Peptides, polypeptides and proteins are all made the same way. Amino acids join end to end through peptide bonds to form a chain. The names mostly describe how long that chain is.
The smallest peptide is a dipeptide, made of two amino acids. Three make a tripeptide. Short chains of a few up to a few dozen amino acids are often called oligopeptides, or simply peptides. The prefix oligo means few.
Longer chains are called polypeptides. Many biochemistry textbooks note that most natural polypeptide chains contain between 50 and 2,000 amino acids, and these are usually called proteins. So a common rule of thumb draws the line near 50. Below it, people tend to say peptide. Above it, they tend to say protein.
This line is a convention, not a law of nature. Some molecules near the boundary get called both. A protein can also be made of more than one chain held together, while a peptide is usually a single short chain. Many people also expect a protein to fold into a stable three-dimensional shape, which short peptides often lack.
You will also meet the word residue. When an amino acid joins a chain, it loses a few atoms in the bond-forming reaction. What remains in the chain is called a residue. Scientists describe chain length in residues, so a 30-residue peptide is a chain of 30 linked amino acids.
References
- Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th ed., Section 3.2: Primary Structure: Amino Acids Are Linked by Peptide Bonds (2002)
- RCSB PDB-101, Guide to Understanding PDB Data: Protein Hierarchical Structure (n.d.)
Educational content about laboratory research. Not medical advice. All Orion compounds are for laboratory research use only.
