Lecture 14 Post-translational Modification Flashcards
What is proteolytic cleavage?
Breaking peptide bonds to remove part of the protein.
What is chemical modification?
Adding functional groups to amino acid residues.
What is required for protein sorting?
An intrinsic signal from protein.
A receptor that recognises the protein & directs it to the correct membrane.
A translocation machinery (to move protein).
Energy to move the protein.
What is constitutive secretion?
Secretory product is packaged into small vesicles and continuously released to the cells surface. Proteins are not modified by the Golgi.
What is regulated secretion?
Secretory granules accumulate in large vesicles. Proteins are modified by Golgi and are released by exocytosis upon stimulation - requires Ca2+ ions.
What is a signal sequence?
Found in the N-terminal of the amino acid sequence.
Has a central region rich in hydrophobic residues.
Is able to form an alpha-helix.
What is a signal recognition particle?
Composed of proteins and RNA. It recognises the signal peptide/sequence and the ribosome.
What are the key functions of the endoplasmic reticulum?
Glycosylation - adding a carbohydrate residue onto proteins.
Formation of disulfide bonds - holds structures together, increases resistance to degradation.
Proper folding of proteins.
What is N-linked glycosylation? Why is it important?
Sugars added onto a nitrogen atom of an amino group.
It allows for:
Correct protein folding (if incorrect folding, incorrect function).
Protein stability.
Interaction with other molecules.
What is the role of protein disulphide isomerase(PDI)?
Ensures the protein adopts the right conformation for the correct disulphide bonds to form
What happens if the mis-folding of proteins is not corrected?
Protein can be returned to cytosol for degradation or may accumulate to toxic levels in the ER and cause disease.
Describe the structure of collagen.
Basic unit is tropocollagen.
Rod-shaped protein.
Made up of 3 polypeptide alpha chains forming a right-handed triple helix.
Every 3rd amino acid is glycine (glycine is the only aa with a side chain small enough to fit in the middle of helix.
H-bonds between the alpha chains stabilise the structure.
Collagen is non-extensible, non-compressible and has high tensile strength.
How is collagen synthesised?
- Preprocollagen alpha chains are synthesized and enter the lumen of the ER. The signal peptide is cleaved so they become procollagen alpha chains.
- Undergo modification - hydroxylation of proline and lysine residues by prolyl hydroxylase and N-linked glycosylation occurs.
- Towards the C-terminus chains align and disulfide bonds form, forming a triple helix.
- Transported to the Golgi where more glycosylation occurs. Procollagen is packaged and exocytosed in transport vesicles.
- Procollagen peptidases remove the N and C-terminals to give mature tropocollagen.
- Lateral association of tropocollagen molecules followed by covalent cross-linking to form collagen fibrils.
- Aggregation of fibrils to form collagen fibers.
Why is tropocollagen secreted before final processing occurs?
Assembly of the collagen fibres would cause the cell to explode due to their size.
What is prolyl hydroxylase?
Enzyme that adds hydroxyl groups to proline and lysine residues.
Associated with PDI and requires vitamin C and Fe2+ ions.
It allows increased H bonding to stabilise the triple helix. Without it, tropocollagen triple helices are weak = scurvy.
What is Ehlers-Danlos syndrome(EDS)?
Mutation in collagen type V causing weak connective tissue and very flexible skin.
