Chapter 4 of 5 - Protein Synthesis Course
A polypeptide fresh off the ribosome is not yet functional. It must fold correctly and often undergo chemical modifications before it can carry out its biological role.
Proteins are linear chains of amino acids when they leave the ribosome, but their function depends on their three-dimensional shape. Post-translational modifications (PTMs) expand the functional diversity of the proteome far beyond what the genome alone can encode. With roughly 20,000 human genes but over 1,000,000 distinct protein forms, PTMs account for much of this complexity.
Proteins fold into four levels of structure:
Primary
Amino acid sequence
Secondary
Alpha helices, beta sheets
Tertiary
3D shape of one chain
Quaternary
Multiple subunits
Folding begins even before translation is complete - the emerging polypeptide starts folding as it exits the ribosome. Chaperone proteins (such as Hsp70 and chaperonins like GroEL/GroES) prevent improper folding and aggregation by providing a protected environment for the polypeptide to fold correctly.
"A protein is a sentence written in amino acids. Post-translational modifications are the punctuation that gives it meaning."
- adapted from biochemistry teaching tradition
Quick Check
What is the role of chaperone proteins in protein folding?
| Modification | What Happens | Purpose |
|---|---|---|
| Phosphorylation | A phosphate group is added (by kinases) | Activates/deactivates enzymes, cell signaling |
| Glycosylation | Sugar chains are attached | Cell recognition, protein stability, secretion |
| Ubiquitination | Ubiquitin tags are attached | Marks proteins for degradation by the proteasome |
| Acetylation | Acetyl group added (often to histones) | Gene regulation, protein stability |
| Proteolytic cleavage | Portions of the polypeptide are cut away | Activates zymogens (e.g., insulin from proinsulin) |
| Disulfide bond formation | Covalent bonds between cysteine residues | Stabilizes 3D structure (common in secreted proteins) |
Fill in the Blank
When a protein is tagged with ubiquitin molecules, it is targeted for degradation by the________, a large barrel-shaped protein complex that breaks polypeptides back into amino acids.
Many proteins need to be sent to specific locations in the cell - the plasma membrane, lysosomes, mitochondria, or secreted outside the cell. A signal peptide (usually 15-30 amino acids at the N-terminus) acts as an address label:
When proteins fail to fold correctly, they can aggregate and become toxic. Several major diseases are linked to protein misfolding:
Quick Check
Which post-translational modification involves adding sugar chains to a protein?
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