LEC 7: Post-Translational Modifications Flashcards
Post Translational Modifications
– Protein folding – Proteolytic Cleavage of Proteins – Acylation – Glycosylation – Methylation – Phosphorylation – Ubiquitin and Targeted Protein Degradation
Purposes of Post Translational Modifications
- Stability/Protection
- Function
- Activity
Regulation of Gene Expression
- Folding of the polypeptide into the correct 3D structure.
- Transport to the correct location
- Post translational processing. e.g., phosphorylation, glycosylation
- Association with other protein co-factors
Protein Folding
Process by which a protein structure assumes its functional shape or conformation
- AA interact to produce well defined 3D structure (known as a Native State)
Folding and AA Side Chains
Polypeptide chains will spontaneously fold so that hydrophobic amino acids are on the inside of the structure
- Spontaneous folding leads to a variety of structures, most of which are inactive
Forms of AA Side Chain Folds
- Hydrophobic bond
- Ionic bond
- Disulfide bond
- Hydrogen bond
- Hydrophobic cluster
Folding directed by Chaperones
Chaperones help other proteins fold correctly
- Prevent protein-protein interactions during the folding process
- Prevents formation of insoluble aggregates
Unfolded Protein Response
- When unfolded proteins accumulate in the ER, BIP is released. This initiates signal transduction pathways leading to induction of gene expression in the nucleus.
- In an attempt to alleviate the accumulation of unfolded proteins, the synthesis of chaperones in increased.
- If unfolded proteins continue to accumulate APOPTOSIS is induced.
Proteolytic Cleavage of Proteins
- Breaking the peptide bonds between amino acids in proteins
- Carried out by enzymes called peptidases, proteases or proteolytic cleavage enzymes.
- Inactive precursor proteins that are activated by removal of polypeptides are termed pro-proteins
Pro-opiomelanocortin (POMC)
Pro-opiomelanocortin (POMC) is a precursor polypeptide
- The processing of POMC involves glycosylations, acetylations, and extensive proteolytic cleavage
- Synthesised by corticotroph and melanotroph cells of the pituitary gland in response to specific feedback signals
- The products of this cleavage are 2 main hormones critical in ensuring regulation of the body and endocrine system (ACTH and MSH)
Glycosylation
The addition of polysaccharide side chains to proteins (such proteins are called GLYCOPROTEINS)
- Protects protein from degradation, more protein more susceptible to being accumulated
- For a given protein the pattern of glycosylation differs for different species/groups and from tissue to tissue
Structures of Oligosaccharides
Monosaccharides can join together by different types of bonds to form complex branched structures
- 1-2
- 1-3
- 1-4
N-Linked and O-Linked Glycans
N-linked glycans: Covalently attached to Asn residues within a consensus sequence (Asn-Xaa-Ser/Thr)
- Enabling prediction of the modification sites by protein sequence analysis
O-Linked Glycnas
Biological Significance of Glycans
- Oligosaccharides may be a tissue-specific marker
- Carbohydrates may alter the polarity and solubility
- The bulkiness and negative charge of oligosaccharide chain may protect protein from the attack by proteolytic enzymes
Notch Signalling Pathways
Different types of glycan side chains are found attached to the EGF repeats
- Receptors lacking the O-glycans or the N-glycans are functional, however lack of the O-fucose glycan destroys activity
