Protein Biosynthesis

1
Protein Biosynthesis

• Away from the Ribosome Genetic
  code Charging tRNA Ribosomes
• On the Ribosome Initiation
  complex Elongation factors Peptide bond
  formation Termination

2
Characteristics of the Code

• non-overlapping
• degenerate
• triplet
• basically universal over all living species
• polar
• no punctuation
• subject to miscues
• defines a reading frame

3
Framing the Code
OLD MAN AND THE SEA
Ernest Hemmingway
Insert A
OLD MAA NAN DTH ESE A
Insert A A
OLD MAA ANA NDT HES EA
OLD MAA ANA AND THE SEA
Insert AAA
Insert A delete N
OLD MAA NAD THE SEA
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tRNA
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Charging tRNA
Q What is meant by Charging
A Charging means placing an amino acid on the
3 (acceptor) end of the tRNA
Q So, whats the big deal?
A There are 20 amino acids the code is
degenerate There could be 4 isoaccepting tRNAs
competing for one
Q I still dont see a problem
A One enzyme must recognize 4 different tRNA
species and select the correct amino acid.
6
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.
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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 the terminal ribose on tRNA, Class II
only the 3.
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Q So, how does aminoacyl-tRNA synthetase
discriminate amino 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 bases called 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 synthetase actually hydrolyzes the
valine-AMP precursor.
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Reaction
ATP
L-Leucine
Enzyme Bound
tRNA
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Codon-Anticodon Interactions
Polarity
Anticodon on tRNA
Codon on mRNA
3
5
Anticodon loop
Alanine
mRNAs are always read 5 to 3.
mRNA
5
3
(C, U)
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Crevice
tRNA sites
Ribosomes The Staging Areas of Protein
Synthesis
mRNA
Ribonucleoprotein Particles
30S
70S (80S) Monosomes
50S
30S (40S)
16S RNA (18S)
23 Peptides (33)
50S (60S)
23S RNA (28S)
31 Peptides (49)
5S RNA (5S 5.8S)
Mammalian
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Polysomes
Groups of ribosomes attached to a single mRNA