein/

Proteins are the most abundant macro-molecules, occurring in each cell and each part of a cell. They are generally formed by one or more long chain(s) of amino acid residues.

Classification of protein

1) Based on source

There are two types of protein.

a) Animal protein: They are derived from animal sources. These are usually high quality protein. E.g: eggs, milk, fish.

b) Plant protein: These are derived from plants and lower quantity protein. e.g: legumes.

 

2) Based on the shape of protein molecule

These are of two type.

a) Globular protein: Polypeptide chain folded into globular form, soluble in water. E.g: Insulin, albumin etc.

    • Soluble in water.

b) Fibrous: Polypeptide chains arranged in a long strands. E.g: hair, skin etc.

    • Insoluble in water.
    • Also called scleroprotein.
    • Resistant to proteolytic enzymes.
    • Examples: Collagen, actin, myosin, keratin of hair, claws etc.

 

3) Based on shape of composition and solubility

Three major groups are present, such as: a)simple protein, b) conjugated protein, c) Derived protein.

a) Simple protein: These are of globular type and contain only amino acids as structural components. These are further classified on the basis of solubility into several types:

Protein namePropertiesExamples

Albumins

  • Soluble in water.
  • Heat coagulable.
Egg albumin.

Globulin

  • Insoluble in water.
  • Soluble in dilute salt solution.
  • Heat coagulable.
Egg, milk, serum

Glutelins

  • Soluble in dilute acids and alkalies but in neutral solvents.
Glutenin of wheat, oryzenine of rice

Prolamines

  • Soluble in 70% alcohol.
  • Rich in proline.
Zein of corn

Scleroprotein

  • Insoluble in all solvents considered so far.
  • Animal protein present in hair, nails.
Keratin of hair

Histone protein

  • Soluble in water, dilute acid and salt solution.
The globin of hemoglobin

Protamines

  • They are the simplest protein with low molecular weight, strongly basic with argenine.
Clupeine from herring sperm.

 

b) Conjugated protein: They are also globular protein combined with a non protein group, called prosthetic group. These are further classified on the basis of nature of prosthetic groups present.

ProteinPropertiesExamples
NucleoproteinHave nucleic acids as their prosthetic group.Protamins
ProteoglycansAre polysaccharides combined with small amount of protein.Mucoproteins
GlycoproteinProteins are conjugated with carbohydrates.
ChromoproteinsThe prosthetic group are color compound.Flavoenzymes
LipoproteinProteins are combined with phospholipids.Egg yolk

 

c) Derived proteins: They are products of denaturation or partial digestion of proteins.

  • Proteans: Fibre from fibrinogen.
  • Coagulated proteins: Coagulated egg white.