Protein Synthesis and Processing Flashcards
Steps in Synthesis of Protein– Initiation (common to all organisms)
- 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
Steps in Synthesis of Protein-Elongation (common to all organisms)
- 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
Steps in Synthesis of Protein- Termination (common to all organisms)
- Release factors recognize termination codons
- Peptide chain is released from ribosome
Initiation in Prokaryotes
-fMet used
-Initiation factors: IF1, IF3 (keeps ribosome subunits apart)
IF2–binds aminoacyl-fMet-tRNA and GTP
Elongation factors in E.coli
- Eu-Tu
- EF-Ts
- EF-G
EF-Tu
binds GTP and aminoacyl tRNA (the GTP is converted to GDP)
EF-Ts
exchanges the GDP on Tu for GTP
EF-G
mediates translocation step
RFs
Release factors. Recognize termination or STOP codons
Release factors in E. coli
RF1, RF2, RF3
RF1
recognizes UAA, UAG
RF2
recognizes UAA, UGA
RF2
GTPase, helps bind RF1 or RF2 to ribosome
Initiation factors-Humans
eIF-2, eIF-3, eIF4E, eIF-4F
eIF-2
binds Met-tRNA, GTP
eIF-3
ribosome dissociation
eiF-4E
binds mRNA cap structure
eIF-4F
mRNA binding and helicase unwinding activity
eEF1-alpha
analogous to EF-Tu (aa-tRNA binding)
eEF1betagamma
analogous to EF-Ts (GTP exchange)
eEF-2
analogous to EF-G - translocation
Post-translational modifications
- Removal of signal sequence
- Disulfide bond formation
- Glycosylation
- Amino acid side-chain modification
- Proteolytic cleavage
- —Insulin, glucagon, and other polypeptide hormones
Dolichol phosphate
Lipid carrier molecule, transfers high mannose blocks of sugar
