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TranslationStep 2 of Protein Synthesis
Transfer RNA: takes amino acids from the cytoplasm to a ribosome
Each tRNA carries a specific amino acid at one end and has an anticodon on the other end.
Anticodon: a nucleotide triplet which base-pairs with a complementary codon on mRNA.
tRNA is a translator because it reads a nucleic acid word (mRNA codon) and interprets it as a protein word (amino acid).
Molecular Components
The structure and function of tRNA:◦ Job – to take a specific amino acid to the ribosome,
then go pick up another from the cytosol◦ Looks like a cloverleaf if flattened into a 2D structure
due to the H-bonding between bases◦ Its 3D shape is roughly an “L.”◦ An enzyme called aminoacyl-tRNA synthetase
attaches a given amino acid to the appropriate tRNA. There is a different synthetase for each amino acid.
◦ tRNAs can sometimes match multiple codons because of the third base pair not needing to match exactly (“wobble”)
Molecular Components
Ribosomes◦ Facilitate the coupling of tRNA anticodons with
mRNA codons during protein synthesis◦ 2 subunits: large and small◦ Made of proteins and rRNA (ribosomal RNA)◦ Cells contain thousands of ribosomes, so rRNA is
the most abundant type of RNA.◦ Since prokaryotic ribosomes are different from
eukaryotic ribosomes, antibiotic drugs are able to inactivate them without inhibiting the ability of eukaryotic cells to make proteins.
Molecular Components
Ribosomes◦ Each has 3 binding sites:◦ P site: (peptidyl-tRNA site) holds the tRNA
carrying the growing polypeptide chain◦ A site: (aminoacyl-tRNA site) holds the tRNA
carrying the next amino acid to be added to the chain
◦ E site: (exit site) where discharged tRNAs leave the ribosome
◦ The ribosome forms peptide bonds between the amino acids.
Molecular Components
3 stages: Initiation – The small ribosomal unit binds to an
initiator tRNA (carrying Methionine) and mRNA, and then finds the start codon (AUG);
Next, proteins called initiation factors bring this complex and the large ribosomal unit together;
Then, the initiator tRNA is in the P site.
Building a Polypeptide
Elongation◦ Involves proteins called elongation factors◦ Uses energy from GTP (like ATP)◦ mRNA is moved through the ribosome in the 5’ to
3’ direction only◦ Many ribosomes may trail along one mRNA at a
time (polyribosomes)
Building a Polypeptide
Termination◦ Stop codon reaches the A site (triplets do not
code for amino acids)◦ A protein called a release factor binds to the stop
codon and causes the addition of a water molecule instead of an amino acid
◦ The polypeptide is released◦ Translation assembly comes apart
Building a Polypeptide
Protein Folding and Post-translational modifications:◦ A protein may begin to coil and fold
spontaneously.◦ Chaperone proteins (“chaperonins”) usually
help the polypeptide fold correctly.◦ Examples of post-translational modifications
before the protein can do its job… attachment of sugars, lipids, phosphate groups; removal of amino acids from the leading end; enzymatic cleaving into 2 or more pieces
Completing and Targeting the Functional Protein
Targeting Polypeptides to Specific Locations:◦ 2 kinds of ribosomes –◦ Free: suspended in cytosol and mostly synthesize
proteins that dissolve in the cytosol and function there
◦ Bound: attached to the cytosolic side of the endoplasmic reticulum or nuclear envelope; make proteins of the endomembrane system as well as proteins secreted from the cell
Completing and Targeting the Functional Protein
Targeting Polypeptides to Specific Locations:◦ The above ribosomes are identical and can switch
their status from free to bound.◦ Polypeptide synthesis always starts in the cytosol
with a free ribosome.◦ If a protein that are destined for the
endomembrane system are marked by a signal peptide.
◦ As it emerges from the ribosome, a signal-recognition particle (SRP) recognizes it and brings the ribosome to a receptor protein built into the ER.
Completing and Targeting the Functional Protein