"tertiary structure of a protein is maintained by (list the interactions)"

Protein tertiary structures are known to be a three dimensional structure of a protein with a single polypeptide chain (backbone) and one or more protein secondary structures known as protein domains.

1) Hydrophobic Interactions: they are non- covalent bonds and very important in the formation of tertiary structure. 

2) Ionic Bonds: the interaction of both positive and negative amino acids forms a bond that helps to stabilize the protein molecules. 

3) Hydrogen Bonds: this bond exit between the amino acid with hydrophilic side chain found on the surface of the molecules and water molecules in a solution.

4) Disulfide Bridges: it is a strong covalent bond commonly found between cysteine residues in close proximity space.