Lecture 15 - Translation of RNA to protein

Question 1

How are amino acids attached to tRNA?

Answer 1

by esterification
Aminoacyl-tRNA Synthetases
amino acyl tRNA synthetases (aaRS) esterify amino acids to tRNAs
one aaRS per amino acid, but usually can aminoacylate >1 tRNA

Question 2

Describe aaRS fidelity and editing

Answer 2

Accuracy of DNA, RNA & protein synth relies on base complementarity (DNA:DNA, DNA:RNA, or mRNA:tRNA)

Matching tRNAs to amino acids by aaRS is base-pairing-independent

Identity of aa is not checked on ribosome, so matching correct aa to tRNA is essential

Fidelity is achieved by editing

challenging for aaRS to distinguish aas – tRNAs are larger, with more distinguishing features

aaRs have evolved editing sites to cope with problem

editing sites hydrolyze aas from misacylated tRNAs

Question 3

What are sources of error in protein synthesis?

Answer 3

1. tRNA Mischarging
   Coupling the wrong amino acid to a tRNA
   Mischarging generally involves similar amino acids – reduces impact of error
2. tRNA Mismatching
   matching mRNA codon to wrong tRNA anticodon
   wobble base-pairs increase mismatching frequency

Question 4

Describe protein synthesis in the ribosome

Answer 4

There are 3 sites that can accommodate the tRNA within the ribosome:
A, P, and E.

Peptidyl-tRNA occupies P site; aminoacyl-tRNA enters A site

Two tRNAs occupy adjacent codons on mRNA within ribosome

Growing polypeptide transfers between tRNAs as peptide bond forms

Translocation of ribosome moves peptidyl-tRNA into P site & ejects previous tRNA from E site

Polypeptide extends through tunnel in large ribosomal subunit

Question 5

Describe the role of peptidyl transferase

Answer 5

Peptidyl transferase centre within ribosome catalyses peptide bond formation
Amino group of aminoacyl-tRNA in A site attacks ester linkage between peptide chain & tRNA in P site
Catalysed by 28S rRNA in peptidyl transferase centre of ribosome
tRNA in A site receives peptide chain & is transferred to P site as ribosome moves along mRNA
Uncharged tRNA is released from E site

Question 6

Describe eIF4

Answer 6

eIF4 complex is composed of four factors

eIF4E binds mRNA cap – highly regulated - deregulated in many cancers

eIF4G serves as a scaffold – eIF4G mutations in familial Parkinson disease compromise its binding to eIF4A & eIF4E

eIF4A is a helicase that unwinds secondary structures in 5’ region of mRNA to allow access by PIC

eIF4B interacts with PIC to bring complexes together