Levels of protein structure – primary

The primary protein structure refers to the sequence of amino acids and the location of disulfide bonds (Figure 10).

The amino acids, when linked by peptide bonds, are referred to as residues. Short chains of amino acid residues are often called (oligo-)peptides. 

Figure 10 Chemical structure (bottom) and the 3D structure (top) of a peptide bond between two adjacent amino acid residues; the N-terminal end of a polypeptide contains a free amino group and the C-terminal end contains a free carboxylate group (image from Wikipedia).