The amino acid sequence is called the primary structure of a protein.
gly-his-lys is the primary structure of a tripeptide used in cosmetics ("anti-aging")
The local folding of the peptide chain to an α helix, β sheet or random coil is called secondary structure.
The secondary structure is due to the attraction between polar carbonyl $C=O$ and amine groups $-NH-$
The shape due to the folding of the polypeptide chain in space is called tertiary structure of a protein.
The tertiary structure is due to
- the disulfide bridges formed by oxidation between remote cysteines:
- the elbow angles of the rigid amino acid proline
- the dipole-dipole attractions between distant parts of the chain:
The three-dimensional tertiary structure of a protein determines its function: When this structure is disturbed by the use of a denaturant, the protein loses its function: it is denatured.