In proline and hydroxyproline, this free rotation is prevented. Cystine can form covalent bonds to another cystine. Protein shape is determined mostly by the amino acid sequence. Amino acids are covalently bonded from the carboxyl carbon of one amino acid to the amino nitrogen of another amino acid. The planar arrangement of of 6 atoms in the result of resonance.
Proteins consist of a polypeptide backbone with attached side chains.
The three dimension folding of proteins is the result of specific interactions between side chains. Globular proteins fold with most hydrophobic side changes on the inside and most hydrophilic side changes on the inside. But, globular proteins inserted in membranes often have a band of hydrophobic side chains on the outside of the molecule. This equatorial band fits in the hydrophobic interior of the cell membrane. Hydrogen bonds help stabilize the three-dimensional structure of many types of globular and structural proteins.