lecture 22 Flashcards
ribosomes are the
protein factories within cells
ribosomes are present in the
cytosol of all cells
matrix of mitochondria
stroma of chloroplast
ribosome provides environemnt for
interaction of tRNAs and mRNA and catalyzes peptide bond formation
RNA-protein complex
ribosomal proteins (r-proteins) and ribosomal RNAs (rRNA)
eukaryotic ribosome
80S
- 60 S
- 40 S
bacterial ribosome
70 S
- 50S
- 30S
small subunit contains the
deconding center
decoding center
where aminoacetylated tRNAs “read” the genetic code by base pairing with each triplet codon in the mRNA
large subunit contains the
peptidyl transferase center
(active site)
peptidyl transferase
catalyzes peptide bond formation between adjacent amino acids in a growing polypeptide chain
the ribosome is a ribozyme
ribosomal RNA has a fundamental importance in catalyzing protein synthesis
- deproteinized ribosome retains peptidyl transferase activity
- there is no protein within the peptidyl transferase active site in the cyrstal structure of the 50S ribosomal subunit
ribosomal RNA, not protein
is responsible for catalysis of peptide bond formation
stages of translation
- activation of amino acids
- initiation
- elongation
- termination
- protein folding
initation
involves assembly if active ribosome with mRNA and the first aminoacyl-tRNA before 1st peptide bond formaiton
elongation
all reactions from 1st to last peptide bond formation
termination
ribosome reaches a stop codon; release of completed peptide and dissociation of ribosome from mRNA
ribosome has three tRNA binding sites
APE
A (aminoacyl)
Aminoacyl tRNA binding
P (peptidyl tRNA binding)
P binding site
E (exit)
occupied by the tRNA molecule released after the growing polypeptide chain is transferred to the aminoacyl-tRNA
shine dalgarno sequence (translaiton in prokaryotes)
positions the mRNA on the bacterial ribosome by binding to the 16S rRNA
also called the ribosome binding site (RBS)
- consensus sequence - initation signal - 5 to 9 purines, situated 8 to 13 nucleodes on the 5’ side of the start codon
- base pairing with a complementary pyrimidine rich sequence near the 3’ end of the 16S rRNA of the 30 S ribosomal subunit
- mRNA-rRNA int. positions the initating 5’AUG sequence of the mRNA in the precise location on the 30S subunit where it is required for translation initaiton.
all protein synteses start with same aa
methionine, which may be removed later
AUG codon
represents Met, two tRNAs
fMet-tRNA^fmet
charged, used only for initiation
initation factors in bacteria
IF3
IF1
IF2
IF3
prevents premature association of the 30S and 50S
IF1
binds at the A site and blocks tRNA binding during initaiton
IF2 (GTPase)
facilitates binding of charged initiator tRNA to partial P site
translation initation in bacteria
- mRNA, fMet-tRNA^fmet, and IFs bind to 30S
- IF3 leaves when fMet-tRNA^fmet pairs with mRNA
- 50S joins in as IF2 hydrolyzes GTP and IF2-GDP and IF1 leave
- 70S initation complex is ready for elongation
translation in eukaroytes
similar to prokaryotic
- Met-tRNAi^met (not formylated)
- more eIFs required (atleast 12)
- Met-tRNAiMet Always binds to 40S before mRNA binding
- ribosomes are recruited to mRNA by 5’ Cap (NO RBS)
initiation sites in eukaryotic mRNA
NO SD sequence
- modification at both ends are important for initation
- 43S recognizes 5’ cap and then migrates along mRNA
- leader region is scanned in the 5’-3’ direction
- AUG is recognized by 43S complex
Kozak sequence
enhance translation (increase translation efficiency)
Presence of purine three residues before the start codon and G residue immediately following the start codon
translation initation in eukaryotes
displacement of eIFs from 40S (requires eIF5 and eIF5B)
- hydrolysis of the eIF2-bound GTP (eIF2 dissocation)
- association of 60S to form functional 80S ribosome
- initaiton is now complete
Poly A tail contributes to efficient translation by
“circularization of mRNA”
- eIF4F: eIF4A, eIF4E, eIF4G
circularization of the eukaryotic mRNA
eIF4G-PABP interaction
increase the efficiency of translation
