Proteins consist of one or more chains of amino acids, linked together by peptide bonds. There are 20 naturally occurring amino acids that make up the building blocks of proteins. Shorter chains of amino acids, generally less than 50, are called peptides. Insulin for example, consists of 51 amino acids, and is therefore borderline a peptide and a protein.

Proteins are characterised by their amino acid sequence and higher order structure. They can also have secondary structures such as α-helixes and β-sheets formed by hydrogen bonds; tertiary structures formed by di-sulfide bonds; and a quaternary structure formed by protein sub-units. These structures are important as they determine the activity of the protein. Since proteins are large molecules, they are generally not cell permeable and hence restricted to extracellular targets and injectable administration routes.

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