Tertiary Structure (3^o Structure)

1. Highly folded protein.
2. Globular in structure.
3. Tertiary structure is stabilised by five types of bonds :

(i)      Peptide Bonds

1. Ist strongest bond of the protein.

(ii)     Disulphide Bonds

1. IInd strongest bond of the protein
2. It is present in sulphur containing amino acids
3. Between hydrogen sulphide groups of two cysteine or methionine amino acids.

(iii)    Hydrogen Bonds.

1. Weakest bond of the protein
2. They occur between hydrogen and oxygen atoms of various groups.

(iv)     Ionic Bonds/Electro static bond/ Salt linkage.

1. Present between the ends of protein
2. They appear between oppositely charged ionised groups of two amino acids, e.g., NH ⁺   and - COO^- .

(v)      Hydrophobic Interactions.

1. They are not true bonds but involve coming together of non-polar R-groups of two amino acids.
2. Most of proteins of protoplasm have tertiary structure.
3. Protein of tertiary structure maintain sol state of protoplasm.