Disulfide Bonds &
Post-Translational Modifications
Explore how covalent \(\text{S–S}\) bonds and chemical modifications after translation shape protein structure and function.
Biochemistry
What Are Post-Translational Modifications?
After a ribosome translates mRNA into a polypeptide chain, the protein is not yet finished. A wide range of chemical modifications are made to the protein after translation: these are called post-translational modifications (PTMs).
Major Types of PTMs
Phosphorylation
Addition of \(\text{PO}_4^{3-}\) to Ser, Thr, or Tyr residues by kinases
Glycosylation
Attachment of sugar chains (N-linked or O-linked) in the ER and Golgi
Disulfide Bonds
Covalent \(\text{S–S}\) linkage between two cysteine residues
Ubiquitination
Tagging with ubiquitin to mark proteins for proteasomal degradation
Why Do PTMs Matter?
1
Regulate activity: Phosphorylation can switch an enzyme on or off like a molecular toggle.
2
Direct localization: Glycosylation tags proteins for secretion; lipidation anchors them to membranes.
3
Control lifespan: Ubiquitin marks damaged or unneeded proteins for destruction by the proteasome.
4
Stabilize structure: Disulfide bonds lock a protein's 3D shape, critical for extracellular proteins.
The Disulfide Bond: Formation
Where Do Disulfide Bonds Form?
Protein Disulfide Isomerase: The Matchmaker
Protein Disulfide Isomerase (PDI) is the key enzyme in the ER that catalyzes the formation, breakage, and rearrangement of disulfide bonds. It ensures the correct cysteine pairs are linked.
1
Oxidase activity: PDI donates its own disulfide bond to the substrate, forming the \(\text{S–S}\) bond in the target protein.
2
Isomerase activity: If incorrect pairs form, PDI breaks them and re-shuffles until the thermodynamically stable arrangement is found.
3
Regeneration: Ero1 re-oxidizes PDI using \(\text{O}_2\) as the final electron acceptor, allowing the catalytic cycle to continue.
Disulfide vs Hydrogen Bonds in Proteins
| Disulfide Bond | Hydrogen Bond | |
|---|---|---|
| Bond type | Covalent | Non-covalent |
| Strength | \(\sim 250 \text{ kJ/mol}\) | \(\sim 2\text{-}40 \text{ kJ/mol}\) |
| Residues involved | Cys-Cys only | Many backbone and side-chain groups |
| Location | Extracellular/ER proteins | Everywhere (alpha-helices, beta-sheets) |
| Reversibility | Requires reducing agent | Breaks easily with heat |
| Structure level | Tertiary/Quaternary | Secondary/Tertiary |
Phosphorylation: The Master Switch
Glycosylation: Sugar Coating Proteins
N-linked
Sugar attached to the nitrogen of asparagine (Asn) in the sequence Asn-X-Ser/Thr. Begins in the ER, completed in Golgi.
O-linked
Sugar attached to the oxygen of serine or threonine. Occurs mainly in the Golgi apparatus. Simpler chains than N-linked.
How to Identify PTM Type: A Checklist
1
Check the amino acid: Is it Cys (disulfide), Ser/Thr/Tyr (phosphorylation), Asn (N-glycosylation), or Lys (ubiquitination/acetylation)?
2
Check the cellular location: ER/extracellular suggests disulfide bonds or glycosylation. Cytoplasm suggests phosphorylation or ubiquitination.
3
Check reversibility: Is the modification regulated by an opposing enzyme (kinase/phosphatase)? If so, it is likely a signaling switch.
4
Check the function: Structural stability = disulfide bond. Signaling = phosphorylation. Degradation = ubiquitination. Trafficking = glycosylation.
Example 1: Insulin and Disulfide Bonds
Example 2: Antibodies (IgG Structure)
Example 3: Phosphorylation Cascade
Common Mistakes
✕ Disulfide = Ionic
Disulfide bonds are covalent, not ionic. The \(\text{S–S}\) bond involves electron sharing between two sulfur atoms.
✕ Cytoplasm has \(\text{S–S}\) bonds
The reducing cytoplasm prevents disulfide formation. \(\text{S–S}\) bonds form in the ER lumen or extracellularly.
✕ Phosphorylation always activates
Phosphorylation can activate or inhibit a protein, depending on the specific site and conformational change.
PTM Summary Table
| PTM | What Is Added | Key Residue(s) | Primary Function |
|---|---|---|---|
| Disulfide Bond | \(\text{S–S}\) covalent link | Cys | Structural stability |
| Phosphorylation | \(\text{PO}_4^{3-}\) | Ser, Thr, Tyr | Signal transduction |
| Glycosylation | Sugar chains | Asn (N), Ser/Thr (O) | Folding, recognition |
| Ubiquitination | Ubiquitin protein | Lys | Protein degradation |
| Acetylation | Acetyl group | Lys (histones) | Gene regulation |
Question 1 of 5
Disulfide bonds form between the side chains of which amino acid?
Cysteine is the only amino acid with a thiol (\(\text{-SH}\)) group that can be oxidized to form a covalent \(\text{S–S}\) bond. Methionine also contains sulfur, but its sulfur is in a thioether linkage and cannot form disulfide bonds.
Question 2 of 5
Why are disulfide bonds rarely found in cytoplasmic proteins?
The cytoplasm contains glutathione and thioredoxin systems that maintain a reducing environment, actively breaking any \(\text{S–S}\) bonds that form. Disulfide bonds form in the oxidizing ER lumen instead.
Question 3 of 5
Which enzyme catalyzes the addition of a phosphate group to a protein during phosphorylation?
Kinases transfer a phosphate group from ATP to a substrate protein. Phosphatases do the reverse: they remove the phosphate group. PDI handles disulfide bonds, and glycosyltransferases attach sugars.
Question 4 of 5
Insulin has two polypeptide chains (A and B) held together by disulfide bonds. If all disulfide bonds were reduced with a reagent like DTT, what would happen?
Disulfide bonds are the primary force holding insulin's two chains together. Reducing them with DTT (dithiothreitol) breaks the \(\text{S–S}\) bonds back to free \(\text{-SH}\) groups, causing the chains to dissociate and the protein to lose its functional 3D structure.
Question 5 of 5
A researcher observes that a secreted protein has sugars attached to asparagine residues in an Asn-X-Ser motif. Which PTM is this?
N-linked glycosylation attaches sugars to the nitrogen of asparagine (Asn) in the consensus sequence Asn-X-Ser/Thr (where X is any amino acid except proline). O-linked glycosylation attaches to Ser/Thr directly, without a consensus motif.