Protein Synthesis and Processing

Question 1

Steps in Synthesis of Protein– Initiation (common to all organisms)

Answer 1

• Initiation complex including mRNA and initiator tRNA(met) is assembled on small subunit of ribosome, then large subunit binds
• All proteins start with methionine (which can be removed later)
• In bacteria, formyl-methionine (fMet) is used for initiation

Question 2

Steps in Synthesis of Protein-Elongation (common to all organisms)

Answer 2

• Aminoacyl tRNA recognizes a code and delivers an amino acid
• Peptide bond is formed
• The growing peptide is translocated by three nucleotides
• Above 3 steps are repeated as ribosome moves along mRNA

Question 3

Steps in Synthesis of Protein- Termination (common to all organisms)

Answer 3

• Release factors recognize termination codons

- Peptide chain is released from ribosome

Question 4

Initiation in Prokaryotes

Answer 4

-fMet used
-Initiation factors: IF1, IF3 (keeps ribosome subunits apart)
IF2–binds aminoacyl-fMet-tRNA and GTP

Question 5

Elongation factors in E.coli

Answer 5

• Eu-Tu
• EF-Ts
• EF-G

Question 6

EF-Tu

Answer 6

binds GTP and aminoacyl tRNA (the GTP is converted to GDP)

Question 7

EF-Ts

Answer 7

exchanges the GDP on Tu for GTP

Question 8

EF-G

Answer 8

mediates translocation step

Question 9

RFs

Answer 9

Release factors. Recognize termination or STOP codons

Question 10

Release factors in E. coli

Answer 10

RF1, RF2, RF3

Question 11

RF1

Answer 11

recognizes UAA, UAG

Question 12

RF2

Answer 12

recognizes UAA, UGA

Question 13

RF2

Answer 13

GTPase, helps bind RF1 or RF2 to ribosome

Question 14

Initiation factors-Humans

Answer 14

eIF-2, eIF-3, eIF4E, eIF-4F

Question 15

eIF-2

Answer 15

binds Met-tRNA, GTP

Question 16

eIF-3

Answer 16

ribosome dissociation

Question 17

eiF-4E

Answer 17

binds mRNA cap structure

Question 18

eIF-4F

Answer 18

mRNA binding and helicase unwinding activity

Question 19

eEF1-alpha

Answer 19

analogous to EF-Tu (aa-tRNA binding)

Question 20

eEF1betagamma

Answer 20

analogous to EF-Ts (GTP exchange)

Question 21

eEF-2

Answer 21

analogous to EF-G - translocation

Question 22

Post-translational modifications

Answer 22

• Removal of signal sequence
• Disulfide bond formation
• Glycosylation
• Amino acid side-chain modification
• Proteolytic cleavage
• —Insulin, glucagon, and other polypeptide hormones

Question 23

Dolichol phosphate

Answer 23

Lipid carrier molecule, transfers high mannose blocks of sugar