These side chains confer different chemical, physical, and structural properties to the final peptide or protein. The amino acids differ in structure by the substituent on their side chains. The terms, amino-terminus and N-terminus, describe the end of the sequence with a free α-amino group. The end of the peptide or protein sequence with a free carboxyl group is called the carboxy-terminus or C-terminus. A protein can be made up of one or more polypeptide molecules. Sequences with fewer than 50 amino acids are generally referred to as peptides, while the terms, protein and polypeptide, are used for longer sequences. This difunctionality allows the individual amino acids to join in long chains by forming peptide bonds : amide bonds between the -NH 2 of one amino acid and the -COOH of another. Amino acids, as their name indicates, contain both a basic amino group and an acidic carboxyl group. There are 20 different standard L-α-amino acids used by cells for protein construction.
Proteins are macromolecules and have four different levels of structure – primary, secondary, tertiary and quaternary. The term, structure, when used in relation to proteins, takes on a much more complex meaning than it does for small molecules. It will also cover briefly how protein structure can be affected during formulation and some of the analytical methods which can be used both to determine the structure and analyze the stability of the protein. This technical brief aims to give the reader a quick overview of protein structure.
Formulation of a protein drug product can be quite a challenge, and without a good understanding of the nature of protein structure and the conformational characteristics of the specific protein being formulated, the results can be ruinous. Increasingly, drug developers are looking to large molecules, particularly proteins, as a therapeutic option.
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