PROTEIN SYNTHESIS -- TRANSLATION
I.
CHEMISTRY
A. Amine groups of amino acids are added to carboxyl groups of the peptide chain
1. Dehydration synthesis reaction
2. Peptide bond formation
3. Catalyzed by ribosomes
II. AMINO ACID SEQUENCE
A. General
1. The amino acid sequence of every protein is encoded for by the nucleotide sequence
of DNA
a) There are 20 amino acids that are commonly found in proteins
2. DNA’s nucleotide sequence is not used directly, but is transcribed into mRNA
B. Codons
1. Definition
a) Nucleotide sequences (codons) correspond to specific amino acid types and
translational signals
b) Codons are sequences of three nucleotides
(1) Three bases are read sequentially to code for an amino acid
2. Redundancy
a) There are 64 combinations possible for 4 nucleotides read 3 at a time, enough to
code for 20 amino acids and translational signals
b) Several differRedundancyent nucleotide sequences (codons) can code for the
same amino acid
c) Silent mutations
(1) A mutation which changes a codon, but so that it still codes for the same amino acid, is referred
to as a silent mutation
3. Special codons
a) Stop codons
(a) Three codons do not code for any amino acid
(b) UAA, UAG, UGA
b) Start codons
(1) AUG is referred to as a start codon
(a) This is where translation begins
(b) AUG codes for methionine
4. Reading frame
a) Since codons are read sequentially, an addition or deletion of just one nucleotide
would result in a “shift in reading frame”
(1) Example
UUUCCCAAAGGG codes for Phe-Pro-Lys-Gly
AUUUCCCAAAGGG codes for Ile-Ser-Gln-Arg
UUCCCAAAGGG codes for Phe-Pro-Lys
b) Hence, a single base pair addition of deletion in DNA would result in a protein with
a completely different amino acid sequence
(1) Such a protein would be non-functional
(2) This is referred to as a missense mutation
c) Frame shift mutations can often change a codon from coding for an amino acid to
a stop sequence
(1) Such a protein would be shorter than the normal protein
(2) This is referred to as a nonsense mutation
5. Universality of the genetic code
a) A specific codon codes for the same amino acid, regardless of the species
(1) The genetic code is thought of as almost universal
III. TRANSFER RNA AS ADAPTER MOLECULES
A. Amino acids do not have direct affinity for nucleic acids
1. Therefore, mRNA can not directly serve as a template for protein synthesis
2. There must exist “adapter molecules” which can read the RNA sequence (codons)
and bring with it the correct amino acids
a) This is the function of tRNA molecules
B. General structure
1. 73 to 93 nucleotides long
2. Intrastrand complimentarily causes three stem-loop structures to form (often drawn as
a clover leaf)
C. Regions of tRNA molecules
1. Anticodon
a) One loop contains three nucleotides that interact with mRNA during translation
(1) These three nucleotides of tRNA are referred to as an anticodon
(2) The loop containing the anticodon is referred to as the anticodon loop
2. Amino acid attachment site
a) Near the 3’ terminus is a sequence CCA
b) Specific amino acids are covalently attached here by enzymes called aminoacyltRNA synthetases
3. Aminoacyl-tRNA synthetase recognition regions
a) Different aminoacyl-tRNA synthetases must match a specific amino acid with a
specific tRNAs and their anticodons
(1) Therefore, there should be a region of tRNA that will bind only with the proper aminoacyl-tRNA
synthetase
D. Sixty-one different tRNAs
1. Differ in their anticodon sequence, amino acid attached, and their aminoacyl-tRNA
synthetase recognition sequence
a) There are no tRNA molecules with anticodons for the stop codons (UAG, UAA,
UGA)
E. Aminoacyl-tRNA synthetases
1. More than twenty different enzymes
a) At least one for each amino acid
(1) Aminoacyl-tRNA synthetases and tRNAs are named for the amino acid associated with them
(2) Example, leucyl-tRNA synthetase attaches leucine to tRNALeu
IV. RIBOSOMES
A. Composition
1. Two subunits that can dissociate
2. Contains many proteins and several RNA molecules
B. Prokaryotes
1. 70S ribosome
a) 50S subunit
b) 30S subunit
C. Eukaryotes
1. 80 ribosome (more variability between species)
a) 60S subunit
b) 40S subunit
D. Enzyme
1. Form peptide bonds
E. Scaffold
1. P (peptidyl) tRNA binding site
2. A (aminoacyl) tRNA binding site
V. PROTEIN SYNTHESIS
A. INITIATION
1. 30S subunit of ribosome binds to mRNA
2. Site of initiation
a) Ribosomes bind to the Shine-Delgaro sequence upstream from AUG
(1) Sometimes referred to as the ribosomal binding sequence
3. tRNA-methionine binds to mRNA-ribosome (30S or 40S) complex
4. 50S subunit binds to complex
a) tRNA-methionine occupies the P site
B. ELONGATION
1. Amino acid-tRNA binds to A-site
a) tRNA must have proper anticodon
b) H-bonds between codon and anticodon nucleotides
2. Peptide bond formation
(1) Amino acid in P-site is attached to the amino acid in A site
(2) Covalent bond
(3) Catalyzed by peptidyl transferase
3. Translocation
a) Ribosome moves down mRNA to next codon
(1) tRNA in P-site is released
(2) tRNA in A-site is moved to P-site
(a) A-site is now empty
4. Elongation steps repeat, adding more amino acid
C. TERMINATION
1. Ribosome stalls when it reaches a stop codon (UAA, UAG, or AGA)
a) Interactions with another protein, the releasing factor causes the ribosome to
dissociate, releasing the newly synthesized protein