protein biosynthesis away from the ribosome genetic code charging trna ribosomes on the ribosome...
TRANSCRIPT
Protein Biosynthesis
• Away from the RibosomeGenetic code
Charging tRNARibosomes
• On the RibosomeInitiation complex
Elongation factorsPeptide bond
formation Termination
Characteristics of the Code• non-overlapping
• degenerate
• triplet
• basically universal over all living species
• polar
• no punctuation
• subject to miscues
• defines a reading frame
OLD MAN AND THE SEAErnest Hemmingway
Insert AOLD MAA NAN DTH ESE A
OLD MAA ANA NDT HES EA
OLD MAA ANA AND THE SEA
OLD MAA NAD THE SEA
Insert A A
Insert AAA
Insert Adelete N
N
^
Framing the Code
Garbled
In frame
In frame
Garbled
tRNA
Charging tRNAQ: What is meant by ChargingA: Charging means placing an amino acid on the
3’ (acceptor) end of the tRNA
Q: So, what’s the big deal?
A: There are 20 amino acids; the code is degenerateThere could be 4 “isoaccepting tRNAs” competing for one
Q: I still don’t see a problem
A: One enzyme must recognize 4 different tRNA species and select the correct amino acid.
Q: One enzyme does all that?
A: No, each tRNA has its own enzyme
Q: What is this enzyme called?
A: Its call Aminoacyl-tRNA Synthetase
Q: So, there are 20 of these enzymes
A: Yes
Q: That makes the job a recognition a little easier then?
A: Yes, but the enzymes still have to distinguish between look-alikes such as leucine and valine, glutamine and glutamate, tyrosine and phenylalanine.
Q: Are all aminoacyl-tRNA synthetases alike?
A: Yes and no. Yes, they perform the same function,i.e., to recognize and transfer the correct amino acid to tRNA.Q: Why no?
A: Because one class (Class I) looks for the anticodon on the tRNA, the other (Class II) looks for other features.
Q: What else?
A: Class I puts the amino acid on the 2’ position of theterminal ribose on tRNA, Class II only the 3’.
Q: So, how does aminoacyl-tRNA synthetase discriminateamino acids and different tRNA species?
A: The key lies in the tRNA itself. Besides the anticodon,tRNAs have other bases that set them apart. These basescalled “identity elements” are found in the terminal ends(acceptor stem) and internal in the tRNA.
Q: Do they also proofreading?A: Yes, but sparingly
Q: How sparingly?
A: Enough to keep errors down to isoleucine mistaken for a valine once every 50,000 times. Ile-tRNA synthetaseactually hydrolyzes the valine-AMP precursor.
Reaction:
CH3
CH3
CH2-CH2CH-COO-
NH3+
+ ATP
CH3
CH3
CH2-CH2CH-CNH3
+
~O
O-P-O-CH2
O Ad
OHHO
O
O
CH3
CH3
CH2 CH-COO-
NH3+
L-Valine
Enzyme Bound
tRNA
L-Leucine
CH3
CH3CH2
CH-COO-
NH3+
CH2
L-Isoleucine
PPi
Codon-Anticodon Interactions
Polarity
5’ 3’3’ 5’
Codon on mRNA
Anticodon on tRNA
Anticodon loop
3’ 5’
C G I
(C, U)G C AmRNA 5’ 3’
Wobble base on anticodon
3rd position
mRNAs are always read 5’ to 3’.
mRNAs are always read 5’ to 3’.
Alanine
Ribonucleoprotein Particles
Ribosomes: The Staging Areas of Protein Synthesis
30S (40S)
16S RNA (18S)23 Peptides (33)
50S (60S)
70S (80S) Monosomes
23S RNA (28S)31 Peptides (49) 5S RNA (5S + 5.8S)
* Mammalian
50S30S
tRNA sites
mRNA
Crevice
Polysomes
Groups of ribosomes attached to a single mRNA