Classification of protein; on the basis of structure, composition and function

September 10, 2019 Gaurab Karki Biochemistry 0




I. Classification of protein on the basis of Structure and composition:


• This Classification of protein is based on shape or structure and composition. They are classified into three types; fibrous, globular and derived protein.

1. Fibrous protein:

  • They are elongated or fiber like protein.
  • Axial ratio (length: breadth ratio) is more than 10
  • They are static in nature with simple structure.
  • They have less biological functions
  • They are mostly present in animals
  • Examples;
  • Fibrous proteins are further classified as- simple and conjugated
  • i. Simple fibrous protein:
  • Examples; Scleroprotein (Keratine, elastin, collagen, fibroin etc)
  • Scleroprotein or Albuminoids: they make animal skeleton and they are water insoluble.
  • ii. Conjugated fibrous proteins:
  • Examples; pigments present in chicken feather.

2. Globular protein:

  • They are spherical or globular in shape.
  • Axial ratio is always less than 10
  • They are dynamic in nature (can flow or move) with higher degree of complexity in structure.
  • They have variety of biological functions
  • Examples; enzymes, hormones etc
  • Globular protein is further classified on the basis of composition or solubility.

i. Simple or homo globular protein:

  • They are composed of amino acids only.
  • Some examples are;

a. Protamine:

  • They are positively charged (basic) proteins mostly present in animals and fishes (sperm)
  • Protamines binds with DNA in embryonic stage and later replaced by histone
  • It is soluble in water and ammonium hydroxide solution
  • It is not coagulated by heat
  • It precipitate out in aqueous solution of alcohol
  • Protamine are rich in arginine and lysine whereas devoid of sulfur containing and aromatic amino acids.

b. histone:

  • They are basic protein but weak base in comparison to protamine.
  • Histone is low molecular weight protein and are water soluble.
  • It is not coagulated by heat.
  • Histone is present in nucleic acids as nucleohistone binding with DNA.

c. Albumin:

  • It is the most abundant protein in nature
  • It is most commonly found in seeds in plants and in blood and muscles in animals.
  • Molecular weight of albumin is 65000 KD
  • It is water soluble and can be coagulated by heat
  • Plant albumins; Leucosine, Legumelins etc
  • Animal albumins; serum albumin, myosin, lactalbumin, ova-albumin etc

d. Globulin:

  • Pseudoglobulin (water soluble) and Euglobulin (water insoluble)

e. Glutelins:

  • Water insoluble. Eg. Glttenin (wheat), glutelin (corn), oryzenin (rice)

f. Prolamine:

  • They are storage protein found in seeds.
  • They are water insoluble. But soluble in dilute acid or detergents and 60-80% alcohol.
  • They are coagulated by heat
  • Prolamine is rich in proline and glutamine
  • Examples; Gliadin (wheat), zein (corn), Hordein (barley), Avenin (oats)

ii. Complex or conjugate or hetero globular protein:

  • These proteins in which protein are always linked by non-protein moiety to become functional. So, they are composed of both protein and non- protein components. The non-protein component is known as prosthetic group.
  • On the basis of prosthetic group, they are classified as follows;

a. Metalloprotein:

  • They have metal prosthetic group.
  • Some metals such as Hg, Ag, CU, Zn etc, strongly binds with proteins such as collagen, albumin, casein by –SH group of side chain of amino acids.
  • Eg. Ceruloplasmin; contains copper as prosthetic group
  • Some other metals such as Calcium weakly binds with protein. Eg. Calsequestrin, calmodulin
  • Some metals such as Na, K etc do not binds with protein but associate with nucleic acids protein.

b. Chromoprotein:

  • They have colored prosthetic group.
  • Some examples are;
  • Haemoprotein: Haemoglobin, myoglobin, chlorophyll, cytochrome, peroxidase, haemocyanin
  • Flavoprotein: Riboflavin (Vit B2) give yellow/orange color to FAD requiring enzymes

c. Glycoprotein/Mucoprotein:

  • They have carbohydrate as prosthetic group
  • Eg. Antibody, complement proteins, Heparin, Hyaluronic acid

d. Phosphoprotein:

  • They have phosphate group as prosthetic group.
  • Eg. Caesein (milk protein binds with calcium ion to form calcium salt of caseinate)
  • Ovovitellin; present in egg yolk
  • Calcineurin

e. Lipoprotein:

  • They have lipid as prosthetic group.
  • Eg. Lipovitelline, chylomicrons

3. Derived protein:

  • These protein are the derivatives of either simple or complex protein resulting from the action of heat, enzymes and chemicals.
  • Some artificially produced protein are included in this group.
  • They are classified as primary derived protein and secondary derived protein.

i. Primary derived protein:

  • The derived protein in which the size of protein molecules are not altered materially but only the arrangement is changed.
  • Some examples are;

a. Proteans:

  • Obtained as a first product after the action of acid or enzymes or water on protein.
  • They are insoluble in water.
  • Eg. Edestan, myosin

b. Metaprotein:

  • They are produced by further action of acid or alkali on protein at 30-60°C.
  • They are water insoluble but soluble in dil acid or alkali.
  • Also known as Infraprotein.
  • Eg. Curd

c. Coagulated protein:

  • They are produced by the action of heat or alcohol on protein.
  • They are insoluble in water.
  • Eg. Coagulated egg

ii. Secondary derived protein:

  • The derived protein in which size of original protein are altered.
  • Hydrolysis has occurred due to which size of protein molecule are smaller than original one.
  • Examples; a) Proteoses:
  • They are produced by the action of dilute acid or digestive enzymes when the hydrolysis proceeds beyond the level of metaprotein.
  • They are soluble in water
  • They are not coagulated by heat. • Eg. Albumose, Globulose etc.

II. Classification of protein on the basis of biological functions:

  1. Catalytic protein:
  • They catalyze biochemical reaction in cells. Eg. Enzymes and co-enzymes

2. Structural protein;

  • They make various structural component of living beings.
  • Eg. Collagen make bone, Elastin make ligamnets and keratin make hair and nails

3. Nutrient protein:

  • They have nutritional value and provide nutrition when consumed.
  • Eg. Casein in milk

4. Regulatory protein:

  • They regulate metabolic and cellular activities in cell and tissue.
  • Eg. Hormones

5. Defense protein:

  • They provide defensive mechanism against pathogens.
  • Eg. Antibodies, complement proteins

6. Transport protein:

  • They transport nutrients and other molecules from one organ to other.
  • Eg. Haemoglobin

7. Storage protein:

  • They stores various molecules and ions in cells.
  • Eg. Ferritin store Iron

8. Contractile or mobile protein:

  • They help in movement and locomotion of various body parts.
  • Eg. Actin, myosin, tubulin etc

9. Toxic protein:

  • They are toxic and can damage tissues.
  • Eg. Snake venom, bacterial exotoxins etc

Classification of protein; on the basis of structure, composition and function