Polypeptide
A polypeptide
(from the Greek πολύς poly "a lot" and πεπτός peptos "digested") is the name used to designate a large peptide; As an orientation, one can speak of more than 10 amino acids.
When the polypeptide is large enough, and in particular when it has a unique and stable three-dimensional structure, it is called a protein.
Structure
The classification of peptides according to the number of amino acids in their chain is neither clear nor universally accepted.
Polypeptides of about 50 amino acids (aa) are commonly known as proteins, but authors differ widely as to where they begin to use the latter term.
| Size | Bibliographical source | Molecular weight |
| 10-49 amino acids | University of Murcia | 5,000-100,000 Daltons (Da) |
| University of Tours | ||
| +51 amino acids | National Human Genome Research Institute NIH | |
| ~100 amino acids | University of Queensland | |
| Encyclopedie Larousse | ||
| +100 amino acids | University of Montpellier | ~10.000 Daltons (Da) |
Usually a polypeptide is considered to be a chain of 10 to 49 amino acids.
A longer chain of amino acids (51 or more aa) is a polypeptide. Proteins made inside cells are made from one or more polypeptides.
For other authors, 50-100 amino acids is the limit between a peptide and a protein.
A third position is that proteins are large polypeptides and the two terms can be used interchangeably.
Examples of Polypeptides
- Pancreatic polypeptide 36 amino acids.
- Polypeptide of the cyclase adenylate of the pituitary 176 amino acids.
- vasoactive intestinal peptide 28 amino acids.
- Gastrin 101 amino acids.
- Motiline.
- Secret.
- Calcitonin.
- Glucagon 29 amino acids.
- Insuline chain A (21 amino acids) and chain B (30 amino acids).
Proteins with a single polypeptide chain are called monomeric proteins, while those composed of more than one polypeptide chain are known as multimeric (dimeric, trimetic) proteins.