Protein Processing

Question 1

Translation Initiation

Answer 1

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

Question 2

Translation Elongation

Answer 2

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)

Question 3

Translation Termination

Answer 3

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

Question 4

Sickle Cell Anemia

Answer 4

1. Arises from a missense mutation of 6th codon in the allele of the gene for human B-globin, a subunit of adult hemoglobin
2. Changes glutamic acid (hydrophilic) to valine (hydrophobic)
3. Conformation of HbA which causes it to aggregate a rigid, rod-like structures
4. Have poor oxygen capacity and tend to clog capillaries

Question 5

ii. Duchenne muscular dystrophy

Answer 5

1. Frameshift mutation to the dystrophin gene leads to partially or non-functioning dystrophin protein
2. 1/3,500 males
3. Leads to muscle wasting, confinement to wheel chair
4. In-frame deletions result in expression of truncated forms of dystrophin giving rise to a milder form of the disease called Becker muscular dystrophy

Question 6

Cytoplasmic pathway

Answer 6

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

Question 7

Secretory Pathway

Answer 7

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

Question 8

role of chaperones

Answer 8

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

Question 9

b. Proteolytic cleavage

Answer 9

i. Converts inactive forms to active enzymes by unmasking active site
ii. Converts nascent precursor proteins to mature ones( proinsulin to insulin)

Question 10

1. O-glycosidic (gylcosylation)

Answer 10

a. Formed with the hydroxyl groups of Ser or Thr residues

Question 11

2. N-glycosidic (glycosylation)

Answer 11

a. Always with Asparagine

b. Precursor sugar transferred from phosphor Dolichol

Question 12

Phosphorylation

Answer 12

1. Formation of ester bond between phosphate and OH
2. Uses activity of serine/threonine and tyrosine kinase
3. Phosphate removed by phosphatases
4. Regulates enzyme activity and protein function, cell growth, proliferation, differentiation, oncogenesis

Question 13

Disulfide Bond formation

Answer 13

1. Inter and intra molecular disulfide bonds stabilize many proteins
2. Form between thiol (SH) group of 2 cysteine residues
3. Occurs in ER lumen
4. Facilitated by protein disulfide isomerases

Question 14

Acetylaction

Answer 14

1. Lysine residues
2. Acetyl CoA as the donor group
3. Acetylated/deactylated on their N-terminal lysines
4. Reactions catalyzed by HAT or HDAC enzymes

Question 15

a. Alzheimer’s disease (β-amyloid)

Answer 15

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

Question 16

a. Creutzfeldt-Jakob/Kuru/Mad Cow disease (prion proteins)

Answer 16

o Misfolding of prion proteins
o Transmissible
o Converts normal proteins to misfolded form
o Transmissible spongiform encephalopathies
o Spongiform-appearance of infected brains, filled with holes and resemble sponges under a microscope

Question 17

a. Huntington disease (poly-glutamine repeats)

Answer 17

o Mutation in huntingtin gene results in CAG triplet repeats
o Polyglutamine repeats
o Selective death of cells in basal ganglia cause the symptoms

Question 18

a. Parkinson’s disease (α-synuclein)

Answer 18

o Aggregation of alpha-synuclein protein forms insoluble fibrils which deposit as Lewy bodies in dopaminergic neurons in sustantia nigra
o Reduced availability of dopamine
o Mutations cause familial forms
o Brain aging is common denominator for sporadic form
o Loses sense of smell years before