Peptides and proteins are often very discrete terms; however, they refer to biological molecules that often overlap in terms of function and other factors. A peptide is a ‘chain’ of amino acids that is expressed as a result of mRNA translation. This is often thought of as the ‘primary’ dimension of protein structure. Many scientists throughout history may not have thought of a single chain as a complete protein. This is due to the theories of protein structure as composed of multiple chains. In addition, even individual chains may attain additional structural complexity. This complexity is conferred by interactions between the side-chains of the amino acids within the chain. These mainly give rise to alpha-helices or beta-pleated sheets. Shapes such as these are regarded as the secondary degree of protein structures. Complicated chains may then proceed to interact (i.e. ‘interlock’ or form interfaces) with others expressed from the same gene to form certain structures. Common examples of these are known as globular structures and zinc-finger structures. This is known as tertiary protein structures, and also as proteins domains (specific subunits or characteristics). One or more domains may then form interactions or bonds (e.g. sulphide bridges) to form proteins with a quaternary structure.

This is the classical view of proteins, which gives rise to their perception as complex molecules that may have a higher weight and three-dimensional size. There are a number of other properties that are also associated with proteins. An example of these is the structure-to-function relationship; essentially how a protein’s structure, once it has been produced as above, defines its role in a living system. However, some peptides present arguments that may dispute these classical characteristics.

At the outset of biological research, it may have been thought that proteins required chains with very large numbers of amino acids to carry out the roles increasingly associated with this class of molecule. This is true for some proteins, which require many specific domains in a definitive conformation to interact with their targets. On the other hand, some peptides with relatively short chains may elicit at least some of the functions of larger proteins from which they have been derived1. For example, a twelve-residue peptide fragment may bind to the receptor of the full insulin protein2. Therefore, a peptide may have some protein-like functions. Similarly, IGF-1 is a protein with several domains that has various regulatory functions in the body. Mechano-growth factor (MGF) is a peptide that is an equivalent to just one domain (the E-domain) of IGF-13. Despite its reduced complexity, however, it is active at receptors of its own and is specifically expressed from the IGF1 gene in response to mechanical damage to muscle tissue3.

In other words, a typical defining feature of proteins is that it is biologically active, i.e. that it binds a receptor or another protein to affect signaling within a cell. This may require a particularly-shaped protein with at least one domain – i.e. a complex and specific 3D structure. However, GRF is a relatively short peptide that can bind a receptor to enhance growth hormone (GH) release1. In addition, there are even smaller peptides – only six residues long – that may be comparably effective, and are thus known as synthetic GH secretagogues (GHSs)4. This is due to a certain motif, (or specific sequence of amino acids) contained within the six-amino-acid chain, that interacts effectively with the GHS receptor5. Small peptides, containing relevant motifs, also have other advantages over proteins such as increased absorption due to their small size. In general, all proteins may be considered peptide chains, but not all functional peptides need be proteins.