A polypeptide is a long chain of amino acids linked together by peptide bonds — typically more than 20 amino acids, but it can include up to several hundred.
- The word “poly” means “many”, so polypeptide = many amino acids.
- Polypeptides are precursors to proteins — and when one or more polypeptides are folded into a specific 3D shape, they become functional proteins.
🧠 Peptides vs. Polypeptides vs. Proteins
| Term | Size (Amino Acids) | Structure | Functionality |
|---|---|---|---|
| Peptide | ~2–20 | Short chain | Often signaling |
| Oligopeptide | ~2–20 | “Few” amino acids | Signaling, metabolic roles |
| Polypeptide | ~20–1000+ | Long chain, may fold | Can become proteins |
| Protein | One or more folded polypeptides | Functional 3D structure | Structural or enzymatic |
So, every protein is made of polypeptides, but not every polypeptide is a full protein (some may not fold or function independently).
🧬 How Are Polypeptides Made?
- Transcription: DNA is copied into messenger RNA (mRNA).
- Translation: Ribosomes read the mRNA and link amino acids in the correct order.
- Polypeptide chain is formed.
- Folding and modifications occur → producing a functional protein.
🧪 Examples of Polypeptides:
| Polypeptide | Amino Acids | Role |
|---|---|---|
| Insulin (A+B chains) | ~51 | Regulates blood sugar |
| Hemoglobin chains | ~146 (each) | Carry oxygen in red blood cells |
| ACTH | 39 | Stimulates adrenal cortisol production |
| Growth hormone (GH) | 191 | Stimulates growth and metabolism |
| Collagen chains | ~1000 | Provides structure to skin and tissue |
🛠️ Functions of Polypeptides (As Part of Proteins):
- Enzymes (e.g., amylase, DNA polymerase)
- Hormones (e.g., insulin, GH)
- Structural proteins (e.g., keratin, collagen)
- Transport proteins (e.g., hemoglobin)
- Antibodies (immune defense)
- Muscle proteins (e.g., actin, myosin)
🔍 Can Polypeptides Function on Their Own?
Sometimes, yes — some smaller polypeptides (like ACTH or glucagon) can act as functional hormones without forming a larger protein.
But most polypeptides need to fold and/or combine with other chains to become fully functional proteins.
🧠 Summary Table:
| Feature | Polypeptide |
|---|---|
| Composition | 20+ amino acids linked by peptide bonds |
| Role | Protein precursor or active biomolecule |
| Can be a protein? | Yes, if it folds properly or joins with others |
| Functions | Structure, signaling, transport, enzymes, hormones |
| Built by | Ribosomes during translation |