(1) Primary configuration or structure :- A straight chain of amino acids linked by peptide bonds from primary structure of proteins. This structure of proteins is most unstable. Newly formed proteins on ribosomes have primary structure.

(2) Secondary configuration :- Protein molecules of sec. structure are spirally coiled. In addition to peptide bond, amino acids are linked by hydrogen bonds form between oxygen of one amide group and hydrogen of another amide group. This structure is of two types

(i) `alpha`-Helix :- Right handed rotation of spirally coiled chain with approximately `3(1/ 2)` amino acids in each turn. This structure has intramolecular hydrogen bonding i. e. between two amino acids of same chain e.g. Keratin ,Myosin, Tropomyosin.
(ii) `beta` Helix or pleated sheath structure :- Protein molecule has zig - zag structure. Two or more protein molecules are held together by intermolecular hydrogen bonding. e.g. Fibroin (silk).

`->` Proteins of sec. structure are insoluble in water and fibrous in appearance.

`-> ` Keratin is a fibrous , tough, resistant to digestion. sclero protein. Hard ness of keratin is due to abundance of cysteine amino acid in its structure.

(3) Tertiary Structure :- Proteins of tertiary structure are highly folded to give a globular appearance. They are soluble in water (colloid solution). This structure of protein has following bonds-

(i) Peptide bonds = strongest bond in proteins.
(ii) Hydrogen bonds
(iii) Disulphide bond :- These bonds are formed between - SH group of arnino acid (Cysteine). These bonds are second strongest bond and stabilise tertiary structure of protein.
(iv) Hydrophobic bond : Between amino acids which have hydrophobic side chains for e.g. Aromatic amino acid


(v) Ionic bond : Formation of ionic bond occurs between two opposite ends of protein molecule due to electrostatic attractio Majority of proteins and enzymes in protoplasm exhibit tertiary structure.

(4) Quaternary Structure :- Two or more poly peptide chains of tertiary structure unite by different types of bond to form quaternary structure of protein. Different polypeptide chains may be similar (lactic-dehydrogenase) or disimilar types (Haemoglobin, insulin).

Quaternary structure is most stable structure of protein.

`->` Besides changes in pH, salts, heavy metals, temperature, pressure, etc. also cause precipitation of proteins. Because of these changes, the secondary and tertiary configuration of proteins is destroyed. Such alternations in the physical state of proteins is called denaturation. If the change in the medium of protein is mild and for a short period, then denaturation of the protein is also temporary, however. if the change in medium is strong and prolonged then denaturation is permanent and the protein becomes coagulated. For example, the white or albumen of egg is a soluble globular protein but on heating it permanently coagulates into fibrous insoluble form. It is clear, that strong alternations result in the denaturation of proteins and they lose their biological properties and significance. It is this reason, that cells of organisms are unable to bear strong changes and they ultimately die.

Elasticity in wheat flour is due to Glutelline.

1. Nucleoprotein :- Prosthetic group is nucleic acid.
eg. Chromosome = DNA + HNA + Protein
Ribosome = rENA + Protein
Virus

e.g.

Lipoprotein :- Prosthetic group is lipid

eg. Plasma membrane
Lipovitel!ine membrane on egg surface.

Phosphoprotein :- Prosthetic group is phosphoric acid `(H_(3)PO_(4))`

`->` Caseinogen - Milk
`->` Pepsin - Protein digesting emzyme.

`tt( (text ( Phosvitin)) , (text(Ovovitelline)) ) ]`Egg

Lecithoprotein :- Prosthetic group is Lecithin

eg. Fibrinogen - Blood

Metalloprotein :- Prosthetic group is metal
eg. Enzyme with its co-factor

Glycoprotein :- Prosthetic group is carbohydrate (less than 4% carbohydrate)
eg. (1) Mucin - Saliva
(2) Erythropoelin - Kidney.
(3) A & B antigen of RBC.
(4) `alpha, beta, gamma` globulin of blood.
(5) FSH - Follicular stimulating hormone
(6) LH - Leutinizing hormone

Glycoproteins which are present on cell surface are helpful in cell recognition.
Human = Egg surface - Fertilizin - Glycoprotein
Sperm surface - Antifertilizin - Simple protein.

Mucoprotein Prosthetic group is carbohydrate (more than `4%` carbohydrate)
e.g. Mucoids of synovial fluid, Osteomucoprotein of bones,
Tendomucoprotein of tendons, Chondromucoprotein of cartilage.

`->` Monomeric protein : Protein composed of one polypeptide chain.
`->` Oligomeric/Polymeric/Multimeric protein : Protein composed of more then one polypeptide chains.