Peptides vs proteins - what's the difference?
If you’re looking for the difference between peptides and proteins, the short answer is ‘size’.
Both peptides and proteins are made up of strings of the body’s basic building blocks – amino acids – and held together by peptide bonds. In basic terms, the difference is that peptides are made up of smaller chains of amino acids than proteins.
But the definition, and the way scientists use each term, is a little loose. As a general rule, a peptide contains two or more amino acids. And just to make it a little more complicated, you will often hear scientists refer to polypeptides – a chain of 10 or more amino acids.
Dr Mark Blaskovich from the Institute for Molecular Bioscience (IMB) at The University of Queensland in Australia says approximately 50-100 amino acids is the cut-off between a peptide and a protein. But most peptides found in the human body are much shorter than that – chains of around 20 amino acids.
There’s also an important variant of peptide called the cyclotide. As with the peptide and the protein, the cyclotide is also comprised of a string of amino acids, but unlike the others, the ends of a cyclotide are joined together to form a circle.
As we’ll discuss below, this structure is important in the manufacture of therapeutic peptide-based drugs.
As for proteins, biochemists generally reserve the term for large peptide molecules, which can either be one long chain of 100 or more amino acids – a ‘complex polypeptide’, if you like – or they can be comprised of several amino acid chains joined together.
Haemoglobin, found in your red blood cells and essential for carrying oxygen, is such a protein. It’s made up of four different amino acid chains – two with 141 amino acids each and two with 146 amino acids each.
Why peptides are the ‘next big thing’ in medical research
Biochemists are excited by the possibilities presented by peptides and proteins as pharmaceuticals because they so often mimic exactly the behaviour of a natural ligand – the substance that interacts with the receptor on an enzyme or cell to cause a biological process.
This gives peptide drugs the potential to be more precisely targeted, with fewer side effects than small-molecule drugs.
Within the body, there are lot of different hormones that react with cells and trigger different biological processes. Often these are peptides, either cyclic versions or straight, linear ones.
And then there’s the matter of how fast that peptide breaks down, which causes some stability issues, but in terms of safety, can be a positive.
“We think peptides are the future of drugs for reasons of being more selective, more potent and potentially safer, because when a peptide eventually breaks down it just breaks down into amino acids, and amino acids are food, basically,” says Professor David Craik, who leads IMB’s Clive and Vera Ramaciotti Facility for Producing Pharmaceuticals in Plants.
There are also manufacturing considerations that make peptides attractive – their length allows them to be chemically synthesised, as opposed to proteins that are generally expressed in yeast or mammalian cells.
So that’s peptides. What are the applications for proteins?