Functions and Dysfunctions of Protein Processing Flashcards
Types of Mutations (4)
- Silent: does not change the amino acid.
- Missense: changes amino acid within the protein.
- Nonsense: changes codon into stop codon.
- Frameshift: changes the amino acid sequence.
Duchenne Muscular Dystrophy
Frameshift mutation leading to dystrophin gene, resulting in little/no expression of dystrophin protein.
OOF mutations lead to severe DMD. In frame deletions result in more mild forms.
Leads to muscle wasting. Patients usually die from respiratory failure.
Eukaryotic mRNA contains (3)
- Codons.
- 7-methylguanosine cap at 5’ end.
- Poly A tail at 3’ end.
Aminoacyl tRNAs
Complex of tRNA with amino acid. AA is esterified at the 3’-end of cognate tRNA.
Aminoacyl tRNA synthetases
Catalyzes activation of AA with tRNA.
Ribosomal complex sites
A: acceptor site. mRNA accepts the tRNA.
P: peptidyl site. Aminoacyl tRNA is attached.
E: empty site. Occupied by empty tRNA before exiting ribosome.
Sickle-Cell Anemia
Missense mutation that substitutes Val for Glu. This alters conformation of HbA that deforms the RBCs. These RBCs have poor oxygen capacity and will clog capillaries.
Translation steps (3)
Occurs in 5’ —> 3’ direction.
- Initiation: formation of mRNA, small ribosomal subunit and initiator tRNA.
- Elongation: activated AA attached to initiating Met via peptide bod.
- Termination: peptide chain is released from ribosome.
Polysomes
Clusters of ribosomes that simultaneously translate a single mRNA, each synthesizing a polypeptide.
Prokaryotic Elongation Inhibitors (5)
- Tetracycline
- Chloramphenicol
- Clindamycin
- Erythromycin
- Streptomycin
Tetracycline
Binds to the small subunit blocking entry of aminoacyl-tRNA to ribsomal complex.
Chloramphenicol
Inhibits peptidyl transferase.
Clindamycin and Erythromycin
Binds to large subunit which blocks the translocation of the ribosome.
Streptomycin
Binds to small subunit and interferes with the binding of fmet-tRNA. Interferes with the small and large unit association.
Eukaryotic Elongation Inhibitors (4)
- Cycloheximide
- Diphtheria toxin
- Shiga toxin
- Ricin
Cyclohexamide
Inhibits peptidyl transferase.
Diphtheria toxin
Inactivates GTP-bound eEF-2, interfering with ribosomal translocation.
Shiga toxin and Ricin
Binds to large subuit, blocking the entry of aminoacyl-tRNA to ribosomal complex.
Euk/Prok Elongation Inhibitor
Puromycin. Causes premature chain termination. Resembles the 3’ end of aminoacylated-tRNA. Enters the A site and is added to the growing chain, casing a premature chain release.
Cytoplasmic pathway for protein sorting
For proteins destined for cytocol, mitochondria, nucleus and peroxisomes.
Protein synthesis is solely done on a free ribosome in the cytoplasm.
Secretory pathway for protein sorting
For proteins destined for ER, lysosomes, plasma membranes or secretion.
Translation begins on free ribosomes but terminates on ribosomes sent to ER. First 20 AAs of of the polypeptide have ER targeting signal sequences.
TOM and TIM
TOM - transporter outer membrane of mito.
TIM - transporter inner membrane of mito.
Heat shock proteins 70 (HSP70)
Chaperone protein that protects unfolded proteins.
Mann 6-P signal
To the lysosome (SP).
Trp-rich domain signal
To a secretory vesicle (SP).
N terminal hydrophobic a-helix signal
To the mitochondria (CP).
K and R rich signal
To the nucleus (CP).
SKL signal
To peroxisome (CP).
KDEL signal
To the ER lumen (SP).
N terminal nonpolar region signal
To the cell membrane - not for vesicular secretion (SP).
Two properties of secretory pathway proteins
- 1-2 basic AAs near the N terminus (R, K).
2. Extremely hydrophobic sequence (10-15 residues) on the C terminus side of the basic AAs.
Signal recognition particle (SRP)
Binds ot the ER-targeted signal and the ribosome during translation. SRP wraps around ribosome-mRNA-peptide complex halting translation temporarily (SP).
I-cell disease
Lysosomal storage disease. Mann 6-P tagging is defective. Failure to thrive, developmental delays, abnormal skeleton, hepatomegaly, recurrent RTIs, death by 7.
Protein folding
Small proteins can fold spontaneously, but large proteins require chaperones.
Chaperonins
Catalyze folding of proteins using ATP.
Proteolytic cleavage
Converts inactive forms of enzymes to active forms (e.g. proisulin to insulin).
Covalent modifications (4)
- Glycosylation
- Phosphorylation
- Disulfide bond formation
- Acetylation
Glycosylation
Extracellular proteins are glycosylated (glycoproteins) via O-linked or N-linked.
O-linked glycolsylation
Formed with OH- groups of S, T.
N-linked glycosylation
Formed with N always.
Phosphorylation
Turns on/off proteins. Phosphatases/kinases add or subtract phosphates.
Disulfide bonds
Can be inter or intra molecular to stabilize proteins via a thiol groups of 2 Cys. Occurs in ER lumen and is facilitated by protein disulfide isomerases.
Acetylation
Typically occurs on K residues and use ACoA as acetyl group donor. Histones commonly acetylated and catalyzed by HAT or HDAC enzymes. Histone acetylation patterns are inheritable.
Post-translational modifications of collagen
Done via lysyl and prolyl hydroxylases, which requires Vit C. Defects result in skin, bone, joint disorders.
Alzheimer’s Disease
APP breaks down to form amyloid beta peptide (AB). Misfolding of AB forms plaques in brain (extracellular) and hyperphosphorylation of Tau (intracellular).
Parkinson’s Disease
Aggregation of a-synclein (AS) protein deposit as Lewy bodies in dopaminergic neurons of the substantia negra, causing death of these neurons.
Huntington’s Disease
Mutation of Huntington gene causing expansion of CAG triplets repeats which results in polyglutamine repeat. Selective death of cells in basal ganglia causes the symptoms.
Creutzfeldt-Jakob Disease
Caused by misfolding of prions. It is transmissable, as it converts normally folded proteins to misfolded ones. Belongs to TSEs (transmissable spongiform ecephalopathies).
