Functions and Dysfunctions of Protein Processing (3)

Question 1

Genetic Code

Answer 1

• nucleotide sequence of gene into aa sequnece of protein using mRNA
• start: AUG
• stop: UAG, UGA, UAA

Question 2

Sickle Cell Anemia

Answer 2

• arises from missense mutation of 6th codon in allele of gene for human B-globin (HBB), subunit of adult hemoglobin
• mutation changes GAG–> GTG (substitutes Val which is hydrophobic for Glu which is negatively charged and hydrophilic)
• alters conformation of HbA –> causes it to aggregate and form rigid, rod-like structures
• deforms RBC into sickle-like shape; deformed = poor oxygen capacity and tend to clog capillaries, further restricting blood supply to tissues

Question 3

Duchenne Muscular Dystrophy (DMD)

Answer 3

• large in-frame and out-of frame deletions to the dystrophin gene leads to partially or nonfunctioning dystrophin protein
• OOF deletion result in little/no expression of dystrophin protein, giving rise to severe form of DMD
• 1:3500 males
• leads to muscle wasting (wheel chair confinement)

Question 4

Activation of Amino Acids

Answer 4

Two steps:
-Aminoacyl tRNA synthetase catalyzes addition of AMP to COOH end of AA
-AA is transferred to cognate tRNA
NET RESULT: amino acid is selected by its codon

Question 5

RIbosomal Subunits

Answer 5

PROKARYOTES: 70S: 50S large and 30S small
EUKARYOTES: 80S: 60S large and 40S small

Question 6

Polysome

Answer 6

clusters of ribosomes simultaneously translating a single mRNA molecule

• each synthesizing a polypeptide
• makes protein synthesis more efficient

Question 7

Streptomycin

Answer 7

binds to the 30S subunit to disrupt the initiation of translation; interferes with 30S subunit association with 50S su

Question 8

Shiga Toxin

Answer 8

binds to the 60S subunit to disrupt elongation

Question 9

Clindamycin and erythromycin

Answer 9

bind to large 50S subunit (prok), blocking translation of ribosome

-erythromycin (commonly used to treat pertussis)

Question 10

Tetracyclines

Answer 10

bind to 30S subunit, blocking entry of aminoacyl-tRNA to ribosomal complex (disrupt elongation)

Question 11

Chloramphenicol

Answer 11

inhibits peptide transferase (prokaryote/mitoch.)

Question 12

Puromycin

Answer 12

causes premature chain termination

• resembles the 3’ end of the aminoacylated- tRNA
• enters A site and adds to the growing chain
• forms a puromycylated chain, leads to premature chain release
• exact mechanism is unknown
• more resistant to hydrolysis
• stops the ribosome

Question 13

Cytoplasmic Pathway

Answer 13

for proteins destined for cytosol, mitochondria, nucleus, and peroxisomes

-protein synthesis begins and ends one see ribosomes in cytoplasm

no signal= stays in cytoplasm

N terminal hydrophobic alpha helix= mitochondrion

Lys Arg rich= nucleus

C-terminal SKL= peroxisome

Question 14

Secretory Pathway

Answer 14

for proteins destined for ER, lysosomes, plasma membranes, or for secretion

• translation begins on free ribosomes but terminates on ribosomes sent to ER
• first 20 aa residues of polypeptide has ER targeting signal sequences

NH3 coiled= ER signal

Lys Asp Glu Leu (c-terminal KDEL)= ribosomes/ER lumen

Trp-Rich domain= secretory vesicle

Mannose 6-phosphate= lysosome

N-terminal apolar region (stop transfer sequence)= membrane

Question 15

I-cell disease

Answer 15

severe form of lysosomal storage disease

• tagging of lysosomal proteins with mannose 6P is defective (high plasma levels of lysosomal enzymes)
• developmental delays and physical manifestations
• hepatomegaly; developmental delays; death by 7

Question 16

Chaperones

Answer 16

protect protein and help fold into property tertiary structure

-example: HSP70

Question 17

chaperonins

Answer 17

barrel shaped compartments that admit unfolded proteins and catalyze their folding in an ATP-dependent manner

-Example:HSP60

Question 18

proteolytic cleaveage

Answer 18

converts inactive forms to active enzymes by unmasking alive site (e.g. trypsinogen and chymotripsinogen to trypsin and chymotrypsin)

converts nascent precursor proteins to mature ones (ex. proinsulin to insulin)

Question 19

Glycosylation

Answer 19

O-glycosylation= -OH bound -residue affected= Ser, Thr

N-glycosylation= acid-amide (-CONH2)
-residue affected= Asn, Gln

Question 20

Phosphorylation

Answer 20

phosphate linked via esterification (-OH functional group)

-residue affected= Ser, Tyr, The; also Asp and His

Question 21

Disulfide Bond formation

Answer 21

oxidation to achieve covalent linkage of cysteine residues (-SH groups)
-residue affected= Cys

Question 22

Acetylation

Answer 22

covalent linkage to amine (-NH3+)

-residue affected= Lysine

Question 23

Post-translational modifications of collagen

Answer 23

most abundant structural protein in vertebrates which undergoes a lot of modifications after synthesis

• modifications important for assembly of collagen
• Ehlers-Danlos syndrome= overly flexible joints, walls of blodo vessels, intesetines or uterus may rupture
• epidermolysis Bulls Simplex= blisters on skin
• Nevo Syndrome and Bruck Syndrome

Question 24

Alzheimers Disease

Answer 24

• loss of memory, cognitive function, language
• APP–> AB(aggregation)–> plaques in brain
• hyperphosphorylation of Tau (neurofibrillary tangles)- intracellular
• mutations in APP and Tau cause familial forms of AD
• Brain aging is the common denominator for Sporadic form

Question 25

Parkinsons Disease

Answer 25

• impairment of fine motor control
• problem with gait and shuffling
• aggregation of alpha-synuclein (AS) protein forms insoluble fibrils which deposit as lewy bodies in dopaminergic neurons in substantial nigra
• results in selective death of these neurons
• symptoms due to reduced availability of dopamine
• mutations in AS cause familial forms of PD
• brain aging is the common denominator for sporadic form

Question 26

Huntington Disease

Answer 26

• loss of movement and cognitive functions and psychiatric problems
• mutation in Huntington gene–> CAG triplet repeats
• results in polyglutamine repeats in abnormal HTT protein. forms intramolecular H-bonds, which eventually misfold and aggregate
• selective death of cells in basal ganglia cause the symptoms
• 10-26 normal; 36-121=HD gene

Question 27

Creutzfeldt-Jakob/Kuru/Mad Cow disease

Answer 27

• failing memory, behavioral changes, lack of coordination and visual disturbances
• late stages involve mental deterioration blindness, weakness of extremities, and coma
• memory, visual disturbances
• caused by misfiling of prion proteins
• transmissible- infection by misfiled proteins converts normal proteins to misfiled form
• belongs to transmissible spongiform encephalopathies (TSEs)
• Spongiform- appearance of infected brains, filled with holes and resemble sponges under a microscope