Translation Flashcards
What are the stop sequences?
UAA, UAG, UGA
What is the start codon?
AUG
How many combinations of bases are there?
64
What orientation does translation occur in?
- Ribosomes scan from 7MeG cap of 5’ end of mRNA - added to the 3’ end
- Translation starts at Met and continues in frame
- Protein synthesis from N-terminus to C terminus in cytoplasm and rER
- Stops at 1st in frame of stop codon
What is the function of aminoacyl tRNA synthetases?
- (1 per amino acid) - hooks amino acid to tRNA,
- makes sure correct amino acid bound to tRNA
- Mutated in cancers, neuropathies, autoimmunity and metabolic disease
What is the aminoacyl tRNA synthetases mechanism?
- Using ATP energy the enzyme forms an adenylated amino acid (intermediate compound)
- Has enzyme bound to AMP
- AMP bound to correct amino acid
- Adenylated AA binds to appropriate tRNA - AMP comes off and enzyme dissociates
- AA transferred to 3’ tRNA end - ready to go ribosomes and perform function
Adenlyate–>bind–>translation at 3’
What are the steps of translation?
A) initiation:
- dissociation of ribosome subunits (40S + 60S)
- assembly of preinitiation complex containing Met-tRNA + Initiation Factors + 40S subunit
- binding of mRNA to preinitiation complex
- binding of 60S subunitB) Elongation
- binding of new tRNA to A site
- catalysis of peptide bond between two amino acids by peptidyl transferase
- translocation of tRNA to P site and dissociation of first tRNAC) Termination
- recognition of stop codon
- release of peptide chain
- dissociation of release factors and ribosomes
What is the role of polyribosomes in translation?
- translating mRNA not carried out by ribosome at time
- several polyribosomes on mRNA at same time and move protein production in same time
Why do antibiotics target protein synthesis?
- Translational machinery complex and easily disrupted - common target for antibiotics
- Exploit eukaryote and prokaryote differences
- Naturally produced by bacteria and fungi giving them selective advantages
Explain how antibiotics inhibit translation?
- Streptomycin: inhibits initiation
- Chloramphenicol: inhibits peptidyl transferase
- Tetracycline inhibits AA-tRNA binding
- Puromycin: inhibits elongation prematurely
- Erythromycin: inhibits translocation
What is the signal sequence?
- special sequence in 1’st 20-24AA present at N terminus
- rich in hydrophobic AA which like to sit in lipid bilayer
How do proteins enter the secretory pathway?
- Signal sequence recognized by protein-RNA complex: signal recognition particle (SRP) binds to sequence and halts translation
- SRP bind to SRP receptor on rER membrane
- Translation resumes
- Translocation of growing polypeptide into rER lumen
- bound SRP triggers protein channel assembly in rER membrane - the growing polypeptide chain is threaded through the channel across the membrane and into rER lumen
- If the protein should be transmembrane it will have an extra hydrophobic sequence anchoring it to the membrane - Once signal protein in rER it is cleaved by signal peptidase and degraded resulting in a folded main protein
How are proteins modified post translationally?
- Disulphide bond formation
- Proteolytic cleavage
- Glycosylation
- Phosphorylation
- Prenylation/acylation (addition of lipid)
- Hydroxylation
How is insulin translated post translationally?
- Synthesized on membrane bound ribosomes in rER as preproisulin
- Signal sequence removed and degrades
- Disulphide bonds form between Cyt - 3 form while protein folds into proinsulin
- Before vesicle packaging is proteolytically cleaved in 2 positions releasing a C chain
- Leaves as active fully functional insulin consisting of A and B chains held together by 3 disulphide bonds
