Protein Processing Flashcards
Translation Initiation
a. Assembly of preinitiation complex: small subunit , initiating tRNA, mRNA
i. Inhibited by: Streptomycin (binds to the 30S subunit/prokaryotic)
b. Hydrolyze GTP, add large subunit and other initiation factors to form complete initiation complex
Translation Elongation
a. Hydrolyze GTP, load second aminoacyl tRNA onto A site
i. Inhibited by: Tetracycline (binds to the 30S subunit), Shiga toxin (binds to 60S subunit), Ricin, Puromycin (premature chain termination)
b. Form peptide bond w/ peptidyl transferase
i. Inhibited by: Chloramphenicol (inhibits peptidyl transferase activity/Prokaryotic) and Cycloheximide (inhibits peptidyl transferase/Eukaryotic)
c. Hydrolyze GTP, translocate ribosome, third aminoacyl tRNA enters at A site
i. Inhibited by: Diptheria toxin (inactivates EF2-GTP/eukaryotic)
Translation Termination
a. After UAG stop codon, load release factor onto A site
b. Hydrolyze GTP, ester bond between peptide and tRNA w/peptidyl transferase, complex dissociates
Sickle Cell Anemia
- Arises from a missense mutation of 6th codon in the allele of the gene for human B-globin, a subunit of adult hemoglobin
- Changes glutamic acid (hydrophilic) to valine (hydrophobic)
- Conformation of HbA which causes it to aggregate a rigid, rod-like structures
- Have poor oxygen capacity and tend to clog capillaries
ii. Duchenne muscular dystrophy
- Frameshift mutation to the dystrophin gene leads to partially or non-functioning dystrophin protein
- 1/3,500 males
- Leads to muscle wasting, confinement to wheel chair
- In-frame deletions result in expression of truncated forms of dystrophin giving rise to a milder form of the disease called Becker muscular dystrophy
Cytoplasmic pathway
o Proteins destined for cytosol, mitochondria, nucleus, and peroxisomes
o Protein synthesis begins and ends on free ribosomes in cytoplasm
Mitochondrial signals and import into mitochondria (Fig. 19.7)
o N terminal hydrophobic
alpha-helix
Nuclear localization signals
Lys, Arg rich
Peroxisomes
o Ser, Lys, Leu
Secretory Pathway
o For proteins destined for ER, lysosomes, plasma membranes, or for secretion
o Translation begins on free ribosomes but terminates on ribosomes sent to ER
o Signal recognition particle (SRP) bonds to the ER-targeting signal and the ribosome during translation
Wraps itself around ribosome-mRNA-peptide complex, tethering it to ER membrane and halting translation temporarily
Translation on the ER (Fig. 19.8) o Basic followed by hydrophobic Amino Acids o Lys Asp Glu Leu Mann 6-P for Lysosome N terminal apolar for cell membrane Trp-rich domain for secretory vesicle
role of chaperones
Nuclear-coded mitochondrial protein is protected by binding to chaperones (heat shock proteins 70)
ii. Recognized by TIM and TOM
iii. Chaperonins have barrel shaped compartments that admit unfolded proteins and catalyze their folding in an ATP-dependent manner
b. Proteolytic cleavage
i. Converts inactive forms to active enzymes by unmasking active site
ii. Converts nascent precursor proteins to mature ones( proinsulin to insulin)
- O-glycosidic (gylcosylation)
a. Formed with the hydroxyl groups of Ser or Thr residues
- N-glycosidic (glycosylation)
a. Always with Asparagine
b. Precursor sugar transferred from phosphor Dolichol
Phosphorylation
- Formation of ester bond between phosphate and OH
- Uses activity of serine/threonine and tyrosine kinase
- Phosphate removed by phosphatases
- Regulates enzyme activity and protein function, cell growth, proliferation, differentiation, oncogenesis
Disulfide Bond formation
- Inter and intra molecular disulfide bonds stabilize many proteins
- Form between thiol (SH) group of 2 cysteine residues
- Occurs in ER lumen
- Facilitated by protein disulfide isomerases
Acetylaction
- Lysine residues
- Acetyl CoA as the donor group
- Acetylated/deactylated on their N-terminal lysines
- Reactions catalyzed by HAT or HDAC enzymes
a. Alzheimer’s disease (β-amyloid)
o Amyloid precursor protein breaks down to form amyloid beta peptide (AB)
o Misfolding/aggregation of AB forms plaques in brain
o Hyperphosphorylation of Tau
o Mutations in APP and Tau cause familial forms of AD
o Brain aging is common denominator for sporadic form